Abstract:
The disclosure is directed to a chronic implantable neurostimulator that supports trial and chronic modes of operation. The implantable neurostimulator can alternatively include one or more sensors that may or may not function differently in trial and chronic modes. In particular, the device is designed to be used as both a trial neurostimulator and a permanent, or chronic, neurostimulator. A trial neurostimulation period is generally desired to evaluate the efficacy of the therapy. A percutaneous or implantable trial neurostimulator is used for the trial neurostimulation period. In most cases, the trial period is successful, in which case the trial stimulator is explanted and replaced with a permanent, i.e., “chronic,” implantable stimulator. In accordance with the disclosure, an implantable neurostimulator supports both trial neurostimulation and chronic neurostimulation in the event trial stimulation is successful. In this manner, the additional surgery ordinarily required for replacement of the trial stimulator can be avoided. Instead, the implanted neurostimulation device remains implanted and is reconfigured to transition from trial stimulation to chronic stimulation.

Description:
PRIORITY OF INVENTION 
     This invention claims priority from U.S. Provisional Application No. 60/655,557, filed on Feb. 23, 2005, entitled “IMPLANTABLE NEUROSTIMULATOR SUPPORTING TRIAL AND CHRONIC MODES”, the disclosure of which is incorporated in its entirety by reference herein 
     TECHNICAL FIELD 
     The invention relates generally to implantable medical devices and, more particularly, to devices for delivery of neurostimulation therapy. 
     BACKGROUND 
     A variety of pelvic floor disorders such as urinary control disorders, fecal control disorders, interstitial cystitis, sexual dysfunction and pelvic pain are influenced by the sacral nerves. In particular, the organs involved in various bodily functions receive much of their control via the second, third, and fourth sacral nerves, commonly referred to as S2, S3, and S4, respectively. The sacrum, in general, is a large, triangular bone situated at the lower part of the vertebral column, and at the upper and back part of the pelvic cavity. The spinal canal runs throughout the sacrum. The sacral nerves pass through the sacrum via the anterior and posterior sacral foramina. These organs are also innervated via other nerves, such as the pudendal nerve. 
     Electrical stimulation of the sacral nerves, pudendal nerves, and other nerves of the pelvic floor has been found to offer relief for many pelvic floor disorders. For example, medical leads having discrete electrodes are implanted on and near the sacral nerves. An implantable pulse generator drives the electrodes with an electrical signal to stimulate the sacral nerves, and thereby restore or control bodily functions affected by pelvic floor disorders. Several techniques of electrical stimulation may be used, including stimulation of nerve bundles within the sacrum. 
     Chronic implantation of a pulse generator and lead for sacral nerve stimulation is typically preceded by a trial period. The trial period ordinarily has a prescribed maximum duration, but sometimes is exceeded by the patient or the physician. 
     During the trial period, a clinician evaluates the efficacy of sacral nerve stimulation in alleviating the patient&#39;s disorder to determine whether the patient is a good candidate for chronic implantation. 
     The trial period ordinarily involves implantation of a temporary or chronic lead, and percutaneous connection of the lead to an external trial stimulator. Often, connection of the lead to the trial stimulator involves extensive subcutaneous tunneling of the lead to a percutaneous exit site. In addition, the percutaneous connection presents a significant risk of infection. To reduce infection risk, the lead is ordinarily tunneled away from the site selected for chronic implant, requiring added time and effort by the surgeon. 
     Neurostimulation efficacy and patient response can also be determined using a fully implantable neurostimulator specifically designed to operate during a brief trial period. Implantation of a neurostimulator with a limited power source or timed termination feature is disclosed in U.S. Publication 20040215287, to Swoyer et al., the entire content of which is incorporated herein by reference. The implantable trial neurostimulator described in the Swoyer et al. application is removed after the trial period and replaced with a chronic neurostimulator if the patient responds positively to the trial therapy. 
     SUMMARY 
     The invention is directed to a chronic implantable neurostimulator that supports both trial and chronic modes of operation. In particular, the neurostimulator is designed to be used as both a trial neurostimulator and a permanent, or chronic, neurostimulator. 
     A trial neurostimulation period is generally desired to evaluate the efficacy of the therapy. A percutaneous or implantable trial neurostimulator is used for the trial neurostimulation period. In most cases, the trial period is successful, in which case the trial stimulator is explanted and replaced with a permanent, i.e., “chronic,” implantable stimulator. 
