Patent Publication Number: US-2021177337-A1

Title: Neurophysiologic Monitoring System and Related Methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is an international patent application claiming the benefit of priority from commonly owned and co-pending U.S. Provisional Patent Application Ser. No. 61/196,264, entitled “Neurophysiologic Monitoring System,” and filed on Oct. 14, 2008, the entire contents of which is hereby expressly incorporated by reference into this disclosure as if set forth in its entirety herein. 
    
    
     FIELD 
     The present invention relates to a system and methods generally aimed at surgery. More particularly, the present invention is directed at a system and related methods for performing surgical procedures and assessments involving the use of neurophysiologic recordings. 
     BACKGROUND 
     Neurophysiology monitoring has become an increasingly important adjunct to surgical procedures where neural tissue may be at risk. Spinal surgery, in particular, involves working close to delicate tissue in and surrounding the spine, which can be damaged in any number of different ways. For example, an exiting nerve root may be comprised if surgical instruments have to pass near or close to the nerve while accessing the surgical target site in the spine. A spinal nerve and/or exiting nerve root may also be compromised if a pedicle screw, used often to secure fixation of multiple vertebra relative to each other, breaches the cortical layer of the pedicle. Surgeries targeting the spine may also require the retraction of nerve and/or vascular tissue out of the operative corridor. While doing so is necessary, there is a possibility of damaging nerve tissue through over retraction and/or a decreased supply of blood reaching the tissue due to the impingement of the retractor against the vascular tissue. Various neurophysiological techniques have been attempted and developed to monitor delicate nerve tissue during surgery in attempts to reduce the risk inherent in spine surgery (and surgery in general). Because of the complex structure of the spine and nervous system no single monitoring technique has been developed that may adequately assess the risk to nervous tissue in all situations and complex techniques are often utilized in conjunction one or more other complex monitoring techniques. EMG monitoring, for example, may be used to detect the presence of nerve roots near a surgical instrument or a breach formed in a pedicle wall. EMG monitoring is not, however, very effective when spinal cord monitoring is required. 
     When spinal cord monitoring is required, either or both motor evoked potential (MEP) or somatosensory evoked potential (SSEP) monitoring are often chosen. While both MEP and SSEP monitoring can be quite effective at detecting changes in the health of the spinal cord, MEP is limited because it only monitors the ventral column of the spinal cord and SSEP is limited because it only monitors the dorsal column of the spinal cord. Danger to nerve tissue that might then be detected using one these methods may be missed by the other, and vice versa. Thus, it may be most effective to use both MEP and SSEP monitoring during the same procedure, while still potentially needing EMG monitoring as well. 
     EMG, MEP, and SSEP involve complex analysis and specially trained neurophysiologists are generally called upon to perform the monitoring. Even though performed by specialists, interpreting the complex waveforms in this fashion is nonetheless disadvantageously prone to human error and can be disadvantageously time consuming, adding to the duration of the operation and translating into increased health care costs. Even more costly is the fact that the neurophysiologist is required in addition to the actual surgeon performing the spinal operation. Putting the difficulties associated with human interpretation of EMG, MEP, and SSEP monitoring aside, combining such testing in the OR generally requires multiple products to accommodate the differing requirements of each. This is disadvantageous when space is often at such a premium in the operating rooms of today. The present invention is directed at eliminating, or at least reducing the effects of, the above-described problems with the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention includes a system and methods for avoiding harm to neural tissue during surgery. According to a broad aspect, the present invention includes instruments capable of stimulating either the peripheral nerves of a patient, the spinal cord of a patient, or both, additional instruments capable of recording the evoked somatosensory responses, and a processing system. The instrument is configured to deliver a stimulation signal preoperatively, perioperatively, and postoperatively. The processing system is programmed with a set of at least three threshold ranges and configured to receive first stimulation signal to said instrument at a first magnitude. The first magnitude corresponds to a boundary between the pair of ranges. The processing system further receives a second stimulation signal at a second magnitude corresponding to a boundary between a different pair of the ranges. The processing unit is still further programmed to and measure the response of nerves depolarized by said stimulation signals as received by the somatosensory cortex to indicate spinal cord health. 
     According to another broad aspect, the present invention includes a control unit, a patient module, and a plurality of surgical accessories adapted to couple to the patient module. The control unit includes a power supply and is programmed to receive user commands, activate stimulation in a plurality of predetermined modes, process signal data according to defined algorithms, display received parameters and processed data, and monitor system status. The patient module is in communication with the control unit. The patient module is within the sterile field. The patient module includes signal conditioning circuitry, stimulator drive circuitry, and signal conditioning circuitry required to perform said stimulation in said predetermined modes. The patient module includes a processor programmed to perform a plurality of predetermined functions including at least two of static pedicle integrity testing, dynamic pedicle integrity testing, nerve proximity detection, neuromuscular pathway assessment, manual motor evoked potential monitoring, automatic motor evoked potential monitoring, manual somatosensory evoked potential monitoring, automatic motor evoked potential monitoring, non-evoked monitoring, and surgical navigation. 
     According to still another broad aspect, the present invention includes an instrument and a processing system. The instrument is in communication with the processing unit. The instrument is capable of advancement to a surgical target site and is configured to deliver a stimulation signal at least one of while advancing to said target site and after reaching said target site. The processing unit is programmed to perform a plurality of predetermined functions using said instrument including at least two of static pedicle integrity testing, dynamic pedicle integrity testing, nerve proximity detection, neuromuscular pathway assessment, manual motor evoked potential monitoring, automatic motor evoked potential monitoring, manual somatosensory evoked potential monitoring, automatic somatosensory evoked potential monitoring, non-evoked monitoring, and surgical navigation. The processing system has a pre-established profile for at least one of said predetermined functions so as to facilitate the initiation of said at least one predetermined function. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein: 
         FIG. 1  is a block diagram of an exemplary surgical system capable of conducting multiple nerve and spinal cord monitoring functions including but not necessarily limited to SSEP Manual, SSEP Automatic, MEP Manual, MEP Automatic, neuromuscular pathway, bone integrity, nerve detection, and nerve pathology (evoked or free-run EMG) assessments; 
         FIG. 2  is a perspective view showing examples of several components of the neurophysiology system of  FIG. 1 ; 
         FIG. 3  is a perspective view of an example of a control unit forming pail of the neurophysiology system of  FIG. 1 ; 
         FIGS. 4-6  are perspective, top, and side views, respectively, of an example of a patient module forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 7  is a top view of an electrode harness forming part of the neurophysiology system of  FIG. 1 ; 
         FIGS. 8A-8C  are side views of various examples of harness ports forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 9  is a plan view of an example of a label affixed to an electrode connector forming part of the neurophysiology system of  FIG. 1 ; 
         FIGS. 10A-10B  are top views of examples of electrode caps forming part of the neurophysiology system of  FIG. 1 ; 
         FIGS. 11-12  are perspective views of an example of a secondary display forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 13  is an exemplary screen display illustrating one embodiment of a general system setup screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 14  is an exemplary screen display illustrating one embodiment of a detailed profile screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 15  is an exemplary screen display illustrating one embodiment of a custom profile selection screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 16  is an exemplary screen display with features of an electrode test as implemented in one embodiment of an electrode test screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 17  is an exemplary screen display illustrating one embodiment of an SSEP profile selection screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 18  is an exemplary screen display illustrating a second embodiment of a SSEP Manual Stimulus Mode setting with a Left Ulnar Nerve (LUN) Breakout screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 19  is an exemplary screen display illustrating one embodiment of an SSEP Manual Run screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 20  is an exemplary screen display illustrating a second embodiment of an SSEP Manual Run screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 21  is an exemplary screen display illustrating a third embodiment of an SSEP Manual Run screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 22  is an exemplary screen display illustrating a fourth embodiment of an SSEP Manual Run screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 23  is an exemplary screen display illustrating one embodiment of an SSEP Automatic Test Setting screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 24  is an exemplary screen display illustrating one embodiment of an SSEP Automatic Run screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 25  is an exemplary screen display illustrating a second embodiment of an SSEP Automatic Run screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 26  is an exemplary screen display illustrating a third embodiment of an SSEP Automatic Run screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 27  is a screen shot of an example of a Manual MEP monitoring screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 28  is a screen shot of an example of an Automatic MEP monitoring screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 29  is a screenshot of an example of a Twitch Test monitoring screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 30  is a screenshot of an example of a Basic Stimulation EMG monitoring screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 31  is a screenshot of an example of a dynamic stimulation EMG monitoring screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 32  is a screenshot of an example of a Nerve Surveillance EMG monitoring screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 33  is a screenshot of an example of a Free-Run EMG monitoring screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIG. 34  is a screenshot of an example of a Navigated Guidance screen forming part of the neurophysiology system of  FIG. 1 ; 
         FIGS. 35A-D  are graphs illustrating the fundamental steps of a rapid current threshold-hunting algorithm according to one embodiment of the present invention; 
         FIG. 36  is block diagram illustrating the steps of an initiation sequence for determining a relevant safety level prior to determining a precise threshold value according to an alternate embodiment of the threshold hunting algorithm of  FIG. 35A-D ; 
         FIG. 37  is a flowchart illustrating the method by which a multi-channel hunting algorithm determines whether to perform or omit a stimulation; 
         FIG. 38A-C  are graphs illustrating use of the threshold hunting algorithm of  FIG. 39  and further omitting stimulations when the likely result is already clear from previous data; 
         FIG. 39A  is a flowchart illustrating the sequence employed by the algorithm to determine and monitor I thresh ; 
         FIG. 39B  is a graph illustrating the confirmation step employed by the algorithm to determine whether I thresh  has changed from a previous determination; 
         FIG. 40  is a flow chart indicating the steps used to automatically determine optimized parameters for SSEP peripheral nerve stimulation for all four limbs; and 
         FIG. 41  is a flow chart indicating the steps used to automatically determine optimized parameters for SSEP peripheral nerve stimulation for one limb utilizing a threshold determination algorithm. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The systems disclosed herein boast a variety of inventive features and components that warrant patent protection, both individually and in combination. It is also expressly noted that, although described herein largely in terms of use in spinal surgery, the surgical system and related methods described herein are suitable for use in any number of additional procedures, surgical or otherwise, wherein assessing the health of the spinal cord and/or various other nerve tissue may prove beneficial. 
     A surgeon operable neurophysiology system  10  is described herein and is capable of performing a number of neurophysiological and/or guidance assessments at the direction of the surgeon (and/or other members of the surgical team). By way of example only,  FIGS. 1-2  illustrate the basic components of the neurophysiology system  10 . The system comprises a control unit  12  (including a main display  34  preferably equipped with a graphical user interface (GUI) and a processing unit  36  that collectively contain the essential processing capabilities for controlling the system  10 ), a patient module  14 , a stimulation accessory (e.g. a stimulation probe  16 , stimulation clip  18  for connection to various surgical instruments, an inline stimulation hub  20 , and stimulation electrodes  22 ), and a plurality of recording electrodes  24  for detecting electrical potentials. The stimulation clip  18  may be used to connect any of a variety of surgical instruments to the system  10 , including, but not necessarily limited to a pedicle access needle  26 , k-wire  27 , tap  28 , dilator(s)  30 , tissue retractor  32 , etc. One or more secondary feedback devices (e.g. secondary display  46  in  FIG. 11-12 ) may also be provided for additional expression of output to a user and/or receiving input from the user. 
     In one embodiment, the neurophysiology system  10  may be configured to execute any of the functional modes including, but not necessarily limited to, static pedicle integrity testing (“Basic Stimulated EMG”), dynamic pedicle integrity testing (“Dynamic Stimulated EMG”), nerve proximity detection (“XLIF®”), neuromuscular pathway assessment (“Twitch Test”), motor evoked potential monitoring (“MEP Manual” and “MEP Automatic”), somatosensory evoked potential monitoring (“SSEP Manual” and “SSEP Automatic”), non-evoked monitoring (“Free-run EMG”) and surgical navigation (“Navigated Guidance”). The neurophysiology system  10  may also be configured for performance in any of the lumbar, thoracolumbar, and cervical regions of the spine. 
     Before further addressing the various functional modes of the surgical system  10 , the hardware components and features of the system  10  will be describe in further detail. The control unit  12  of the neurophysiology system  10 , illustrated by way of example only in  FIG. 3 , includes a main display  34  and a processing unit  36 , which collectively contain the essential processing capabilities for controlling the neurophysiology system  10 . The main display  34  is preferably equipped with a graphical user interface (GUI) capable of graphically communicating information to the user and receiving instructions from the user. The processing unit  36  contains computer hardware and software that commands the stimulation source (e.g. patient module  14 ,  FIGS. 4-6 ), receives digital and/or analog signals and other information from the patient module  14 , processes EMG and SSEP response signals, and displays the processed data to the user via the display  34 . The primary functions of the software within the control unit  12  include receiving user commands via the touch screen main display  34 , activating stimulation in the appropriate mode (Basic Stimulated EMG, Dynamic Stimulated EMG, XLIF, MEP automatic, MEP manual, SSEP manual, SSEP auto, and Twitch Test), processing signal data according to defined algorithms, displaying received parameters and processed data, and monitoring system status. According to one example embodiment, the main display  34  may comprise a  15 ″ LCD display equipped with suitable touch screen technology and the processing unit  36  may comprise a 2 GHz. The processing unit  36  shown in  FIG. 3  further includes a powered USB port  38  for connection to the patient module  14 , a media drive  40  (e.g. CD, CD-RW, DVD, DVD-RW, etc. . . . ), a network port, wireless network card, and a plurality of additional ports  42  (e.g. USB, IEEE 1394, infrared, etc. . . . ) for attaching additional accessories, such as for example only, navigated guidance sensors, auxiliary stimulation anodes, and external devices (e.g. printer, keyboard, mouse, etc. . . . ). Preferably, during use the control unit  12  sits near the surgical table but outside the surgical field, such as for example, on a table top or a mobile stand. It will be appreciated, however, that if properly draped and protected, the control unit  12  may be located within the surgical (sterile) field. 
     The patient module  14 , shown by way of example only in  FIGS. 4-6 , is communicatively linked to the control unit  12 . In this embodiment the patient module  14  is communicatively linked with and receives power from the control unit  12  via a USB data cable  44 . However, it will be appreciated that the patient module  14  may be supplied with its own power source and other known data cables, as well as wireless technology, may be utilized to establish communication between the patient module  14  and control unit  12 . The patient module  14  contains a digital communications interface to communicate with the control unit  12 , as well as the electrical connections to all recording and stimulation electrodes, signal conditioning circuitry, stimulator drive and steering circuitry, and signal conditioning circuitry required to perform all of the functional modes of the neurophysiology system  10 , including but not necessarily limited to Basic Stimulated EMG, Dynamic Stimulated EMG, XLIF®, Twitch Test, MEP Manual and MEP Automatic, and SSEP. In one example, the patient module  14  includes thirty-two recording channels and eleven stimulation channels. A display (e.g. an LCD screen) may be provided on the face of the patient module  14 , and may be utilized for showing simple status readouts (for example, results of a power on test, the electrode harnesses attached, and impedance data, etc. . . . ) or more procedure related data (for example, a stimulation threshold result, current stimulation level, selected function, etc. . . . ). The patient module  14  may be positioned near the patient in the sterile field during surgery. By way of example, the patient module  14  may be attached to bed rail with the aid of a hook  48  attached to, or forming a part of, the patient module  14  casing. 
     With reference to  FIGS. 4-6 , patient module  14  comprises a multitude of ports and indicators for connecting and verifying connections between the patient module  14  and other system components. A control unit port  50  is provided for data and power communication with the control unit  12 , via USB data cable  44  as previously described. There are four accessory ports  52  provided for connecting up to the same number of surgical accessories, including, but not necessarily limited to, stimulation probe  16 , stimulation clip  18 , inline stimulation hub  20 , and navigated guidance sensor (or tilt sensor)  54 . The accessory ports  52  include a stimulation cathode and transmit digital communication signals, tri-color LED drive signals, button status signals, identification signals, and power between the patient module  14  and the attached accessory. A pair of anode ports  56 , preferably comprising 2 wire DIN connectors, may be used to attach auxiliary stimulation anodes should it become desirable or necessary to do so during a procedure. A pair of USB ports  58  are connected as a USB hub to the control unit  12  and may be used to make any number of connections, such as for example only, a portable storage drive. 
     As soon as a device is plugged into any one of ports  50 ,  52 ,  56 , or  58 , the neurophysiology system  10  automatically performs a circuit continuity check to ensure the associated device will work properly. Each device forms a separate closed circuit with the patient module such that the devices may be checked independent of each other. If one device is not working properly the device may be identified individually while the remaining devices continue indicate their valid status. An indicator LED is provided for each port to convey the results of the continuity check to the user. Thus, according to the example embodiment of  FIGS. 7-9 , the patient module  14  includes one control unit indicator  60 , four accessory indicators  62 , two anode indicators  64 , and two USB indicators  66 . According to a preferred embodiment, if the system detects an incomplete circuit during the continuity check, the appropriate indicator will turn red alerting the user that the device might not work properly. On the other hand, if a complete circuit is detected, the indicator will appear green signifying that the device should work as desired. Additional indicator LEDs are provided to indicate the status of the system and the MEP stimulation. The system indicator  68  will appear green when the system is ready and red when the system is not ready. The MEP stim indicator  70  lights up when the patient module is ready to deliver and MEP stimulation signal. In one embodiment, the MEP stim indicator  68  appears yellow to indicate a ready status. 
     To connect the array of recording electrodes  24  and stimulation electrodes  22  utilized by the system  10 , the patient module  14  also includes a plurality of electrode harness ports. In the embodiment shown, the patient module  14  includes an EMG/MEP harness port  72 , SSEP harness port  74 , and an Auxiliary harness port  76  (for expansion and/or custom harnesses). Each harness port  72 ,  74 , and  76  includes a shaped socket  78  that corresponds to a matching shaped connector  82  on the appropriate electrode harness  80 . In addition, the neurophysiology system  10  may preferably employ a color code system wherein each modality (e.g. EMG, EMG/MEP, and SSEP) has a unique color associated with it. By way of example only and as shown herein, EMG monitoring (including, screw tests, detection, and nerve retractor) may be associated with the color green, MEP monitoring with the color blue, and SSEP monitoring may be associated with the color orange. Thus, each harness port  72 ,  74 ,  76  is marked with the appropriate color which will also correspond to the appropriate harness  80 . Utilizing the combination of the dedicated color code and the shaped socket/connector interface simplifies the setup of the system, reduces errors, and can greatly minimize the amount of pre-operative preparation necessary. The patient module  14 , and especially the configuration of quantity and layout of the various ports and indicators, has been described according to one example embodiment of the present invention. It should be appreciated, however, that the patient module  14  could be configured with any number of different arrangements without departing from the scope of the invention. 
     As mentioned above, to simplify setup of the system  10 , all of the recording electrodes  24  and stimulation electrodes  22  that are required to perform one of the various functional modes (including a common electrode  23  providing a ground reference to pre-amplifiers in the patient module  14 , and an anode electrode  25  providing a return path for the stimulation current) are bundled together and provided in single electrode harness  80 , as illustrated, by way of example only, in  FIG. 7 . Depending on the desired function or functions to be used during a particular procedure, different groupings of recoding electrodes  24  and stimulation electrodes  22  may be required. By way of example, the SSEP function requires more stimulating electrodes  22  than either the EMG or MEP functions, but also requires fewer recording electrodes than either of the EMG and MEP functions. To account for the differing electrode needs of the various functional modes, the neurophysiology system  10  may employ different harnesses  80  tailored for the desired modes. According to one embodiment, three different electrode harnesses  80  may be provided for use with the system  10 , an EMG harness, an EMG/MEP harness, and an SSEP harness. 
     At one end of the harness  80  is the shaped connector  82 . As described above, the shaped connector  82  interfaces with the shaped socket  72 ,  74 , or  76  (depending on the functions harness  80  is provided for). Each harness  80  utilizes a shaped connector  82  that corresponds to the appropriate shaped socket  72 ,  74 ,  76  on the patient module  14 . If the shapes of the socket and connector do not match the harness  80 , connection to the patient module  14  cannot be established. According to one embodiment, the EMG and the EMG/MEP harnesses both plug into the EMG/MEP harness port  72  and thus they both utilize the same shaped connector  82 . By way of example only,  FIGS. 8A-8C  illustrate the various shape profiles used by the different harness ports  72 ,  74 ,  76  and connectors  82 .  FIG. 8A  illustrates the half circular shape associated with the EMG and EMG/MEP harness and port  72 .  FIG. 8B  illustrates the rectangular shape utilized by the SSEP harness and port  74 . Finally,  FIG. 8C  illustrates the triangular shape utilized by the Auxiliary harness and port  76 . Each harness connector  82  includes a digital identification signal that identifies the type of harness  80  to the patient module  14 . At the opposite end of the electrode harness  80  are a plurality of electrode connectors  102  linked to the harness connector  82  via a wire lead. Using the electrode connector  102 , any of a variety of known electrodes may be used, such as by way of example only, surface dry gel electrodes, surface wet gel electrodes, and needle electrodes. 
     To facilitate easy placement of scalp electrodes used during MEP and SSEP modes, an electrode cap  81 , depicted by way of example only in  FIG. 10A  may be used. The electrode cap  81  includes two recording electrodes  23  for SSEP monitoring, two stimulation electrodes  22  for MEP stimulation delivery, and an anode  23 . Graphic indicators may be used on the electrode cap  81  to delineate the different electrodes. By way of example, lightning bolts may be used to indicate a stimulation electrode, a circle within a circle may be used to indicate recording electrodes, and a stepped arrow may be used to indicate the anode electrode. The anode electrode wire is colored white to further distinguish it from the other electrodes and is significantly longer that the other electrode wires to allow placement of the anode electrode on the patient&#39;s shoulder. The shape of the electrode cap  81  may also be designed to simplify placement. By way of example only, the cap  81  has a pointed end that may point directly toward the patient&#39;s nose when the cap  81  is centered on the head in the right orientation. A single wire may connect the electrode cap  81  to the patient module  14  or electrode harness  80 , thereby decreasing the wire population around the upper regions of the patient. Alternatively, the cap  81  may be equipped with a power supply and a wireless antenna for communicating with the system  10 .  FIG. 10B  illustrates another example embodiment of an electrode cap  83  similar to cap  81 . Rather than using graphic indicators to differentiate the electrodes, colored wires may be employed. By way of example, the stimulation electrodes  22  are colored yellow, the recording electrodes  24  are gray, and the anode electrode  23  is white. The anode electrode is seen here configured for placement on the patient&#39;s forehead. According to an alternate embodiment, the electrode cap (not shown) may comprise a strap or set of straps configured to be worn on the head of the patient. The appropriate scalp recording and stimulation sites may be indicated on the straps. By way of example, the electrode cap may be imbued with holes overlying each of the scalp recording sites (for SSEP) and scalp stimulation sites (for MEP). According to a further example embodiment, the border around each hole may be color coded to match the color of an electrode lead wire designated for that site. In this instance, the recording and stimulation electrodes designated for the scalp are preferably one of a needle electrode and a corkscrew electrode that can be placed in the scalp through the holes in the cap. 
     In addition to or instead of color coding the electrode lead wires to designated intended placement, the end of each wire lead next to the electrode connector  102  may be tagged with a label  86  that shows or describes the proper positioning of the electrode on the patient. The label  86  preferably demonstrates proper electrode placement graphically and textually. As shown in  FIG. 9 , the label may include, a graphic image showing the relevant body portion  88  and the precise electrode position  90 . Textually, the label  86  may indicate the side  100  and muscle (or anatomic location)  96  for placement, the function of the electrode (e.g. stimulation, recording channel, anode, and reference—not shown), the patient surface (e.g. anterior or posterior), the spinal region  94 , and the type of monitoring  92  (e.g. EMG, MEP, SSEP, by way of example, only). According to one embodiment (set forth by way of example only), the electrode harnesses  80  are designed such that the various electrodes may be positioned about the patient (and preferably labeled accordingly) as described in Table 1 for Lumbar EMG, Table 2 for Cervical EMG, Table 3 for Lumbar/Thoracolumbar EMG and MEP, Table 4 for Cervical EMG and MEP, and Table 5 for SSEP: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Lumbar EMG 
               
