Patent Abstract:
A method for selectively interacting with electrically excitable tissue of a patient is provided. In one configuration, an implantable pulse generator with a number of outputs and an array of electrodes with a number of electrodes being greater than the number of outputs may be implanted in a patient. An extension unit may be implanted between the implantable pulse generator and array. The extension unit acts to electrically couple the inputs of implantable pulse generator with the greater number of electrodes in the array so that the output sources are coupled to a portion of the electrodes.

Full Description:
RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 10/045,122, filed Nov. 9, 2001, now U.S. Pat. No. 7,286,878 which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of implantable systems for stimulating electrically excitable tissue within a patient. More specifically, the present invention provides a system that includes an extension unit that connects an implantable pulse generator to an implantable lead or electrode array. 
     BACKGROUND OF THE INVENTION 
     Electrical stimulation of the spinal cord and deep brain has been used for pain relief and to control movement disorders. Electrical leads having many electrodes are implanted in the body such that one or more cathodes and one or more anodes are in optimal locations to produce benefits or to minimize undesirable side effects.  FIG. 1  shows a typical implantable electrical stimulation system. An implantable pulse generator  10  generates the electrical signals that will provide the stimulation. A cable  20  connects the implantable pulse generator  10  to a lead  30 . Lead  30  contains individual electrodes  31 - 40 . Cable  20  contains ten separate conductors connecting the implantable pulse generator to each of the electrodes  31 - 40 . Implantable electrical stimulation systems are described in co-pending patent application Ser. No. 09/024,162 filed Feb. 17, 1998 and co-pending patent application Ser. No. 08/627,576 filed Apr. 4, 1996. The entire disclosures of both co-pending applications are incorporated herein by reference. 
     It would be desirable to use a lead with a large number of electrodes, such as sixteen or more, for some therapies. The polarity of each electrode could be assigned and the optimal combinations of cathodes and anodes could be selected for each patient. Another advantage to having several electrodes is that it allows for adjusting the stimulation after the components have been implanted. In particular, an implanted spinal cord stimulation lead can shift up to 0.5 cm or more after being inserted in the body. Ideally, the lead should contain enough electrodes so that some electrodes can be switched off and others switch on after the shift has taken place to avoid the patient undergoing another surgical procedure. It may also be desirable to change the location of the stimulation after the lead has been implanted. 
     The use of a large number of electrodes on a lead has been limited, in part, because of the limitations imposed by the conductors that connect the implantable pulse generator to the lead. Typical implantable electrical stimulation systems pass up to 20 milliamperes or more of current through each conductor, involving current densities of 10 microcoulombs per square centimeter per phase or more. As a result, each electrode is connected to a sizable conductor in order to minimize energy losses due to impedance and to provide adequate strength to connect the wire to a power supply without substantial risk of breakage. The size of the conductors has made it impractical to connect a large number of conductors between the implantable pulse generator and the electrodes on the lead. Furthermore, it is difficult to obtain the required reliability when using a large number of conductors. 
     One proposed system places a semiconductor device on the lead to perform a multiplexing operation to minimize the number of conductors connecting the implantable pulse generator to the electrodes. The semiconductor increases the size of the lead and may require significant changes to the standard procedure currently used to implant leads as well as the manufacturing procedures. 
     Therefore, there exists a need in the art for an implantable electrical stimulation system that includes a large number of electrodes without increasing the size of the lead and that minimizes the number of conductors connecting the implantable pulse generator to the electrodes. 
     SUMMARY OF THE INVENTION 
     The present invention provides an implantable electrical stimulation system and method that includes a large number of electrodes with a reduced number of conductors connecting the implantable pulse generator to the electrodes. Minimizing the number of conductors for a given number of electrodes increases reliability and the number of electrodes that can be used. 
     In one embodiment, the advantages of the present invention are realized by an apparatus for selectively interacting with electrically excitable tissue of a patient. The apparatus includes an implantable pulse generator having a number of output sources that transmit electrical signals and an implantable electrode array having a number of electrodes, wherein the number of electrodes is greater than the number of output sources. An extension unit is coupled between the implantable pulse generator and the implantable electrode array and is configured to electrically connect the output sources to a portion of the electrodes. 
     The implantable electrode array may include at least one biomedical sensor. Furthermore, the electrodes may be arranged in a line or in a multi-dimensional array. 
     The advantages of the present invention may also be realized with an extension unit that electrically connects an implantable pulse generator having a number of output sources that transmit pulse signals to an implantable electrode array having a number of electrodes, wherein the number of electrodes is greater than the number of output sources. The extension unit includes an array of programmable switches, each switch being connected between one output lead and at least a portion of the electrodes. The extension unit may also include a programming logic unit, coupled to the array of programmable switches, that receives programming signals and produces signals for configuring the programmable switches. 