     In accordance with the invention, an implantable neurostimulator supports both trial neurostimulation and chronic neurostimulation. In the event trial stimulation is successful, the need for explant and replacement is eliminated. In this manner, using the same neurostimulator for trial and chronic stimulation, the additional surgery ordinarily required for replacement of the trial stimulator can be avoided. Instead, the implanted neurostimulation device remains implanted and is reconfigured to transition from trial stimulation to chronic stimulation. 
     The implantable neurostimulator invokes a trial mode of neurostimulation in which the device is partially or fully operable, but only operates for a trial period of finite duration. Upon expiration of the trial period, the implantable neurostimulator stops operating, unless it receives additional authorization to either extend the trial mode or enter a chronic mode of operation. 
     The trial mode enables the clinician to evaluate the efficacy of the neurostimulation device in terms of treating a disorder and avoiding undesirable side effects. Upon initial implantation, the trial period commences and continues until the trial period has lapsed. The trial period may be tracked by the implanted neurostimulation device or an external patient programmer. Hence, the implanted neurostimulation device may disable itself unilaterally, in response to a disable command from an external programmer, or in the absence of a periodic authorization command from an external programmer. 
     Upon completion of the trial period, a clinician may upload patient information gathered during the trial period, interview the patient, and/or take other steps helpful in evaluating the efficacy of the therapy. If the therapy has been successful, as it most commonly is, the patient may continue to use the implanted neurostimulator and its associated implanted lead without the need for another surgical procedure. In particular, the clinician may authorize continued use of the implanted medical device in a chronic mode of operation. 
     The authorization to continue use in a chronic mode may be provided in a variety of ways. For example, the clinician may simply download an authorization code to the implanted neurostimulator, which authorizes the neurostimulator to continue operation. The authorization code may unlock the neurostimulator from a frozen state, or unlock parameters or programs necessary for chronic operation. Alternatively, the clinician may reprogram the implanted neurostimulation device by downloading new parameters or programs that govern the chronic mode. In general, the chronic mode is intended for relatively long term stimulation therapy over an extended, indefinite time period. However, it may be possible for a patient to discontinue the therapy at any time using a patient programmer. 
     In other embodiments, instead of or, in addition to, authorizing transition from the trial mode to the chronic mode, authorization may serve to unlock additional features of the implanted stimulator. As examples, a clinician or administrator, or a manufacturer of the implanted stimulator, may provide an authorization to activate features such as a voiding diary, different algorithms, different stimulation patterns and the like. The authorization could be provided to any element within the neurostimulation system, such as a physician programmer, patient programmer or the implanted stimulator. 
     In one embodiment, the invention provides a method comprising implanting a neurostimulator in a patient, operating the neurostimulator in a trial mode to evaluate efficacy of the neurostimulator, and operating the neurostimulator in a chronic mode if the trial mode indicates an acceptable level of efficacy. The neurostimulator may be explanted from the patient if the trial mode does not indicate the acceptable level of efficacy. The trial mode extends over a relatively short trial period, such as less than six months, less than one month, or less than one week. The chronic mode may extend for more than six months, and preferably more than one year. 
     In another embodiment, the invention provides an implantable neurostimulator comprising a pulse generator, a memory storing a trial mode program and a chronic mode program, and a processor that controls the pulse generator to apply stimulation pulses according to either the trial mode program or the chronic mode program. 
     One embodiment of the invention also incorporates one or more sensors. In some embodiments, the one or more sensor can gather different information, gather information at different time intervals, or some combination thereof during trial mode and chronic mode. In one embodiment, the one or more sensor gathers information more frequently during the trial mode than it does during the chronic mode. 
     The invention may provide one or more advantages. For example, an implantable neurostimulator that supports both trial and chronic modes of operation can be used in a trial period without the need for percutaneous extensions, and does not require explantation for replacement by a chronic stimulator if the trial period is successful. In this manner, the surgeon benefits from reduced surgical effort and time, while the patient benefits from reduced surgical trauma, recovery and infection risks. In some case, the implantable neurostimulator will be explanted when the trial period is not successful. However, it is expected that explantation will be necessary in only a minority of cases, and is generally outweighed by the advantage of avoiding explantation for those patients for whom the trial period was successful. As a further advantage, the time between the end of the trial period and the beginning of chronic therapy can be reduced because there is no need for explantation and replacement of the trial stimulator. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an implantable neurostimulation system designed to support both a trial mode and a chronic mode of operation. 