            
           
           
               
               
               
               
            
               
                   
                 Electrode Type 
                 Electrode Placement 
                 Spinal Level 
               
               
                   
                   
               
               
                   
                 Ground 
                 Upper Outer Thigh 
                 — 
               
               
                   
                 Anode 
                 Latissimus Dorsi 
                 — 
               
               
                   
                 Stimulation 
                 Knee 
                 — 
               
               
                   
                 Recording 
                 Left Tibialis Anterior 
                 L4, L5 
               
               
                   
                 Recording 
                 Left Gastroc. Medialis 
                 S1, S2 
               
               
                   
                 Recording 
                 Left Vastus Medialis 
                 L2, L3, L4 
               
               
                   
                 Recording 
                 Left Biceps Femoris 
                 L5, S1, S2 
               
               
                   
                 Recording 
                 Right Biceps Femoris 
                 L5, S1, S2 
               
               
                   
                 Recording 
                 Right Vastus Medialis 
                 L2, L3, L4 
               
               
                   
                 Recording 
                 Right Gastroc. Medialis 
                 S1, S2 
               
               
                   
                 Recording 
                 Right Tibialis Anterior 
                 L4, L5 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Cervical EMG 
               
            
           
           
               
               
               
               
            
               
                   
                 Electrode Type 
                 Electrode Placement 
                 Spinal Level 
               
               
                   
                   
               
               
                   
                 Ground 
                 Shoulder 
                 — 
               
               
                   
                 Anode 
                 Mastoid 
                 — 
               
               
                   
                 Stimulation 
                 Inside Elbow 
                 — 
               
               
                   
                 Recording 
                 Left Triceps 
                 C7, C8 
               
               
                   
                 Recording 
                 Left Flexor Carpi Radialis 
                 C6, C7, C8 
               
               
                   
                 Recording 
                 Left Deltoid 
                 C5, C6 
               
               
                   
                 Recording 
                 Left Trapezius 
                 C3, C4 
               
               
                   
                 Recording 
                 Left Vocal Cord 
                 RLN 
               
               
                   
                 Recording 
                 Right Vocal Cord 
                 RLN 
               
               
                   
                 Recording 
                 Right Trapezius 
                 C3, C4 
               
               
                   
                 Recording 
                 Right Deltoid 
                 C5, C6 
               
               
                   
                 Recording 
                 Right Flexor Carpi Radialis 
                 C6, C7, C8 
               