     In yet another embodiment of the invention a method of selectively providing electrical therapeutic treatment to a patient using an electrode array is provided. The method includes the steps of implanting an electrode array having a number of electrodes near electrically excitable tissue of a patient and implanting a pulse generator having a number of output electrode arrays in the patient, the number of output sources being less than the number of electrodes. An extension unit is also implanted between the electrode array and the pulse generator. The extension unit electrically connects the output sources to a portion of the electrodes. After the devices are implanted, it is determined which electrodes are physically positioned to provide optimal therapeutic treatment and the extension unit is configured to electrically couple the output sources to the electrodes identified in the determining step. Alternatively, the method may include implanting an array having a number of biomedical sensors in a patient and implanting a diagnostic device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not limited in the accompanying figures in which like reference numerals indicate similar elements and in which: 
         FIG. 1  shows a related art implantable electrical stimulation system. 
         FIG. 2  is a schematic diagram of an implantable electrical stimulation system in accordance with a preferred embodiment of the invention. 
         FIG. 3  is a schematic diagram of an extension unit in accordance with a preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 2  shows an implantable electrical stimulation system in accordance with an embodiment of the invention. An implantable pulse generator (IPG)  220  generates the stimulation signals. The structure and operation of IPGs is known to those skilled in the art. IPG  220 , which includes housing  220   a , is connected to extension unit  226 , which includes housing  226   a , with three electrical conductors  222 - 224 . Extension unit  226  is connected to implantable electrode array  228  with six conductors  230 - 235 . Extension unit  226  receives three electrical signals on conductors  222 - 224  and transmits those signals to three of conductors  230 - 235 .  FIG. 2  shows an embodiment in which extension unit  226  is located closer to implantable electrode array  228  than to IPG  220  to minimize the overall length of the conductors connecting IPG  220  to implantable electrode array  228 . Extension unit  226  will be described in detail with reference to  FIG. 3 . 
     Electrode array  228  can be implanted at a site within a patient adjacent the tissue to be stimulated. Electrode array  228  has six electrodes  238 - 243  arranged in a straight line for illustration purposes only. Each of the electrodes  238 - 243  are electrically insulated from the other electrodes and can have an area of about 1-6 mm 2 . In operation, several neighboring electrodes can be connected in parallel to have a combined surface area of 6-24 mm 2 . Of course other sizes and configurations can be used to meet the patient&#39;s treatment needs. Electrodes  238 - 243  are electrically conductive and are preferably made from a metal like platinum or iridium. 
     Electrode array  228  can have a variety of different shapes. For example, electrode array  228  and/or electrodes  238 - 243  may be planar or any other shape (e.g., round, oval, and rectangular). Electrodes  238 - 243  also may have three dimensional outer surface (e.g., cylindrical, spherical, semispherical or conical). Electrode array  228  may also have any number of electrodes, such as sixteen or more and may also include one or more biomedical sensors (not shown) in place of or in addition to electrodes  238 - 243 . A diagnostic device (not shown) may be connected to electrodes and/biomedical sensors through an extension unit that is similar to extension unit  226 . Examples of diagnostic devices include glucose sensors, circuits that measure voltage levels and devices that store information. In alternative embodiment (not shown), two or more electrode arrays can be connected to a single extension unit. 
     Extension unit  226  allows a physician or patient to select which electrodes  238 - 243  will receive stimulation pulses. Being able to select and activate electrodes from a large number of possible sites provided by the preferred embodiments is valuable in case any site becomes unusable due to mechanical/electrical problems, scar tissue, electrode array migration, etc. A near neighboring site might give almost as useful a result. Furthermore, one does not always know before implantation what is the best strategy for electrode array placement and electrode polarity. Extension unit  226  allows the choice to be made later, and with additional reprogramming at later dates, to give degrees of freedom in the active electrode positions. For example, it is sometimes useful to have three or more electrodes in a line (especially transverse to the spinal cord axis), so that two or three can be chosen at preferred medial/lateral positions. The present invention enables changes in effective stimulation area after implantation by programming only while minimizing the number of conductors that connect IPG  220  to electrode array  228 . 
     Extension unit  226  can also be used to allow the patient or physician to optimize the diagnostic function of implanted electrodes or biomedical sensors. A large number of electrodes and/or biomedical sensors can be implanted in the patient and the optimum ones can then be selected after implantation. 
       FIG. 3  illustrates a schematic diagram of an extension unit  226  in accordance with a preferred embodiment of the invention. Extension unit  226  includes input lines  302 - 304  that receive the input signals from the IPG  220  (shown in  FIG. 2 ). Input lines  302 - 304  are connected to wave shaping circuits  306 - 308 . Wave shaping circuits  306 - 308  are electrically connected to switches  310 - 312 , which are connected to output lines  314 - 319 . Output lines  314 - 319  are connected to electrode array  228  (shown in  FIG. 2 ). Extension unit  228  may also include a controller  320  for controlling the overall operation of the unit, a communication circuit  322  for communicating with external circuits, a master clock  324  and a battery  326 . 
     The operation of extension unit  226  will now be described. Wave shaping circuits  306 - 308  receive the input signals, which are generally pulses and reshapes the input signals, if necessary. Wave shaping may include changing the voltage level of the pulse or the frequency of the pulse. Wave shaping circuits  306 - 308  may be implemented with a variety of electrical components including potentiometers and integrated circuits. Controller  320  may receive clock signals from clock  324  and control one or more of the wave shaping circuits  306 - 308  to change the polarity, voltage level or frequency of the input signals. 