         FIG. 2  is a block diagram illustrating various components of an implantable neurostimulator. 
         FIG. 3  is a block diagram illustrating various components of another implantable neurostimulator. 
         FIG. 4  is a block diagram illustrating various components of a patient programmer for use with the implantable neurostimulation system of  FIG. 1 . 
         FIG. 5  is a block diagram illustrating various components of another patient programmer for use with the implantable neurostimulation system of  FIG. 1 . 
         FIG. 6  is a diagram of a patient programmer and an implantable neurostimulator with a battery as a power source. 
         FIG. 7  is a diagram of a patient programmer and an implantable neurostimulator with a transcutaneous receiver coil interface for power delivery. 
         FIG. 8  is a flow diagram illustrating implantation and use of an implantable neurostimulator system in a trial mode and a chronic mode. 
     
    
    
     DETAILED DESCRIPTION 
     The invention, as described herein, is directed to an implantable neurostimulator for delivering neurostimulation therapy to a patient in both a trial mode and a chronic mode. In the trial mode, the neurostimulator delivers therapy that permits a clinician and patient to evaluate the efficacy of the therapy. The trial period may run for a relatively short period of time, such as several hours, days, weeks, or months. If the trial mode indicates desirable efficacy, the neurostimulator enters a chronic mode in which the neurostimulator delivers neurostimulation on a permanent basis over an extended period of time, such as several months or years. 
     Although the invention may be described in the context of neurostimulation for pelvic floor disorders, for purposes of illustration, the invention may be readily applicable to other forms of neurostimulation or other neurostimulation applications in which a trial period is desirable, including but not limited to spinal cord stimulation for relief of chronic, intractable pain, gastric stimulation for treatment of gastric mobility disorders or obesity, and stimulation for the treatment of sexual dysfunction. Sacral nerve stimulation will be described herein for purposes of illustration. However, the invention may be applied to applications involving stimulation of other nerves, such as pudendal nerves, perineal nerves, the spinal cord, the stomach, or other areas of the nervous system. 
       FIG. 1  is a diagram illustrating an implantable neurostimulation system  20  for sacral nerve stimulation via at least one lead  10 . Neurostimulation system  20  includes an implantable neurostimulator  24  that supports both a trial stimulation mode and a chronic stimulation mode. Neurostimulation system  20  delivers neurostimulation to the sacral nerves or other regions of the nervous system known to influence pelvic floor disorders, urinary control disorders, fecal control disorders, interstitial cystitis, sexual dysfunction, pelvic pain, or some combination thereof. 
     Neurostimulator  24  includes an implantable pulse generator, and delivers neurostimulation therapy to patient  12  in the form of electrical pulses generated by the implantable pulse generator. In the example of  FIG. 1 , neurostimulator  24  is implanted in the upper left buttock of patient  12 , but may be implanted at other locations. A proximal end of stimulation lead  10  is coupled to a connector block  26  associated with neurostimulator  24 . As shown in  FIG. 1 , lead  10  may be coupled to connector block  26  via a lead extension  28  and connector  30 . Neurostimulator  24  may be implanted within a subcutaneous pocket  31  that serves as the implant site. Subcutaneous pocket  31  is formed by a surgical procedure. 
     Lead  10  carries one or more stimulation electrodes to permit delivery of electrical stimulation to sacral nerves. For example, implantable neurostimulation system  20  may stimulate organs involved in urinary, fecal or sexual function via C-fibers or sacral nerves at the second, third, and fourth sacral nerve positions, commonly referred to as S2, S3, and S4, respectively. Also, in some embodiments, lead  10  may carry one or more sense electrodes to permit neurostimulation device  24  to sense electrical signals within sacrum  16 . 
     Accordingly, lead  10  includes an outer lead body that contains one or more conductors to electrically couple the electrodes to terminals within connector block  26 . In some embodiments, trial neurostimulator  24  may be coupled to two or more leads deployed at different positions relative to the spinal cord or sacral nerves, for example. 
     Implantable neurostimulator  24  eliminates the need for a percutaneous connection for trial stimulation, reducing the risk of infection and affording greater convenience and comfort to the patient. Moreover, the absence of a percutaneous connection makes the neurostimulator easier to tolerate, and presents a reduced infection risk, permitting trial periods to run for extended period of times. The trial period may run for days, weeks or even months, in view of heightened patient tolerance. As examples, the trial period may be less than one month or, in some cases, less than six months. 