               
                   
                 Recording 
                 Right Triceps 
                 C7, C8 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Lumbar/Thoracolumbar EMG + MEP 
               
            
           
           
               
               
               
               
            
               
                   
                 Electrode Type 
                 Electrode Placement 
                 Spinal Level 
               
               
                   
                   
               
               
                   
                 Ground 
                 Upper Outer Thigh 
                 — 
               
               
                   
                 Anode 
                 Latissimus Dorsi 
                 — 
               
               
                   
                 Stimulation 
                 Knee 
                 — 
               
               
                   
                 Recording 
                 Left Tibialis Anterior 
                 L4, L5 
               
               
                   
                 Recording 
                 Left Gastroc. Medialis 
                 S1, S2 
               
               
                   
                 Recording 
                 Left Vastus Medialis 
                 L2, L3, L4 
               
               
                   
                 Recording 
                 Left Biceps Femoris 
                 L5, S1, S2 
               
               
                   
                 Recording 
                 Left APB-ADM 
                 C6, C7, C8, T1 
               
               
                   
                 Recording 
                 Right APB-ADM 
                 C6, C7, C8, T1 
               
               
                   
                 Recording 
                 Right Biceps Femoris 
                 L5, S1, S2 
               
               
                   
                 Recording 
                 Right Vastus Medialis 
                 L2, L3, L4 
               
               
                   
                 Recording 
                 Right Gastroc. Medialis 
                 S1, S2 
               
               
                   
                 Recording 
                 Right Tibialis Anterior 
                 L4, L5 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Cervical EMG + MEP 
               
            
           
           
               
               
               
               
            
               
                   
                 Electrode Type 
                 Electrode Placement 
                 Spinal Level 
               
               
                   
                   
               
               
                   
                 Ground 
                 Shoulder 
                 — 
               
               
                   
                 Anode 
                 Mastoid 
                 — 
               
               
                   
                 Stimulation 
                 Inside Elbow 
                 — 
               
               
                   
                 Recording 
                 Left Tibialis Anterior 
                 L4, L5 
               
               
                   
                 Recording 
                 Left Flexor Carpi Radialis 
                 C6, C7, C8 
               
               
                   
                 Recording 
                 Left Deltoid 
                 C5, C6 
               
               
                   
                 Recording 
                 Left Trapezius 
                 C3,C4 
               
               
                   
                 Recording 
                 Left APB-ADM 
                 C6, C7, C8, T1 
               
               
                   
                 Recording 
                 Left Vocal Cord 
                 RLN 
               
               
                   
                 Recording 
                 Right Vocal Cord 
                 RLN 
               
               
                   
                 Recording 
                 Right APB-ADM 
                 C6, C7, C8, T1 
               
               
                   
                 Recording 
                 Right Trapezius 
                 C3, C4 
               
               
                   
                 Recording 
                 Right Deltoid 
                 C5, C6 
               
               
                   
                 Recording 
                 Right Flexor Carpi Radialis 
                 C6, C7, C8 
               
               
                   
                 Recording 
                 Right Tibialis Anterior 
                 L4, L5 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 SSEP 
               
            
           
           
               
               
               
               
            
               
                   
                 Electrode Type 
                 Electrode Placement 
                 Spinal Level 
               
               
                   
                   
               
               
                   
                 Ground 
                 Shoulder 
                 — 
               
               
                   
                 Stimulation 
                 Left Post Tibial Nerve 
                 — 
               
               
                   
                 Stimulation 
                 Left Ulnar Nerve 
                 — 
               
               
                   
                 Stimulation 
                 Right Post Tibial Nerve 
                 — 
               
               
                   
                 Stimulation 
                 Right Ulnar Nerve 
                 — 
               
               
                   
                 Recording 
                 Left Popliteal Fossa 
                 — 
               
               
                   
                 Recording 
                 Left Erb&#39;s Point 
                 — 
               
               
                   
                 Recording 
                 Left Scalp Cp3 
                 — 
               
               
                   
                 Recording 
                 Right Popliteal Fossa 
                 — 
               
               
                   
                 Recording 
                 Right Erb&#39;s Point 
                 — 
               
               
                   
                 Recording 
                 Right Scalp Cp4 
                 — 
               
               
                   
                 Recording 
                 Center Scalp Fpz 
                 — 
               
               
                   
                 Recording 
                 Center Scalp Cz 
                 — 
               
               
                   
                 Recording 
                 Center Cervical Spine 
                 — 
               
               
                   
                   
               
            
           
         
       
     
     As mentioned above, the neurophysiology monitoring system  10  may include a secondary display, such as for example only, the secondary display  46  illustrated in  FIGS. 11-12 . The secondary display  46  may be configured to display some or all of the information provided on main display  34 . The information displayed to the user on the secondary display  34  may include, but is not necessarily limited to, alpha-numeric and/or graphical information regarding any of the selected function modes (e.g. SSEP Manual, SSEP Automatic, MEP Manual, MEP Automatic, Twitch Test, Basic Stimulated EMG, Dynamic Stimulated EMG, XLIF, Free-Run EMG, and Navigated Guidance), attached accessories (e.g. stimulation probe  16 , stimulation clip  18 , tilt sensor  54 ), electrode harness or harnesses attached, impedance test results, myotome/EMG levels, stimulation levels, history reports, selected parameters, test results, etc . . . . In one embodiment, secondary display  46  may be configured to receive user input in addition to its display function. The secondary display  46  can thus be used as an alternate control point for the system  10 . The control unit  12  and secondary display  46  may be linked such that input may be received on from one display without changing the output shown on the other display. This would allow the surgeon to maintain focus on the patient and test results while still allowing other members of the OR staff to manipulate the system  10  for various purposes (e.g. inputting annotations, viewing history, etc . . . ). The secondary display  46  may be battery powered. Advantageously, the secondary display  46  may be positioned inside the sterile field as well as outside the sterile field. For positioning within the sterile field a disposable sterile case  47  may be provided to house the display. Alternatively, the display  46  may be sterile bagged. Both the sterile case  47  and the secondary display  46  may be mounted to a pole, bed frame, light fixture, or other apparatus found near and/or in the surgical field. It is further contemplated that multiple secondary displays  46  may be linked to the control unit  12 . This may effectively distribute neurophysiology information and control throughout the operating room. By way of example, a secondary display  46  may also be provided for the anesthesiologist. This may be particularly useful in providing the anesthesiologist with results from the Twitch Test and providing reminders about the use of paralytics, which may adversely affect the accuracy of the neurophysiology system  10 . Wired or wireless technology may be utilized to link the secondary display  46  to the control unit  12 . 
     Having described an example embodiment of the system  10  and the hardware components that comprise it, the neurophysiological functionality and methodology of the system  10  will now be described in further detail. Various parameters and configurations of the neuromonitoring system  10  may depend upon the target location (i.e. spinal region) of the surgical procedure and/or user preference. In one embodiment, upon starting the system  10  the software will open to a startup screen, illustrated by way of example only, in  FIG. 13 . The startup screen includes a profile selection window  160  from which the user may select from one of the standard profiles (e.g. “Standard Cervical,” “Standard Thoracolumbar,” and “Standard Lumbar”) or any custom profiles that have been previously saved to the system. Profiles may be arranged for selection, alphabetically, by spinal region, or by other suitable criteria. Profiles may be saved to the control unit hard drive or to a portable memory device, such as for example, a USB memory drive, or on a web server. 
     Selecting a profile configures the system  10  to the parameters assigned for the selected profile (standard or custom). The availability of different function modes may depend upon the profile selected. By way of example only, selecting the cervical and thoracolumbar spinal regions may automatically configure the options to allow selection of the SSEP Manual, SSEP Automatic, MEP Manual, MEP Automatic, Twitch Test, Basic Stimulated EMG, Dynamic Stimulated EMG, XLIF, Free-Run EMG, and Navigated Guidance modes, while selecting the lumbar region may automatically configure the options to allow selection of the Twitch Test, Basic, Difference, and Dynamic Stimulated EMG Tests, XLIF®, and Nerve Retractor modes. Default parameters associated with the various function modes may also depend on the profile selected, for example, the characteristics of the stimulation signal delivered by the system  10  may vary depending on the profile. By way of example, the stimulation signal utilized for the Stimulated EMG modes may be configured differently when a lumbar profile is selected versus when one of a thoracolumbar profile and a cervical profile. 
     As previously described above, each of the hardware components includes an identification tag that allows the control unit  12  to determine which devices are hooked up and ready for operation. In one embodiment, profiles may only be available for selection if the appropriate devices (e.g. proper electrode harness  80  and stimulation accessories) are connected and/or ready for operation. Alternatively, the software could bypass the startup screen and jump straight to one of the functional modes based on the accessories and/or harnesses it knows are plugged in. The ability to select a profile based on standard parameters, and especially on customized preferences, may save significant time at the beginning of a procedure and provides for monitoring availability right from the start. Moving on from the startup screen, the software advances directly to an electrode test screen and impedance tests, which are performed on every electrode as discussed above. When an acceptable impedance test has been completed, the system  10  is ready to begin monitoring and the software advances to a monitoring screen from which the neurophysiological monitoring functions of the system  10  are performed. 
     The information displayed on the monitoring screen may include, but is not necessarily limited to, alpha-numeric and/or graphical information regarding any of the functional modes (e.g. SSEP Manual, SSEP Automatic, MEP Manual, MEP Automatic, Twitch Test, Basic Stimulated EMG, Dynamic Stimulated EMG, XLIF, Free-Run EMG, and Navigated Guidance), attached accessories (e.g. stimulation probe  16 , stimulation clip  18 , tilt sensor  54 ), electrode harness or harnesses attached, impedance test results, myotome/EMG levels, stimulation levels, history reports, selected parameters, test results, etc. . . . . In one embodiment, set forth by way of example only, this information displayed on a main monitoring screen may include, but is not necessarily limited to the following components as set forth in Table 6: 
     
       
         
           
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                 Screen Component 
                 Description 
               
               
                   
               
             
            
               
                 Patient Image/ 
                 An image of the human body or relevant portion thereof showing the 
               
               
                 Electrode layout 
                 electrode placement on the body, with labeled channel number tabs on 
               
               
                   
                 each side (1-4 on the left and right). Left and right labels will show the 
               
               
                   
                 patient orientation. The channel number tabs may be highlighted or 
               
               
                   
                 colored depending on the specific function being performed. 
               
               
                 Myotome &amp; Level 
                 A label to indicate the Myotome name and corresponding Spinal 
               
               
                 Names 
                 Level(s) associated with the channel of interest. 
               
               
                 Test Menu 
                 A hideable menu bar for selecting between the available functional 
               
               
                   
                 modes. 
               
               
                 Device Bar 
                 A hideable bar displaying icons and/or names of devices connected to 
               
               
                   
                 the patient module. 
               
               
                 Display Area 
                 Shows procedure-specific information including stimulation results. 
               
               
                 Color Indication 
                 Enhances stimulation results with a color display of green, yellow, or 
               
               
                   
                 red corresponding to the relative safety level determined by the system. 
               
               
                 Stimulation Bar 
                 A graphical stimulation indicator depicting the present stimulation 
               
               
                   
                 status (i.e. on or off and stimulation current level), as well as providing 
               
               
                   
                 for starting and stopping stimulation 
               
               
                 Event Bar 
                 A hideable bar that shows the last up to a selected number of previous 
               
               
                   
                 stimulation results, provides for annotation of results, and a chat 
               
               
                   
                 dialogue box for communicating with remote participants. 
               
               
                 EMG waveforms 
                 EMG waveforms may be optionally displayed on screen along with the 
               
               
                   
                 stimulation results. 
               