     The outputs of the wave shaping circuits  306 - 308  are transmitted to switches  310 - 312 . Switches  310 - 312  may be implemented with a variety of electrical switches, including semiconductor switches. In one preferred embodiment, switches  310 - 312  are micro-relay switches that retain their switching state after power has been removed. Battery  326  may provide power to the micro-relay switches. Switches  310 - 312  are electrically connected to output lines  314 - 319 . 
     Switches  310 - 312  can be configured to transmit the signals they receive to any three of output lines  314 - 319 . Switches  310 - 312  can be controlled by controller  320 , a source external to the body, communication circuit  322  or any combination of the three. Three input lines  302 , three switches  310 - 312  and six output lines  314 - 319  are shown for illustration purposes only. Extension unit  28  can be configured to interface with any number of input lines and output lines. In one embodiment of the invention, the number of switches corresponds to the number of input lines. In the same embodiment, each switch has a number of output ports that is equal to 1+the number of output lines−the number of switches. For example, if the extension unit interfaces with five input lines and fifty output lines, the extension unit would need five 1×46 switches. Alternatively, each of the switches may be configured to be connectable to fewer of the output lines. 
     There are a number of conventional technologies that can be used to communicate with extension unit  226 . Communication can be accomplished with needles, screwdrivers, telemetry or electromagnetic energy. In one embodiment of the invention, IPG  220  (shown in  FIG. 2 ) can be used to program extension unit  226 . In particular, input lines  302 - 304  are connected to controller  320 . Controller  320  may include hardware or software to recognize programming signals and for programming wave shaping circuits  306 - 308  and/or switches  310 - 312 . Such programming signals can include predetermined programming state pulse sequences or pulses have predetermined characteristics such as pulse length followed by programming pulses having characteristics that are recognized by controller  320 . After receiving the programming signals, controller  320  can then adjust wave shaping circuits  306 - 308  and/or switches  310 - 312 . 
     In one embodiment of the invention, extension unit  226  can be used to increase the number of electrodes that receive pulse signals. Referring to  FIG. 3 , wave shaping circuits  306 - 308  can double the voltage level and frequency of the received pulses. Switches  310 - 312  and then be controlled to route the signals to output lines  314 - 316  during a first clock period and to output lines  317 - 319  during a second clock period. Furthermore, the determination of which electrodes are anodes or cathodes can be chosen by the patient or through investigation by clinicians to maximize the desired effects of stimulation, e.g., maximize pain relief, minimize spasticity, stop seizures, cause contraction of muscles, etc., and also to minimize undesirable side effects. The flexibility provided by the ability to alter the shape and frequency of the input signals allows one to provide numerous types of output signals. 
     The invention is useful in connection with electrically excitable tissue that includes both neural tissue and muscle tissue. Neural tissue includes peripheral nerves, the spinal cord surface, the deep spinal cord, deep brain tissue and brain surface tissue. Muscle tissue includes skeletal (red) muscle, smooth (white) muscle, and cardiac muscle. Furthermore, the invention works especially well for red skeletal muscle, since stimulation on such a muscle can only activate the muscle fibers directly under the cathode. Action potentials do not spread from muscle fiber to fiber, as they do in smooth muscle or cardiac muscle. Hence, a broad array of cathodes is useful to recruit many fibers of a red muscle. 
     Advantageous uses for electrode array L1-L5 described in this specification include: 
     A) Over or in motor cortex or cerebellar cortex, where there are somatotopic maps of the body, and where fine control of the loci of excitation can help affect the movements or control of various body parts. 
     B) Over or in the sensory cortex, which also has a somatotopic map, so that paresthesia and/or motor effects can be adjusted to specific body parts. 
     C) In the thalamus, where there is a three-dimensional body map, and where there are lamina of cells that might best be activated (or partly activated) using many contacts and programming. 
     D) In deep tissue, where stimulation is advantageously achieved by cylindrical leads. 
     E) Transversely and over the cauda equina (nerves in the spinal canal descending from the tip of the cord) to enable great selectivity of stimulation. 
     F) In the cochlea, where there is insufficient space for many wires, but many channels are needed and where fine-tuning which sites along the cochlea get stimulated might lead to much better hearing. 
     G) Over branches of motor nerves or large nerves, to activate distinct fascicles. 
     H) In the retina, where if a patient has no light going to the back of the eye, the preferred embodiment could stimulate in neural patterns as if light were going there in focus and being perceived. 
     Another advantage of the invention is that allows physicians and patients to use IPGs and electrode arrays manufactured by different companies. The disclosed extension unit can be used for interfacing an IPG that was not designed to operate with a particular electrode array. One skilled in the art will appreciate that the extension unit can include additional circuits that are specifically designed to couple a particular extension unit to a particular electrode array. 
     While the present invention has been described in connection with the illustrated embodiments, it will be appreciated and understood that modifications can be made without departing from the true spirit and scope of the invention.

Technology Classification (CPC): 0