               
                   
               
            
           
         
       
     
     From a profile setting window  160 , illustrated by way of example only in  FIG. 14 , custom profiles can be created and saved. Beginning with one of the standard profiles, parameters may be altered by selecting one of the audio  168 , site selection  170 , test selection  172 , and waveform scaling  174  buttons and making the changes until the desired parameters are set. By way of example only, profiles may be generated and saved for particular procedures (e.g. ACDF, XLIF, and Decompression), particular individuals, and combinations thereof. Clicking on each button will display the parameter options specific to the selected button in a parameter window  176 . The parameter options for the Test Selection Window are illustrated by way of example in FIG.  14 . By way of example only, by selecting the Test Selection button, session tests may be added and viewing options may be changed. From within the test selection area, function specific parameters for all available test functions (based on site selection, available devices, etc . . . ) may be accessed and set according to need. One option that is available for multiple functions under the test selection button is the ability to select from three different viewing options. The user may choose to see results displayed in numeric form, on a body panel, and on a label that reflects the labels associated with each electrode, or any combination of the three. The user may also choose to see the actual waveforms. Selecting the Waveform Scaling button  174  allows the user to adjust the scale on which waveforms are displayed. By selecting the audio button  168  both the system audio and Free Run audio may be adjusted. Selecting the site selection button  170  allows the opportunity to change from the site selected initially. Adjusting the site selection of the profile may alter the options available. By way of example, if the user changes the site selection from cervical to lumbar, the MEP function may no longer be selectable as an option.  FIG. 13  is an example of a site selection screen.  FIGS. 19-26 ;  28 - 35  illustrates examples of the test selection tab for each of the test functions (e.g. SSEP Manual, SSEP Automatic, MEP Manual, MEP Automatic, Twitch Test, Basic Stimulated EMG, Dynamic Stimulated EMG, XLIF, Free-Run EMG, and Navigated Guidance). Profiles may be saved directly on the control unit  12  or they may be saved to a portable memory device, or uploaded onto a web-server. 
     Various features of the monitoring screen  200  of the GUI will now be described. The patient module  14  is configured such that the neurophysiology system  10  may conduct an impedance test under the direction of the control unit  12  of all electrodes once the system is set up and the electrode harness is connected and applied to the patient. After choosing the appropriate spinal site upon program startup (described below), the user is automatically directed to an electrode test.  FIG. 16  illustrates, by way of example only, the features of the electrode test by graphical implementations of electrode test screens according to example embodiments of the GUI. The electrode test screen  104  includes a human figure graphic  105  with electrode position indicators  108 . A harness indicator  109  displays the harness or harnesses  80  that are connected to the patient module  14 . For each electrode on the harness or harnesses  80  in use, including the common  25  and anode  23  electrodes, there is a corresponding channel button  110 . Preferably, the common  25  and anode  23  electrodes may be independently checked for acceptable impedance. To accomplish this, the anode  23  and common  25  are both provided as dual electrodes. At least one of the anode leads on the anode electrode is reversible. During the impedance check the reversible anode lead switches to a cathode such that the impedance between the leads can be measured. When the impedance test is complete the reversible lead switches back to an anode. The channel button  110  may be labeled with the muscle name or coverage area of the corresponding electrode. Stimulation electrodes may be denoted with a symbol or other indicator, such as by way of example only, a lightning bolt in order to distinguish the recording and stimulation electrodes. Selecting a channel button  110  will disable the associated channel. Disabled channels will not be tested for impedance and they will not be monitored for responses or errors unless reactivated (e.g. by again selecting the corresponding channel button  110 ). Upon selection of a start button  106  (entitled “Run Electrode Test”), the system  10  tests each electrode individually to determine the impedance value. If the impedance is determined to be within acceptable limits, the channel button  110  and corresponding electrode depiction on the human  FIG. 108  turn green. If the impedance value for any electrode is not determined to be acceptable, the associated channel button  110  and electrode depiction turn red, alerting the user. Once the test is complete, selecting the Accept button  112  will open the main monitoring screen of system  10 . 
     The functions performed by the neuromonitoring system  10  may include, but are not necessarily limited to, the Twitch Test, Free-run EMG, Basic Stimulated EMG, Dynamic Stimulated EMG, XLIF®, Nerve Retractor, MEP Manual, MEP Automatic, and SSEP Manual, SSEP Automatic, and Navigated Guidance modes, all of which will be described briefly below. The Twitch Test mode is designed to assess the neuromuscular pathway via the so-called “train-of-four test” to ensure the neuromuscular pathway is free from muscle relaxants prior to performing neurophysiology-based testing, such as bone integrity (e.g. pedicle) testing, nerve detection, and nerve retraction. This is described in greater detail within PCT Patent App. No. PCT/US2005/036089, entitled “System and Methods for Assessing the Neuromuscular Pathway Prior to Nerve Testing,” filed Oct. 7, 2005, the entire contents of which is hereby incorporated by reference as if set forth fully herein. The Basic Stimulated EMG Dynamic Stimulated EMG tests are designed to assess the integrity of bone (e.g. pedicle) during all aspects of pilot hole formation (e.g., via an awl), pilot hole preparation (e.g. via a tap), and screw introduction (during and after). These modes are described in greater detail in PCT Patent App. No. PCT/US02/35047 entitled “System and Methods for Performing Percutaneous Pedicle Integrity Assessments,” filed on Oct. 30, 2002, and PCT Patent App. No. PCT/US2004/025550, entitled “System and Methods for Performing Dynamic Pedicle Integrity Assessments,” filed on Aug. 5, 2004 the entire contents of which are both hereby incorporated by reference as if set forth fully herein. The XLIF mode is designed to detect the presence of nerves during the use of the various surgical access instruments of the neuromonitoring system  10 , including the pedicle access needle  26 , k-wire  42 , dilator  44 , and retractor assembly  70 . This mode is described in greater detail within PCT Patent App. No. PCT/US2002/22247, entitled “System and Methods for Determining Nerve Proximity, Direction, and Pathology During Surgery,” filed on Jul. 11, 2002, the entire contents of which is hereby incorporated by reference as if set forth fully herein. The Nerve Retractor mode is designed to assess the health or pathology of a nerve before, during, and after retraction of the nerve during a surgical procedure. This mode is described in greater detail within PCT Patent App. No. PCT/US2002/30617, entitled “System and Methods for Performing Surgical Procedures and Assessments,” filed on Sep. 25, 2002, the entire contents of which are hereby incorporated by reference as if set forth fully herein. The MEP Auto and MEP Manual modes are designed to test the motor pathway to detect potential damage to the spinal cord by stimulating the motor cortex in the brain and recording the resulting EMG response of various muscles in the upper and lower extremities. The SSEP function is designed to test the sensory pathway to detect potential damage to the spinal cord by stimulating peripheral nerves inferior to the target spinal level and recording the action potential from sensors superior to the spinal level. The MEP Auto, MEP manual, and SSEP modes are described in greater detail within PCT Patent App. No. PCT/US2006/003966, entitled “System and Methods for Performing Neurophysiologic Assessments During Spine Surgery,” filed on Feb. 2, 2006, the entire contents of which is hereby incorporated by reference as if set forth fully herein. The Navigated Guidance function is designed to facilitate the safe and reproducible use of surgical instruments and/or implants by providing the ability to determine the optimal or desired trajectory for surgical instruments and/or implants and monitor the trajectory of surgical instruments and/or implants during surgery. This mode is described in greater detail within PCT Patent App. No. PCT/US2007/11962, entitled “Surgical Trajectory Monitoring System and Related Methods,” filed on Jul. 30, 2007, and PCT Patent App. No. PCT/US2008/12121, the entire contents of which are each incorporated herein by reference as if set forth fully herein. These functions will be explained now in brief detail. 
     The neuromonitoring system  10  performs assessments of spinal cord health using one or more of MEP Auto, MEP Manual, SSEP Auto, and SSEP manual modes. 
     In the SSEP modes, the neuromonitoring system  10  stimulates peripheral sensory nerves that exit the spinal cord below the level of surgery and then measures the electrical action potential from electrodes located on the nervous system superior to the surgical target site. Recording sites below the applicable target site are also preferably monitored as a positive control measure to ensure variances from normal or expected results are not due to problems with the stimulation signal deliver (e.g. misplaced stimulation electrode, inadequate stimulation signal parameters, etc.). To accomplish this, stimulation electrodes  22  may be placed on the skin over the desired peripheral nerve (such as by way of example only, the left and right Posterior Tibial nerve and/or the left and right Ulnar nerve) and recording electrodes  24  are positioned on the recording sites (such as, by way of example only, C2 vertebra, Cp3 scalp, Cp4 scalp, Erb&#39;s point, Popliteal Fossa) and stimulation signals are delivered from the patient module  14 . 
     Damage in the spinal cord may disrupt the transmission of the signal up along the spinothalamic pathway through the spinal cord resulting in a weakened, delayed, or absent signal at the recording sites superior to the surgery location (e.g. cortical and subcortical sites). To check for these occurrences, the system  10  monitors the amplitude and latency of the evoked signal response. According to one embodiment, the system  10  may perform SSEP in either of two modes: Automatic mode and Manual mode. In SSEP Auto mode, the system  10  compares the difference between the amplitude and latency of the signal response vs. the amplitude and latency of a baseline signal response. The difference is compared against predetermined “safe” and “unsafe” levels and the results are displayed on display  34 . According to one embodiment, the system may determine safe and unsafe levels based on each of the amplitude and latency values for each of the cortical and subcortical sites individually, for each stimulation channel. That is, if either of the subcortical and cortical amplitudes decrease by a predetermined level, or either of the subcortical and cortical latency values increase by a predetermined level, the system may issue a warning. By way of example, the alert may comprise a Red, Yellow, Green type warning associated with the applicable channel wherein Red indicates that at least one of the determined values falls within the unsafe level, the color green may indicate that all of the values fall within the safe level, and the color yellow may indicate that at least one of the values falls between the safe and unsafe levels. To generate more information, the system  10  may analyze the results in combination. With this information, in addition to the Red, Yellow, and Green alerts, the system  10  may indicate possible causes for the results achieved. In SSEP Manual mode, signal response waveforms and amplitude and latency values associated with those waveforms are displayed for the user. The user then makes the comparison between a baseline the signal response. 
       FIGS. 17-22  are exemplary screen displays of the “SSEP Manual” mode according to one embodiment of the neuromonitoring system  10 .  FIG. 17  illustrates an intra-operative monitoring (IOM) setup screen from which various features and parameters of the SSEP Manual mode may be controlled and/or adjusted by the user as desired. Using this screen, the user has the opportunity to toggle between Manual mode and Automatic mode, select a stimulation rate, and change one or more stimulation settings (e.g. stimulation current, pulse width, and polarity) for each stimulation target site (e.g. left ulnar nerve, right ulnar nerve, left tibial nerve, and right tibial nerve). By way of example only, the user may change one or more stimulation settings of each peripheral nerve by first selecting one of the stimulation site tabs  264 . 
     Selecting one of the stimulation site tabs  264  will open a control window  265 , seen in  FIG. 18 , from which various parameters of the SSPE manual test may be adjusted according to user preference. B y way of example only,  FIG. 18  is an illustration of an onscreen display for the SSEP manual test settings of the left ulnar nerve stimulation site. The highlighted “Left Ulnar Nerve” stimulation site tab  264  and the pop-up window title  266  indicate that adjusting any of the settings will alter the stimulation signal delivered to the left ulnar nerve. Multiple adjustment buttons are used to set the parameters of the stimulation signal. According to one example, the stimulation rate may be selected from a range between 2.2 and 6.2 Hz, with a default value of 4.7 Hz. The amplitude setting may be increased or decreased in increments of 10 mA using the amplitude selection buttons  270  labeled (by way of example only) “+10” and “−10”. More precise amplitude selections may be made by increasing or decreasing the amplitude in increments of 1 mA using the amplitude selection buttons  272  labeled (by way of example only) “+1” and “−1”. According to one example, the amplitude may be selected from a range of 1 to 100 mA with a default value of 10 mA. The selected amplitude setting is displayed in box  274 . The pulse width setting may be increased or decreased in increments of 50 μsec using the width selection buttons  276  labeled “+50” and “−50”. According to one example, the pulse width may be selected from a range of 50 to 300 μsec, with a default value of 200 μsec. The precise pulse width setting  278  is indicated in box  278 . Polarity controls  280  may be used to set the desired polarity of the stimulation signal. SSEP stimulation may be initiated at the selected stimulation settings by pressing the SSEP stimulation start button  284  labeled (by way of example only) “Start Stim.” Although stimulation settings adjustments are discussed with respect to the left ulnar nerve, it will be appreciated that stimulation adjustments may be applied to the other stimulation sites, including but not limited to the right ulnar nerve, and left and right tibial nerve. Alternatively, as described below, the system  10  may utilize an automated selection process to quickly determine the optimal stimulation parameters for each stimulation channel. 
     In order to monitor the health of the spinal cord with SSEP, the user must be able to determine if the responses to the stimulation signal are changing. To monitor for this change a baseline is determined, preferably during set-up. This can be accomplished simply by selecting the “set as baseline” button  298  next to the “start stim” button  284  on the setting screen illustrated in  FIG. 18 . Having determined a baseline recording for each stimulation site, subsequent monitoring may be performed as desired throughout the procedure and recovery period to obtain updated amplitude and latency measurements. 
       FIG. 19  depicts an exemplary screen display for Manual mode of the SSEP monitoring function. A mode indicator tab  290  on the test menu  204  indicates that “SSEP Manual” is the selected mode. The center result area  201  is divided into four sub areas or channel windows  294 , each one dedicated to displaying the signal response waveforms for one of the stimulation nerve sites. The channel windows  294  depict information including the nerve stimulation site  295 , and waveform waterfalls for each of the recording locations  291 - 293 . For each stimulated nerve site, the system  10  displays three signal response waveforms, representing the measurements made at three different recording sites. By way of example only, the three recording sites are a peripheral  291  (from a peripheral nerve proximal to the stimulation nerve), subcortical  292  (spine), and cortical  293  (scalp), as indicated for example in Table 5 above. Each section may be associated with a pictorial icon, illustrating the neural/skeletal structure. Although SSEP stimulation and recording is discussed with respect to the nerve stimulation site and the recording sites discussed above, it will be appreciated that SSEP stimulation may be applied to any number of peripheral sensory nerves and the recording sites may be located anywhere along the nervous system superior to the spinal level at risk during the procedure. 
     During SSEP modes (auto and manual), a single waveform response is generated for each stimulation signal run (for each stimulation channel). The waveforms are arranged with stimulation on the extreme left and time increasing to the right. By way of example, the waveforms are captured in a 100 ms window following stimulation. The stimulation signal run is comprised of a predefined number of stimulation pulses firing at the selected stimulation frequency. By way of example only, the stimulation signal may include 300 pulses at a frequency of 4.7 Hz. A 100 ms window of data is acquired on each of three SSEP recording channels: cortical, subcortical, and peripheral. With each successive stimulation on the same channel during a stimulation run, the three acquired waveforms are summed and averaged with the prior waveforms during the same stimulation run for the purpose of filtering out asynchronous events such that only the synchronous evoked response remains after a sufficient number of pulses. Thus, the final waveform displayed by the system  10  represents an averaging of the entire set (e.g. 300) of responses detected. 
     With each subsequent stimulation run, waveforms are drawn slightly lower each time, as depicted in  FIGS. 19-21 , until a total of four waveforms are showing. After more than four stimulation runs, the baseline waveform is retained, as well as the waveforms from the previous four stimulation runs. Older waveforms are removed from the waveform display. According to one embodiment, different colors may be used to represent the different waveforms. For example, the baseline waveforms may be colored purple, the last stimulation run may be colored white, the next-to-last stimulation run may be colored medium gray, and the earliest of the remaining stimulation runs may be colored dark gray. 
     According to one example, the baseline and the latest waveforms may have markers  314 ,  316  placed indicating latency and amplitude values associated with the waveform. The latency is defined as the time from stimulation to the first (earliest) marker. There is one “N”  314  and one “P”  316  marker for each waveform. The N marker is defined as the maximum average sample value within a window and the P value is defined as the minimum average sample value within the window. The markers may comprise cross consisting of a horizontal and a vertical line in the same color as the waveform. Associated with each marker is a text label  317  indicating the value at the marker. The earlier of the two markers is labeled with the latency (e.g. 22.3 ms). The latter of the two markers is labeled with the amplitude (e.g. 4.2 uV). The amplitude is defined as the difference in microvolts between average sample values at the markers. The latency is defined as the time from stimulation to the first (earliest) marker. Preferably, the markers are placed automatically by the system  10  (in both auto an manual modes). In manual mode, the user may select to place (and or move) markers manually. 
     Further selecting one of the channel windows  294  will zoom in on the waveforms contained in that window  294 .  FIG. 22  is an example illustration of the zoom view achieved by selecting one of the channel windows  294 . The zoom view includes waveforms  291 - 293 , the baseline waveform, markers  314  and  316 , and controls for moving markers  318  and waveform scaling  332 . Only the latest waveform is shown. The “Set All as Baseline” button  310  will allow the user to set (or change) all three recorded waveforms as the baselines. Additionally, baselines may be set (or changed) individually by pressing the individual “Set as Baseline” buttons  312 . Furthermore, the user may also move the N marker  314  and P markers  316  to establish new measurement points if desired. Direction control arrows  318  may be selected to move the N and P markers to the desired new locations. Alternatively, the user may touch and drag the marker  314 ,  316  to the new location. Utilizing the waveform controls  332  the user may zoom in and out on the recorded waveform. 
     Referencing  FIGS. 23-26 , Automatic SSEP mode functions similar to Manual SSEP mode except that the system  10  determines the amplitude and latency values and alerts the user if the values deviate.  FIG. 23  shows, by way of example only, an exemplary setup screen for the SSEP Automatic mode. In similar fashion to the setup screen previously described for the SSEP Manual mode, the user may toggle between Manual mode and Automatic mode, select a stimulation rate, and change one or more stimulation settings. By way of example only, the user may change one or more stimulation settings of each peripheral nerve by first selecting one of the stimulation site tabs  264 , as described above with reference to Manual mode and  FIG. 18 . According to one example, the stimulation rate may be selected from a range between 2.2 and 6.2 Hz, with a default value of 4.7 Hz, the amplitude may be selected from a range of 1 to 100 mA, with a default value of 10 mA, the pulse width may be selected from a range of 50 to 300 μsec, with a default value of 200 μsec. 
     In Automatic mode, the surgical system  10  also includes a timer function which can be controlled from the setup screen. Using the timer drop down menu  326 , the user may set and/or change a time interval for the timer application. There are two separate options of the timer function: (1) an automatic stimulation on time out which can be selected by pressing the auto start button  322  labeled (by way of example only) “Auto Start Stim when timed out”; and (2) a prompted stimulation reminder on time out which can be selected by pressing the prompt stimulation button  324  labeled (by way of example only) “Prompt Stim when timed out”. After each SSEP monitoring episode, the system  10  will initiate a timer corresponding to the selected time interval and, when the time has elapsed, the system will either automatically perform the SSEP stimulation or a stimulation reminder will be activated, depending on the selected option. The stimulation reminder may include, by way of example only, any one of, or combination of, an audible tone, voice recording, screen flash, pop up window, scrolling message, or any other such alert to remind the user to test SSEP again. It is also contemplated that the timer function described may be implemented in SSEP Manual mode. 
       FIGS. 24-26  depict exemplary onscreen displays for Automatic mode of the SSEP function. According to one embodiment, the user may select to view a screen with only alpha-numeric information ( FIG. 25 ) and one with alpha-numeric information and recorded waveforms ( FIG. 24 ). A mode indicator tab  290  indicates that “SSEP Auto” is the selected mode. A waveform selection tab  330  allows the user to select whether waveforms will be displayed with the alpha-numeric results. In similar fashion to the onscreen displays previously described for the SSEP Manual mode, the system  10  includes a channel window  294  for each nerve stimulation site. The channel window  294  may display information including the nerve stimulation site  295 , waveform recordings, and associated recording locations  291 - 293  (peripheral, sub cortical, and cortical) and the percentage change between the baseline and amplitude measurements and the baseline and latency measurements. By way of example only, each channel window  294  may optionally also show the baseline waveform and latest waveform for each recording site. In the event the system  10  detects a significant decrease in amplitude or an increase in latency, the associated window may preferably be highlighted with a predetermined color (e.g. red) to indicate the potential danger to the surgeon. Preferably, the stimulation results are displayed to the surgeon along with a color code so that the user may easily comprehend the danger and corrective measures may be taken to avoid or mitigate such danger. This may for example, more readily permit SSEP monitoring results to be interpreted by the surgeon or assistant without requiring dedicated neuromonitoring personnel. By way of example only, red is used when the decrease in amplitude or increase in latency is within a predetermined unsafe level. Green indicates that the measured increase or decrease is within a predetermined safe level. Yellow is used for measurements that are between the predetermined unsafe and safe levels. By way of example only, the system  10  may also notify the user of potential danger through the use of a warning message  334 . Although the warning message is in the form of a pop-up window, it will be appreciated that the warning may be communicated to the user by any one of, or combination of, an audible tone, voice recording, screen flash, scrolling message, or any other such alert to notify the user of potential danger 
     With reference to  FIG. 26  at any time during the procedure, a prior stimulation run may be selected for review. This may be accomplished by, for example, by opening the event bar  208  and selecting the desired event. Details from the event are shown with the historical details denoted on the right side of the menu screen  302  and waveforms shown in the center result screen. Again, the user may chose to reset baselines for one or more nerve stimulation sites by pressing the appropriate “Set As Baseline” button  306 . In the example shown, the system  10  illustrates the waveform history at the 07:51 minute mark which is denoted on the right side of the menu screen  302 . Prior waveform histories are saved by the surgical system  10  and stored in the waveform history toolbar  304 . The describe only in relation to the SSEP Auto function it will be appreciated that the same features may be accessed from SSEP Manual mode, the user may choose to set a recorded stimulation measurement as the baseline for each nerve stimulation site by pressing the “Set As Baseline” button  306 . By way of example only, the system  10  will inform the user if the applicable event is already the current baseline with a “Current Baseline” notification  308 . 
     In addition to alerting the user to any changes in the amplitude and/or latency of the SSEP signal response, it is further contemplated that the neuromonitoring system  10  may assess the data from all the recording sites to interpret possible causes for changes in the SSEP response. Based on that information, the program may suggest potential reasons for the change. Furthermore, it may suggest potential actions to be taken to avoid danger. It is still further contemplated that the neurophysiology system  10  may be communicatively linked with other equipment in the operating room, such as for example, anesthesia monitoring equipment. Data from this other equipment may be considered by the program to generate more accuracy and or better suggestions. By way of example only, Table 7 illustrates the SSEP illustrates various warnings that may be associated with particular SSEP results and result combination, and show to the user. For example, if in response to stimulation of the left ulnar nerve, the peripheral response from Erb&#39;s Point showed no change in amplitude or latency, the subcortical response showed a decrease in amplitude, and the cortical response showed a decrease in amplitude, the event box  206  (shown in  FIG. 25 ) would show either a yellow or a red indicator as well as the text “Possible mechanical insult. Possible spinal cord ischemia.” By way of another example, if there is a decreased amplitude or absent response in all peripheral, subcortical, and cortical recording sites, the system may show a “Check Electrode” warning  332  ( FIG. 26 ). With this date, and the particular circumstances leading to the result (e.g. what surgical maneuver resulted in the warning, etc.) the user may be better equipped to determine the most prudent course of action. 
     
       
         
           
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                   
                 Audio-visual 
                   
               
               
                   
                 Alert 
               
               
                 Neurophysiologic Event 
                 (Color) 
                 SSEP Expert Text 
               
               
                   
               
             
            
               
                 Cortical amplitude decrease: 
                 Green 
                 No Warning 
               
               
                 0-25% from baseline: 
               
               
                 Cortical amplitude decrease: 
                 Yellow 
                 “Some anesthetic agents may reduce 
               
               
                 26-49% from baseline 
                   
                 the cortical response amplitude.” 
               
               
                 Cortical amplitude decrease: 
                 Red 
                 “Some anesthetic agents may reduce 
               
               
                 50%-99% from baseline 
                   
                 the cortical response amplitude.” 
               
               
                 Cortical amplitude decrease: 
                 Red 
                 “Possible cortical ischemia.” 
               
               
                 100% from baseline 
               
               
                 Cortical latency increase: 
                 Green 
                 No Warning 
               
               
                 0-5% from baseline 
               
               
                 Cortical latency increase: 
                 Yellow 
                 “Some anesthetic agents may increase 
               
               
                 6-9% from baseline 
                   
                 the cortical response latency. Possible 
               
               
                   
                   
                 cortical ischemia.” 
               
               
                 Cortical latency increase: 
                 Red 
                 “Some anesthetic agents may increase 
               
               
                 10% or greater from baseline 
                   
                 the cortical response latency. Possible 
               
               
                   
                   
                 cortical ischemia.” 
               
               
                 Cortical response absent: 
                 Red 
                 “Some anesthetic agents may cause the 
               
               
                   
                   
                 cortical response to be absent. 
               
               
                   
                   
                 Possible cortical ischemia.” 
               
               
                 Subcortical amplitude decrease: 
                 Green 
                 No Warning 
               
               
                 0%-25% from baseline 
               
               
                 Subcortical amplitude decrease: 
                 Yellow 
                 “Possible muscle activity artifact. 
               
               
                 25%-49% from baseline 
                   
                 Possible cervical recording electrode 
               
               
                   
                   
                 issue.” 
               
               
                 Subcortical amplitude decrease: 
                 Red 
                 “Possible muscle activity artifact. 
               
               
                 50-99% from baseline or absent 
                   
                 Possible cervical recording issue.” 
               
               
                 50% amplitude decrease, 10% 
                 Red 
                 “Possible mechanical insult. Possible 
               
               
                 latency increase in both cortical 
                   
                 spinal cord ischemia.” 
               
               
                 and subcortical responses, or 
               
               
                 absence in both cortical and 
               
               
                 subcortical responses: 
               
               
                 Peripheral amplitude decrease: 
                 Red 
                 “Possible peripheral recording 
               
               
                 greater than 50% or absent 
                   
                 electrode issue.” 
               
               
                 Peripheral (Erb&#39;s Point) 
                 Green 
                 No Warning (left or right) 
               
               
                 amplitude decrease: 
               
               
                 0-25% from baseline 
               
               
                 Peripheral (Erb&#39;s Point) 
                 Yellow 
                 “Possible peripheral recording 
               
               
                 amplitude decrease: 
                   
                 electrode issue (Left Erb&#39;s Point).” 
               
               
                 26-49% from baseline 
                   
                 “Possible peripheral recording 
               
               
                   
                   
                 electrode issue (Right Erb&#39;s Point).” 
               
               
                 Peripheral (Erb&#39;s Point) 
                 Red 
                 “Possible peripheral recording 
               
               
                 amplitude decrease: 
                   
                 electrode issue (Left Erb&#39;s Point).” 
               
               
                 50%-100% from baseline 
                   
                 “Possible peripheral recording 
               
               
                   
                   
                 electrode issue (Right Erb&#39;s Point).” 
               
               
                 Peripheral (Popliteal Fossa 
                 Green 
                 No Warning (left or right) 
               
               
                 amplitude decrease: 
               
               
                 0-25% from baseline 
               
               
                 Peripheral (Popliteal Fossa) 
                 Yellow 
                 “Possible peripheral recording 
               
               
                 amplitude decrease: 
                   
                 electrode issue (Left Popliteal Fossa).” 
               
               
                 26-49% from baseline 
                   
                 “Possible peripheral recording 
               
               
                   
                   
                 electrode issue (Right Popliteal Fossa).” 
               
               
                 Peripheral (Popliteal Fossa) 
                 Red 
                 “Possible peripheral recording 
               
               
                 amplitude decrease: 
                   
                 electrode issue (Left Popliteal Fossa).” 
               
               
                 50%-100% from baseline 
                   
                 “Possible peripheral recording 
               
               
                   
                   
                 electrode issue (Right Popliteal Fossa).” 
               
               
                 Peripheral (Erb&#39;s Point) latency 
                 Green 
                 No Warning (left or right) 
               
               
                 increase: 
               
               
                 0-5% from baseline 
               
               
                 Peripheral (Erb&#39;s Point) latency 
                 Yellow 
                 No Warning (left or right) 
               
               
                 increase: 6-9% from baseline 
               
               
                 Peripheral (Erb&#39;s Point) latency 
                 Red 
                 No Warning (left or right) 
               
               
                 increase: 
               
               
                 10% or greater from baseline 
               
               
                 Peripheral (Popliteal Fossa) 
                 Green 
                 No Warning (left or right 
               
               
                 latency increase: 
               
               
                 0-5% from baseline 
               
               
                 Peripheral (Popliteal Fossa) 
                 Yellow 
                 No Warning (left or right) 
               
               
                 latency increase: 
               
               
                 6-9% from baseline 
               
               
                 Peripheral (Popliteal Fossa) 
                 Red 
                 No Warning (left or right) 
               
               
                 latency increase: 
               
               
                 10% or greater from baseline 
               
               
                 Peripheral (Popliteal Fossa) and 
                 Green 
                 Possible muscle activity artifact. 
               
               
                 subcortical amplitude decrease: 
                   
                 Possible cervical recording electrode 
               
               
                 0-25% from baseline 
                   
                 issue. (left or right) 
               
               
                 Peripheral (Popliteal Fossa) and 
                 Yellow/ 
                 “Possible cervical muscle activity 
               
               
                 subcortical amplitude decrease: 
                 Red 
                 artifact. Possible cervical recording 
               
               
                 26%-100% from baseline 
                   
                 electrode issue. Possible muscle 
               
               
                   
                   
                 activity artifact (posterior tibial 
               
               
                   
                   
                 nerve).” (left or right) 
               
               
                 Peripheral (Erb&#39;s Point) and 
                 Green 
                 “Possible muscle activity artifact. 
               
               
                 subcortical amplitude decrease: 
                   
                 Possible cervical recording electrode 
               
               
                 0-25% from baseline 
                   
                 issue.” (left or right) 
               
               
                 Peripheral (Erb&#39;s Point) and 
                 Yellow/ 
                 “Possible cervical muscle activity 
               
               
                 subcortical amplitude decrease: 
                 Red 
                 artifact. Possible cervical recording 
               
               
                 26-99% from baseline 
                   
                 electrode issue. Possible muscle 
               
               
                   
                   
                 activity artifact (median nerve).” (left 
               
               
                   
                   
                 or right) 
               
               
                 Decreased amplitude or absent 
                 Yellow/ 
                 “Possible stimulating electrode issue. 
               
               
                 response in all, peripheral (left 
                 Red 
                 (left wrist).” 
               
               
                 Erb&#39;s point), subcortical, and 
               
               
                 cortical: 
               
               
                 Decreased amplitude or absent 
                 Yellow/ 
                 “Possible stimulating electrode issue 
               
               
                 in all, peripheral (right Erb&#39;s 
                 Red 
                 (right wrist).” 
               
               
                 point), subcortical, and cortical: 
               
               
                 Decreased amplitude or absent 
                 Yellow/ 
                 “Possible stimulating electrode issue 
               
               
                 response in all peripheral (left 
                 Red 
                 (left ankle).” 
               
               
                 Popliteal Fossa), subcortical, 
               
               
                 and cortical: 
               
               
                 Decreased amplitude or absent 
                 Yellow/ 
                 Possible stimulating electrode issue 
               
               
                 response in all peripheral (right 
                 Red 
                 (right ankle) 
               
               
                 Popliteal Fossa), subcortical, 
               
               
                 and cortical 
               
               
                 Increased latency or decreased 
                 Yellow/ 
                 “Possible systemic change 
               
               
                 amplitude in all, peripheral, 
                 Red 
                 (hypotension, hypothermia, 
               
               
                 subcortical, and cortical: 
                   
                 hyperthermia). Possible peripheral 
               
               
                   
                   
                 nerve ischemia.” (left or right) 
               
               
                   
                   
                 (posterior tibial or ulnar nerve) 
               
               
                   
               
            
           
         
       
     
     As mentioned above, the system  10  may employ an automated tests to quickly select the optimal stimulus parameters for conducting SSEP testing on each active stimulation channel. This can be done according to any number of algorithms that automatically adjust various parameters until a combination resulting in the most desirable result is achieved. By way of example, the system  10  may utilize an algorithm similar to the hunting algorithm described below for finding I thresh  for EMG and MEP modalities. According to this example, the desired stimulation parameters are determined by first finding the lowest I thresh  (that is the lowest stimulation signal intensity that results in a waveform having a predetermined amplitude, V thresh ) for each stimulation site (e.g., left posterior tibial nerve (LPTN), right posterior tibial nerve (RPTN), left ulnar nerve (LUN), and right ulnar nerve (RUN)). By way of example only, to determine the I thresh  for a LPTN, using polarity A (cathode proximal to the surgical site), an initial, predetermined stimulus intensity is applied transcutaneously to the left PTN stimulation site. If no response is obtained from recording electrodes at the left popliteal fossa with a V pp  greater or equal to V thresh , polarity B is used (anode proximal to the surgical site), and the same stimulus intensity is applied. If no response is obtained at the first stimulus level for either polarity, the polarity is again switched and the stimulation intensity is doubled. Thus, using polarity A, a second stimulus intensity is applied. If there is no response recorded from the left popliteal fossa, the polarity is reversed and a stimulus of the same second intensity is applied. If there is still no response that recruits (results in a V pp  at or above V thresh ), the stimulus intensity is again doubled until there is an evoked potential with a V pp  greater or equal to V thresh . The polarity setting from which the first evoked potential recorded in the left popliteal fossa that achieves V thresh , is set as the polarity for this stimulation site. The first stimulation intensity to achieve V thresh  and the immediately previous stimulation intensity form an initial bracket. 
     After the threshold current I thresh  has been bracketed, the initial bracket is successively reduced via bisection to a predetermined width. This is accomplished by applying a first bisection stimulation current that bisects (i.e. forms the midpoint of) the initial bracket. If this first bisection stimulation current recruits, the bracket is reduced to the lower half of the initial bracket. If this first bisection stimulation current does not recruit, the bracket is reduced to the upper half of the initial bracket. This process is continued for each successive bracket until I thresh  is bracketed by stimulation currents separated by the predetermined width. Once I thresh  is determined for a particular stimulation channel, the stimulus intensity is set as the value 20% greater than the detected threshold. This is repeated for each stimulation channel until the optimal stimulation signal is set for each. The optimal stimulation signal may be determined for each stimulation channel in sequence, or, simultaneously (by proceeding in similar fashion to the multi channel threshold detection algorithm described below. The determined stimulation values will then preferably be used throughout the monitoring procedure. 
     The threshold hunting algorithm for optimizing SSEP stimulation parameter is further described with reference to  FIGS. 40-41 .  FIG. 40  illustrates (in flowchart form) a method by which the stimulus intensity algorithm quickly searches for the optimal stimulation parameters. The algorithm first stimulates at an initial stimulation intensity using polarity A, and determines whether this results in an I recruit  (step  411 ). If the algorithm determines that there has been no recruitment, the algorithm reverses the direction of the polarity and stimulates at the same initial stimulation intensity using polarity B and determines whether this results in an I recruit  (step  412 ). If the algorithm determines that there has been no recruitmentf, the algorithm moves to step  413  and doubles the stimulation intensity. At step  414 , using polarity A, the algorithm stimulates at the second intensity and determines if this is an I recruit  (step  414 ). If the answer is no, the algorithm proceeds to step  415 , reverses to polarity B, and stimulates at the second intensity. If the answer is still no, then step  413  is repeated and the stimulus intensity is doubled again. If at any point during step  411 ,  413 ,  414 , or  415  the answer is yes, the algorithm designates this as the initial bracket and polarity as shown in step  416  and as previously described. The algorithm then moves to step  417  and the bracket is bisected. In other words, the stimulation is performed at the midpoint of the bracket. At step  418 , the algorithm bisects the bracket until a threshold is known and the stimulating intensity required for a predetermined response is obtained to a desired accuracy. At step  419 , the SSEP stimulus intensity is set at 20% above the detected threshold. Once I thresh  is found for Limb 1, as shown in step  420  of  FIG. 41 , the algorithm turns to a The algorithm begins a second step (step  421 ) and processes Limb 2 by mirroring steps  411 - 419 . This same process is repeated for Limb 3 (step  422 ) and Limb 4 (step  423 ). After the stimulus intensity algorithm has determined the optimal stimulus parameters, SSEP neurophysiologic testing may be commenced (step  424 ). 
     With reference to  FIGS. 27-39 , the remaining functions of the neurophysiologic monitoring system  10  will be described in brief detail. In MEP modes, stimulation signals are delivered to the Motor Cortex via patient module  14  and resulting responses are detected from various muscles in the upper and lower extremities. An increase in I thresh  from an earlier test to a later test may indicate a degradation of spinal cord function. Likewise, the absence of a significant EMG response to a given I stim  on a channel that had previously reported a significant response to the same or lesser I stim  is also indicative of a degradation in spinal cord function. These indicators are detected by the system in the MEP modes and reported to the surgeon. In MEP Auto mode the system determines the I thresh  baseline for each channel corresponding to the various monitored muscles, preferably early in the procedure, using the multi-channel algorithm described. Throughout the procedure subsequent tests may be conducted to again determine I thresh  for each channel. The difference between the resulting I thresh  values and the corresponding baseline are computed by the system  10  and compared against predetermined “safe” and “unsafe” difference values. The I thresh , baseline, and difference values are displayed to the user, along with any other indicia of the safety level determined (such as a red, yellow, green color code), on the display  34 , as illustrated in  FIG. 28 . In MEP Manual mode, the user selects the stimulation current level and the system reports whether or not the stimulation signal evokes a significant response on each channel. Stimulation results may be shown on the display  34  in the form of “YES” and “NO” responses, or other equivalent indicia, as depicted in  FIG. 27 . Using either mode the surgeon may thus be alerted to potential complications with the spinal cord and any corrective actions deemed necessary may be undertaken at the discretion of the surgeon. 
     The neuromonitoring system  10  performs neuromuscular pathway (NMP) assessments, via Twitch Test mode, by electrically stimulating a peripheral nerve (preferably the Peroneal Nerve for lumbar and thoracolumbar applications and the Median Nerve for cervical applications) via stimulation electrodes  22  contained in the applicable electrode harness and placed on the skin over the nerve or by direct stimulation of a spinal nerve using a surgical accessory such as the probe  116 . Evoked responses from the muscles innervated by the stimulated nerve are detected and recorded, the results of which are analyzed and a relationship between at least two responses or a stimulation signal and a response is identified. The identified relationship provides an indication of the current state of the NMP. The identified relationship may include, but is not necessarily limited to, one or more of magnitude ratios between multiple evoked responses and the presence or absence of an evoked response relative to a given stimulation signal or signals. With reference to  FIG. 29 , details of the test indicating the state of the NMP and the relative safety of continuing on with nerve testing are conveyed to the surgeon via GUI display  34 . On the monitoring screen  200  utilized by the various functions performed by the system  10 , function specific data is displayed in a center result area  201 . The results may be shown as a numeric value  210 , a highlighted label corresponding to the electrode labels  86 , or (in the case of twitch test only) a bar graph of the stimulation results. On one side of center result area  201  is a collapsible device menu  202 . The device menu displays a graphic representation of each device connected to the patient module  14 . Opposite the device menu  202  there is a collapsible test menu  204 . The test menu  204  highlights each test that is available under the operable setup profile and may be used to navigate between functions. A collapsible stimulation bar  206  indicates the current stimulation status and provides start and stop stimulation buttons (not shown) to activate and control stimulation. The collapsible event bar  208  stores all the stimulation test results obtained throughout a procedure. Clicking on a particular event will open a note box and annotations may be entered and saved with the response, for later inclusion in a procedure report. The event bar  208  also houses a chat box feature when the system  10  is connected to a remote monitoring system as described above. Within the result area  202  the twitch test specific results may be displayed. 
     It should be appreciated that while  FIG. 29  depicts the monitoring screen  200  while the selected function is the Twitch Test, the features of monitoring screen  200  apply equally to all the functions. Result-specific data is displayed in a center result area  201 . A large color saturated numeric value (not shown) is used to show the threshold result. Three different options are provided for showing the stimulation response level. First, the user can view the waveform. Second, a likeness of the color coded electrode harness label  86  may be shown on the display. Third, the color coded label  212  may be integrated with a body image. On one side of center result area  201  there is a collapsible device menu  202 . The device menu displays a graphic representation of each device connected to the patient module  14 . If a device is selected from the device menu  202 , an impedance test may be initiated. Opposite the device menu  202  there is a collapsible test menu  204 . The test menu  204  highlights each test that is available under the operable setup profile and may be used to navigate between functions. A collapsible stimulation bar  206  indicates the current stimulation status and provides start and stop stimulation buttons (not shown) to activate and control stimulation. The collapsible event bar  208  stores all the stimulation test results obtained throughout a procedure so that the user may review the entire case history from the monitoring screen. Clicking on a particular event will open a note box and annotations may be entered and saved with the response, for later inclusion in a procedure report chronicling all nerve monitoring functions conducted during the procedure as well as the results of nerve monitoring. In one embodiment the report may be printed immediately from one or more printers located in the operating room or copied to any of a variety of memory devices known in the prior art, such as, by way of example only, a floppy disk, and/or USB memory stick. The system  10  may generate either a full report or a summary report depending on the particular needs of the user. In one embodiment, the identifiers used to identify the surgical accessories to the patient module may also be encoded to identify their lot number or other identifying information. As soon as the accessory is identified, the lot number may be automatically added to the report. Alternatively, hand held scanners can be provided and linked to the control unit  12  or patient module  14 . The accessory packaging may be scanned and again the information may go directly to the procedure report. The event bar  208  also houses a chat box feature when the system  10  is connected to a remote monitoring system to allow a user in the operating room to contemporaneously communicate with a person performing the associated neuromonitoring in a remote location. 
     The neuromonitoring system  10  may test the integrity of pedicle holes (during and/or after formation) and/or screws (during and/or after introduction) via the Basic Stimulation EMG and Dynamic Stimulation EMG tests. To perform the Basic Stimulation EMG a test probe  116  is placed in the screw hole prior to screw insertion or placed on the installed screw head and a stimulation signal is applied. The insulating character of bone will prevent the stimulation current, up to a certain amplitude, from communicating with the nerve, thus resulting in a relatively high I thresh , as determined via the basic threshold hunting algorithm described below. However, in the event the pedicle wall has been breached by the screw or tap, the current density in the breach area will increase to the point that the stimulation current will pass through to the adjacent nerve roots and they will depolarize at a lower stimulation current, thus I thresh  will be relatively low. The system described herein may exploit this knowledge to inform the practitioner of the current I thresh  of the tested screw to determine if the pilot hole or screw has breached the pedicle wall. 
     In Dynamic Stim EMG mode, test probe  116  may be replaced with a clip  18  which may be utilized to couple a surgical tool, such as for example, a tap member  28  or a pedicle access needle  26 , to the neuromonitoring system  10 . In this manner, a stimulation signal may be passed through the surgical tool and pedicle integrity testing can be performed while the tool is in use. Thus, testing may be performed during pilot hole formation by coupling the access needle  26  to the neuromonitoring system  10 , and during pilot hole preparation by coupling the tap  28  to the system  10 . Likewise, by coupling a pedicle screw to the neuromonitoring system  10  (such as via pedicle screw instrumentation), integrity testing may be performed during screw introduction. 
     In both Basic Stimulation EMG mode and Dynamic Stimulation EMG mode, the signal characteristics used for testing in the lumbar testing may not be effective when monitoring in the thoracic and/or cervical levels because of the proximity of the spinal cord to thoracic and cervical pedicles. Whereas a breach formed in a pedicle of the lumbar spine results in stimulation being applied to a nerve root, a breach in a thoracic or cervical pedicle may result in stimulation of the spinal cord instead, but the spinal cord may not respond to a stimulation signal the same way the nerve root would. To account for this, the surgical system  10  is equipped to deliver stimulation signals having different characteristics based on the region selected. By way of example only, when the lumbar region is selected, stimulation signals for the stimulated EMG modes comprise single pulse signals. On the other hand, when the thoracic and cervical regions are selected the stimulation signals may be configured as multipulse signals. 
     Stimulation results (including but not necessarily limited to at least one of the numerical I thresh  value and color coded safety level indication) and other relevant data are conveyed to the user on at least main display  34 , as illustrated in  FIGS. 29 and 30 .  FIG. 29  illustrates the monitoring screen  200  with the Basic Stimulation EMG test selected.  FIG. 30  illustrates the monitoring screen  200  with the Dynamic Stimulation EMG test selected. In one embodiment of the various screw test functions (e.g. Basic and Dynamic), green corresponds to a threshold range of greater than 10 milliamps (mA), a yellow corresponds to a stimulation threshold range of 7-10 mA, and a red corresponds to a stimulation threshold range of 6 mA or below. EMG channel tabs may be selected via the touch screen display  26  to show the I thresh  of the corresponding nerves. Additionally, the EMG channel possessing the lowest I thresh  may be automatically highlighted and/or colored to clearly indicate this fact to the user. 
     The neuromonitoring system  10  may perform nerve proximity testing, via the XLIF mode, to ensure safe and reproducible access to surgical target sites. Using the surgical access components  26 - 32 , the system  10  detects the existence of neural structures before, during, and after the establishment of an operative corridor through (or near) any of a variety of tissues having such neural structures which, if contacted or impinged, may otherwise result in neural impairment for the patient. The surgical access components  26 - 32  are designed to bluntly dissect the tissue between the patient&#39;s skin and the surgical target site. Dilators of increasing diameter, which are equipped with one or more stimulating electrodes, are advanced towards the target site until a sufficient operating corridor is established to advance retractor  32  to the target site. As the dilators are advanced to the target site electrical stimulation signals are emitted via the stimulation electrodes. The stimulation signal will stimulate nerves in close proximity to the stimulation electrode and the corresponding EMG response is monitored. As a nerve gets closer to the stimulation electrode, the stimulation current required to evoke a muscle response decreases because the resistance caused by human tissue will decrease, and it will take less current to cause nervous tissue to depolarize. I thresh  is calculated, using the basic threshold hunting algorithm described below, providing a measure of the communication between the stimulation signal and the nerve and thus giving a relative indication of the proximity between access components and nerves. An example of the monitoring screen  200  with XLIF mode active is depicted in  FIG. 32 . In a preferred embodiment, a green or safe level corresponds to a stimulation threshold range of 10 milliamps (mA) or greater, a yellow level denotes a stimulation threshold range of 5-9 mA, and a red level denotes a stimulation threshold range of 4 mA or below. 
     The neuromonitoring system  10  may also conduct free-run EMG monitoring while the system is in any of the above-described modes. Free-run EMG monitoring continuously listens for spontaneous muscle activity that may be indicative of potential danger. The system  10  may automatically cycle into free-run monitoring after 5 seconds (by way of example only) of inactivity. Initiating a stimulation signal in the selected mode will interrupt the free-run monitoring until the system  10  has again been inactive for five seconds, at which time the free-run begins again. An example of the monitoring screen  200  with Free-run EMG active is depicted in  FIG. 33 . 
     The neuromonitoring system  10  may also perform a navigated guidance function. The navigated guidance feature may be used by way of example only, to ensure safe and reproducible pedicle screw placement by monitoring the axial trajectory of surgical instruments used during pilot hole formation and/or screw insertion. Preferably, EMG monitoring may be performed simultaneously with the navigated guidance feature. To perform the navigated guidance and angle-measuring device (hereafter “tilt sensor”)  54  is connected to the patient module  14  via one of the accessory ports  62 . The tilt sensor measures its angular orientation with respect to a reference axis (such as, for example, “vertical” or “gravity”) and the control unit displays the measurements. Because the tilt sensor is attached to a surgical instrument the angular orientation of the instrument, may be determined as well, enabling the surgeon to position and maintain the instrument along a desired trajectory during use. In general, to orient and maintain the surgical instrument along a desired trajectory during pilot hole formation, the surgical instrument is advanced to the pedicle (through any of open, mini-open, or percutaneous access) while oriented in the zero-angle position. The instrument is then angulated in the sagittal plane until the proper cranial-caudal angle is reached. Maintaining the proper cranial-caudal angle, the surgical instrument may then be angulated in the transverse plane until the proper medial-lateral angle is attained. Once the control unit  12  indicates that both the medial-lateral and cranial caudal angles are matched correctly, the instrument may be advanced into the pedicle to form the pilot hole, monitoring the angular trajectory of the instrument until the hole formation is complete. 
     The control unit  12  may communicate any of numerical, graphical, and audio feedback corresponding to the orientation of the tilt sensor in the sagittal plane (cranial-caudal angle) and in the transverse plane (medial-lateral angle). The medial-lateral and cranial-caudal angle readouts may be displayed simultaneously and continuously while the tilt sensor is in use, or any other variation thereof (e.g. individually and/or intermittently).  FIG. 34  illustrates, by way of example only, one embodiment of a GUI screen for the Navigated Guidance function. The angular orientation of the instrument is displayed along with a color coded targeting scheme to help the user find the desired angle. 
     To obtain I thresh  and take advantage of the useful information it provides, the system  10  identifies and measures the peak-to-peak voltage (V pp ) of each EMG response corresponding to a given stimulation current (I Stim ). Identifying the true V pp  of a response may be complicated by the existence of stimulation and/or noise artifacts which may create an erroneous V pp  measurement. To overcome this challenge, the neuromonitoring system  10  of the present invention may employ any number of suitable artifact rejection techniques such as those shown and described in full in the above referenced co-pending and commonly assigned PCT App. Ser. No. PCT/US2004/025550, entitled “System and Methods for Performing Dynamic Pedicle Integrity Assessments,” filed on Aug. 5, 2004, the entire contents of which are incorporated by reference into this disclosure as if set forth fully herein. Upon measuring V pp  for each EMG response, the information is analyzed relative to the corresponding stimulation current (I stim ) in order to identify the minimum stimulation current (I Thresh ) capable of resulting in a predetermined V pp  EMG response. According to the present invention, the determination of I Thresh  may be accomplished via any of a variety of suitable algorithms or techniques. 
       FIGS. 35A-D  illustrate, by way of example only, the principles of a threshold hunting algorithm of the present invention used to quickly find I thresh . The method for finding I thresh  utilizes a bracketing method and a bisection method. The bracketing method quickly finds a range (bracket) of stimulation currents that must contain I thresh  and the bisection method narrows the bracket until I thresh  is known within a specified accuracy. If the stimulation current threshold, I thresh , of a channel exceeds a maximum stimulation current, that threshold is considered out of range. 
       FIGS. 35A-D  illustrate the bracketing feature of the threshold hunting algorithm of the present invention. Stimulation begins at a minimum stimulation current, such as (by way of example only) 1 mA. It will be appreciated that the relevant current values depend in part on the function performed (e.g. high currents are used for MEP and low currents are generally used for other functions) and the current values described here are for purposes of example only and may in actuality be adjusted to any scale. The level of each subsequent stimulation is doubled from the preceding stimulation level until a stimulation current recruits (i.e. results in an EMG response with a V pp  greater or equal to V thresh ). The first stimulation current to recruit (8 mA in  FIG. 35B ), together with the last stimulation current to have not recruited (4 mA in  FIG. 35B ), forms the initial bracket. 
       FIGS. 35C-D  illustrate the bisection feature of the threshold hunting algorithm of the present invention. After the threshold current I thresh  has been bracketed ( FIG. 35B ), the initial bracket is successively reduced via bisection to a predetermined width, such as (by way of example only) 0.25 mA. This is accomplished by applying a first bisection stimulation current that bisects (i.e. forms the midpoint of) the initial bracket (6 mA in  FIG. 35C ). If this first bisection stimulation current recruits, the bracket is reduced to the lower half of the initial bracket (e.g. 4 mA and 6 mA in  FIG. 35C ). If this first bisection stimulation current does not recruit, the bracket is reduced to the upper half of the initial bracket (e.g. 6 mA and 8 mA in  FIG. 35C ). This process is continued for each successive bracket until I thresh  is bracketed by stimulation currents separated by the predetermined width (which, in this case, is 0.25 mA). In this example shown, this would be accomplished by applying a second bisection stimulation current (forming the midpoint of the second bracket, or 5 mA in this example). Because this second bisection stimulation current is below I thresh , it will not recruit. As such, the second bracket will be reduced to the upper half thereof (5 mA to 6 mA), forming a third bracket. A third bisection stimulation current forming the mid-point of the third bracket (5.50 mA in this case) will then be applied. Because this third bisection stimulation current is below I thresh , it will not recruit. As such, the third bracket will be reduced to the upper half thereof (5.50 mA to 6 mA), forming a fourth bracket. A fourth bisection stimulation current forming the mid-point of the fourth bracket (5.75 mA in this case) will then be applied. Because the fourth bisection stimulation current is above I thresh , it will recruit. The final bracket is therefore between 5.50 mA and 5.75 mA. Due to the “response” or recruitment at 5.50 mA and “no response” or lack of recruitment at 5.75 mA, it can be inferred that I thresh  is within this range. In one embodiment, the midpoint of this final bracket may be defined as I thresh , however, any value falling within the final bracket may be selected as I thresh  without departing from the scope of the present invention. Depending on the active mode, the algorithm may stop after finding I thresh  for the first responding channel (i.e. the channel with the lowest I thresh ) or the bracketing and bisection steps may be repeated for each channel to determine I thresh  for each channel. In one embodiment, this multiple channel I thresh  determination may be accomplished by employing the additional steps of the multi-channel threshold detection algorithm, described below. 
     Additionally, in the “dynamic” functional modes, including, but not necessarily limited to Dynamic Stimulation EMG and XLIF, the system may continuously update the stimulation threshold level and indicate that level to the user. To do so, the threshold hunting algorithm does not repeatedly determine the I thresh  level anew, but rather, it determines whether stimulation current thresholds are changing. This is accomplished, as illustrated in  FIG. 35D , by a monitoring phase that involves switching between stimulations at lower and upper ends of the final bracket. If the threshold has not changed then the lower stimulation current should not evoke a response, while the upper end of the bracket should. If either of these conditions fail, the bracket is adjusted accordingly. The process is repeated for each of the active channels to continue to assure that each threshold is bracketed. If stimulations fail to evoke the expected response three times in a row, then the algorithm transitions back to the bracketing state in order to reestablish the bracket. In the event a change in I thresh  is detected during the monitoring phase, the user may be alerted immediately via the screen display and/or audio feedback. By way of example only, the color shown on the display corresponding to the previous I thresh  can be altered to a neutral color (e.g. black, grey, etc . . . ) as soon as the change in I thresh  is detected but before the new I thresh  value is determined. If an audio tone is used to represent a particular safety level, the tone can ceased as soon as the change in detected. Once the new I thresh  value is determined the color and/or audio tone can be altered again to signify the value. 
     In an alternative embodiment, rather than beginning by entering the bracketing phase at the minimum stimulation current and bracketing upwards until I thresh  is bracketed, the threshold hunting algorithm may begin by immediately determining the appropriate safety level and then entering the bracketing phase. The algorithm may accomplish this by initiating stimulation at one or more of the boundary current levels. By way of example only, and with reference to  FIG. 36 , the algorithm may begin by delivering a stimulation signal at the boundary between the unsafe (e.g. red) and caution (e.g. yellow) levels. If the safety level is not apparent after the first stimulation, the algorithm may stimulate again at the boundary between the caution (e.g. yellow) and safe (e.g. green) levels. Once the safety level is known (i.e. after the first stimulation if the safety level is red, or, after the second stimulation if the safety level is yellow or green) the screen display may be updated to the appropriate color and/or coded audio signals may be emitted. As the screen display is updated, the algorithm may transition to the bracketing and bisection phases to determine the actual I thresh  value. When the I thresh  value is determined the display may be updated again to reflect the additional information. In dynamic modes, if the monitoring phase detects a change in I thresh , the algorithm will again stimulate at the boundary level(s) as necessary and update the color and/or audio signals before transitioning to the bracketing and bisection phases to determine the new I thresh . 
     For some functions, such as (by way of example) MEP, it may be desirable to obtain I thresh  for each active channel each time the function is performed. This is particularly advantageous when assessing changes in I thresh  over time as a means to detect potential problems (as opposed to detecting an I thresh  below a predetermined level determined to be safe, such as in the Stimulated EMG modes). While I thresh  can be found for each active channel using the algorithm as described above, it requires a potentially large number of stimulations, each of which is associated with a specific time delay, which can add significantly to the response time. Done repeatedly, it could also add significantly to the overall time required to complete the surgical procedure, which may present added risk to the patient and added costs. To overcome this drawback, a preferred embodiment of the neuromonitoring system  10  boasts a multi-channel threshold hunting algorithm so as to quickly determine I thresh  for each channel while minimizing the number of stimulations and thus reduce the time required to perform such determinations. 
     The multi-channel threshold hunting algorithm reduces the number stimulations required to complete the bracketing and bisection steps when I thresh  is being found for multiple channels. The multi-channel algorithm does so by omitting stimulations for which the result is predictable from the data already acquired. When a stimulation signal is omitted, the algorithm proceeds as if the stimulation had taken place. However, instead of reporting an actual recruitment result, the reported result is inferred from previous data. This permits the algorithm to proceed to the next step immediately, without the time delay associated with a stimulation signal. 
     Regardless of what channel is being processed for I thresh , each stimulation signal elicits a response from all active channels. That is to say, every channel either recruits or does not recruit in response to a stimulation signal (again, a channel is said to have recruited if a stimulation signal evokes an EMG response deemed to be significant on that channel, such as V pp  of approximately 100 uV). These recruitment results are recorded and saved for each channel. Later, when a different channel is processed for I thresh  the saved data can be accessed and, based on that data, the algorithm may omit a stimulation signal and infer whether or not the channel would recruit at the given stimulation current. 
     There are two reasons the algorithm may omit a stimulation signal and report previous recruitment results. A stimulation signal may be omitted if the selected stimulation current would be a repeat of a previous stimulation. By way of example only, if a stimulation current of 1 mA was applied to determine I thresh  for one channel, and a stimulation at 1 mA is later required to determine I thresh  for another channel, the algorithm may omit the stimulation and report the previous results. If the specific stimulation current required has not previously been used, a stimulation signal may still be omitted if the results are already clear from the previous data. By way of example only, if a stimulation current of 2 mA was applied to determine I thresh  for a previous channel and the present channel did not recruit, when a stimulation at 1 mA is later required to determine I thresh  for the present channel, the algorithm may infer from the previous stimulation that the present channel will not recruit at 1 mA because it did not recruit at 2 mA. The algorithm may therefore omit the stimulation and report the previous result. 
       FIG. 37  illustrates (in flowchart form) a method by which the multi-channel threshold hunting algorithm determines whether to stimulate, or not stimulate and simply report previous results. The algorithm first determines if the selected stimulation current has already been used (step  302 ). If the stimulation current has been used, the stimulation is omitted and the results of the previous stimulation are reported for the present channel (step  304 ). If the stimulation current has not been used, the algorithm determines I recruit  (step  306 ) and I norecruit  (step  308 ) for the present channel. I recruit  is the lowest stimulation current that has recruited on the present channel. I norecruit  is the highest stimulation current that has failed to recruit on the present channel. The algorithm next determines whether I recruit  is greater than I norecruit  (step  310 ). An I recruit  that is not greater than I norecruit  is an indication that changes have occurred to I thresh  on that channel. Thus, previous results may not be reflective of the present threshold state and the algorithm will not use them to infer the response to a given stimulation current. The algorithm will stimulate at the selected current and report the results for the present channel (step  312 ). If I recruit  is greater than I norecruit , the algorithm determines whether the selected stimulation current is higher than I recruit , lower than I norecruit , or between I recruit  and I norecruit  (step  314 ). If the selected stimulation current is higher than I recruit , the algorithm omits the stimulation and reports that the present channel recruits at the specified current (step  316 ). If the selected stimulation current is lower than I norecruit , the algorithm infers that the present channel will not recruit at the selected current and reports that result (step  318 ). If the selected stimulation current falls between I recruit  and I norecruit , the result of the stimulation cannot be inferred and the algorithm stimulates at the selected current and reports the results for the present channel (step  312 ). This method may be repeated until I thresh  has been determined for every active channel. 
     In the interest of clarity,  FIGS. 38A-C  demonstrate use of the multi-channel threshold hunting algorithm to determine I thresh  on only two channels. It should be appreciated, however, that the multi-channel algorithm is not limited to finding I thresh  for two channels, but rather it may be used to find I thresh  for any number of channels, such as (for example) eight channels according to a preferred embodiment of the neuromonitoring system  10 . With reference to  FIG. 38A , channel 1 has an I thresh  to be found of 6.25 mA and channel 2 has an I thresh  to be found of 4.25 mA. I thresh  for channel 1 is found first as illustrated in  FIG. 38B , using the bracketing and bisection methods discussed above. Bracketing begins at the minimum stimulation current (for the purposes of example only) of 1 mA. As this is the first channel processed and no previous recruitment results exist, no stimulations are omitted. The stimulation current is doubled with each successive stimulation until a significant EMG response is evoked at 8 mA. The initial bracket of 4-8 mA is bisected, using the bisection method described above, until the stimulation threshold, I thresh , is contained within a final bracket separated by the selected width or resolution (again 0.25 mA). In this example, the final bracket is 6 mA-6.25 mA. I thresh  may be defined as any point within the final bracket or as the midpoint of the final bracket (6.125 mA in this case). In either event, I thresh  is selected and reported as I thresh  for channel 1. 
     Once I thresh  is found for channel 1, the algorithm turns to channel 2, as illustrated in  FIG. 38C . The algorithm begins to process channel 2 by determining the initial bracket, which is again 4-8 mA. All the stimulation currents required in the bracketing state were used in determining I thresh  for channel 1. The algorithm refers back to the saved data to determine how channel 1 responded to the previous stimulations. From the saved data, the algorithm may infer that channel 2 will not recruit at stimulation currents of 1, 2, and 4 mA, and will recruit at 8 mA. These stimulations are omitted and the inferred results are displayed. The first bisection stimulation current selected in the bisection process (6 mA in this case), was previously used and, as such, the algorithm may omit the stimulation and report that channel 2 recruits at that stimulation current. The next bisection stimulation current selected (5 mA in this case) has not been previously used and, as such, the algorithm must determine whether the result of a stimulation at 5 mA may still be inferred. In the example shown, I recruit  and I norecruit  are determined to be 6 mA and 4 mA, respectively. Because 5 mA falls in between I recruit  and I norecruit , the algorithm may not infer the result from the previous data and, as such, the stimulation may not be omitted. The algorithm then stimulates at 5 mA and reports that the channel recruits. The bracket is reduced to the lower half (making 4.50 mA the next bisection stimulation current). A stimulation current of 4.5 mA has not previously been used and, as such, the algorithm again determines I recruit  and I norecruit  (5 mA and 4 mA in this case). The selected stimulation current (4.5 mA) falls in between I recruit  an I norecruit  and, as such, the algorithm stimulates at 4.5 mA and reports the results. The bracket now stands at its final width of 0.25 mA (for the purposes of example only). I thresh  may be defined as any point within the final bracket or as the midpoint of the final bracket (4.125 mA in this case). In either event, I thresh  is selected and reported as I thresh  for channel 2. 
     Although the multi-channel threshold hunting algorithm is described above as processing channels in numerical order, it will be understood that the actual order in which channels are processed is immaterial. The channel processing order may be biased to yield the highest or lowest threshold first (discussed below) or an arbitrary processing order may be used. Furthermore, it will be understood that it is not necessary to complete the algorithm for one channel before beginning to process the next channel, provided that the intermediate state of the algorithm is retained for each channel. Channels are still processed one at a time. However, the algorithm may cycle between one or more channels, processing as few as one stimulation current for that channel before moving on to the next channel. By way of example only, the algorithm may stimulate at 10 mA while processing a first channel for I thresh . Before stimulating at 20 mA (the next stimulation current in the bracketing phase), the algorithm may cycle to any other channel and process it for the 10 mA stimulation current (omitting the stimulation if applicable). Any or all of the channels may be processed this way before returning to the first channel to apply the next stimulation. Likewise, the algorithm need not return to the first channel to stimulate at 20 mA, but instead may select a different channel to process first at the 20 mA level. In this manner, the algorithm may advance all channels essentially together and bias the order to find the lower threshold channels first or the higher threshold channels first. By way of example only, the algorithm may stimulate at one current level and process each channel in turn at that level before advancing to the next stimulation current level. The algorithm may continue in this pattern until the channel with the lowest I thresh  is bracketed. The algorithm may then process that channel exclusively until I thresh  is determined, and then return to processing the other channels one stimulation current level at a time until the channel with the next lowest I thresh  is bracketed. This process may be repeated until I thresh  is determined for each channel in order of lowest to highest I thresh . If I thresh  for more than one channel falls within the same bracket, the bracket may be bisected, processing each channel within that bracket in turn until it becomes clear which one has the lowest I thresh . If it becomes more advantageous to determine the highest I thresh  first, the algorithm may continue in the bracketing state until the bracket is found for every channel and then bisect each channel in descending order. 
       FIGS. 39A-B  illustrates a further feature of the threshold hunting algorithm of the present invention, which advantageously provides the ability to further reduce the number of stimulations required to find I thresh  when an I thresh  value has previously been determined for a specific channel. In the event that a previous I thresh  determination exists for a specific channel, the algorithm may begin by merely confirming the previous I thresh  rather than beginning anew with the bracketing and bisection methods. The algorithm first determines whether it is conducting the initial threshold determination for the channel or whether there is a previous I thresh  determination (step  320 ). If it is not the initial determination, the algorithm confirms the previous determination (step  322 ) as described below. If the previous threshold is confirmed, the algorithm reports that value as the present I thresh  (step  324 ). If it is the initial I thresh  determination, or if the previous threshold cannot be confirmed, then the algorithm performs the bracketing function (step  326 ) and bisection function (step  328 ) to determine I thresh  and then reports the value (step  324 ). 
     Although the hunting algorithm is discussed herein in terms of finding I thresh  (the lowest stimulation current that evokes a predetermined EMG response), it is contemplated that alternative stimulation thresholds may be useful in assessing the health of the spinal cord or nerve monitoring functions and may be determined by the hunting algorithm. By way of example only, the hunting algorithm may be employed by the system  10  to determine a stimulation voltage threshold, Vstim thresh . This is the lowest stimulation voltage (as opposed to the lowest stimulation current) necessary to evoke a significant EMG response, V thresh . Bracketing, bisection and monitoring states are conducted as described above for each active channel, with brackets based on voltage being substituted for the current based brackets previously described. Moreover, although described above within the context of MEP monitoring, it will be appreciated that the algorithms described herein may also be used for determining the stimulation threshold (current or voltage) for any other EMG related functions, including but not limited to pedicle integrity (screw test), nerve detection, and nerve root retraction. 
     While this invention has been described in terms of a best mode for achieving this invention&#39;s objectives, it will be appreciated by those skilled in the art that variations may be accomplished in view of these teachings without deviating from the spirit or scope of the present invention. For example, the present invention may be implemented using any combination of computer programming software, firmware or hardware. As a preparatory step to practicing the invention or constructing an apparatus according to the invention, the computer programming code (whether software or firmware) according to the invention will typically be stored in one or more machine readable storage mediums such as fixed (hard) drives, diskettes, optical disks, magnetic tape, semiconductor memories such as ROMs, PROMs, etc., thereby making an article of manufacture in accordance with the invention. The article of manufacture containing the computer programming code is used by either executing the code directly from the storage device, by copying the code from the storage device into another storage device such as a hard disk, RAM, etc. or by transmitting the code on a network for remote execution. As can be envisioned by one of skill in the art, many different combinations of the above may be used and accordingly the present invention is not limited by the specified scope.