     Neurostimulator  24  supports both trial stimulation and chronic stimulation. Consequently, neurostimulator  24  offers the convenience of fewer surgical procedures for patients by enabling both a trial period and chronic operation without the need to remove a trial neurostimulator and replace it with a chronic stimulator after effective trial therapy. 
     In the trial mode, neurostimulator  24  delivers either a full set of stimulation parameters or a limited set of stimulation parameters specified by a clinician. The patient may be permitted to adjust stimulation parameters, such as amplitude, pulse width and pulse rate, i.e., frequency, during the trial mode. Alternatively, the clinician may specify a set of fixed stimulation parameters or a limited range of adjustment. 
     In the chronic mode, neurostimulator  24  may deliver a full set of stimulation parameters, and may permit a full range of adjustment by patient  12 , subject to limits specified by the clinician. In addition, neurostimulator  24  may accept new parameters, adjusted parameters, or new programs containing parameter sets, via a physician programmer. 
     In one embodiment of the invention, a device includes at least one sensor. The one or more sensor may provide a variety of information indicative of the level of efficacy achieved by the neurostimulation therapy delivered by neurostimulator. The information may be any information relating to the function of the bladder, or any other segment of the patient&#39;s urinary tract, in storing releasing and passing urine. For example, the sensor may monitor parameters such as bladder pressure, bladder contractile force, urinary sphincter pressure, urine flow rate, urine flow pressure, voiding amount, and the like. 
     Other examples of sensed information include urine flow velocity, urine or bladder temperature, impedance, urinary pH, or chemical constituency of the urine. Any of such information may reveal the effect of the neurostimulation therapy on the physiological function of the bladder, the urethra, or the urinary sphincter. For example, if the sensor indicates excessive pressure, excessive contractile force, or involuntary urine flow (i.e., leakage) in response to a set of stimulation parameters, the information will be gathered by the one or more sensors. 
     In still other embodiments, the one or more sensors may be implanted within a patient to sense a physiological state of the patient. For example, a sensor may be deployed to sense cardiac activity, respiratory activity, electromyographic activity, or the like, as an indication of patient activity level. Such activity level information, in conjunction with other information, may be useful in determining the efficacy of the stimulation parameters. Other types of sensors may detect a posture or activity level of the patient. For example, an accelerometer may detect an elevated activity level, e.g., during exercise, while other sensors may detect whether the patient is sitting, standing or lying down. In addition, some of the information obtained by such sensors, such as respiration activity, may be analyzed to determine, e.g., whether the patient is sleeping. 
     In an embodiment including one or more sensors, the function of the one or more sensors can be different during the trial mode and the chronic mode. In one embodiment, the one or more sensors gathers information more frequently in the trial mode than it does in the chronic mode. In one embodiment, for example, the one or more sensors gathers information on an hourly or daily basis in the trial mode, and on a weekly basis in the chronic mode. In another embodiment, at one or more sensors gathers information in response to every voiding event, for example, that is indicated by the patient, and in the chronic mode, information is gathered only in response to every fifth, tenth, or twentieth (for example) voiding event. 
     In another embodiment including one or more sensors, the information that is gathered during the trial mode includes more physiological parameters than that gathered during the chronic mode. In yet another embodiment including one or more sensors, both the information that is gathered, and the frequency upon which it is gathered is different in the trial mode and the chronic mode. An example of such an embodiment would be a device where more parameters are gathered more often during the trial mode than in the chronic mode. 
     The one or more sensors, if included in an embodiment of the invention, can carry sufficient battery resources, a rechargeable battery, or an inductive power interface that can permit extended operation. The sensor(s) may be implanted by minimally invasive, endoscopic techniques for example. In some embodiments, the sensor(s) transmits sensed information continuously or periodically to the neurostimulator or the patient programmer. In this case, the sensor(s) can monitor physiological conditions continuously or periodically. Alternatively, the neurostimulator or patient programmer may trigger activation of the sensor(s) to capture information at desired intervals. In some cases, triggered activation may occur when the patient enters information into the patient programmer to indicate a voiding event, for example. Triggered activation of the sensor(s) may be useful in conserving battery life, if applicable, of the sensor(s) or neurostimulator. In one embodiment, multiple sensors may be provided and dedicated to different parameters or different locations within the urinary tract. 
     In one embodiment of the invention, the sensor(s) can be programmed to gather information at a particular frequency (either the same or different in both the trial and chronic mode), and be configured to gather information when prompted by the patient programmer or the physician programmer. In another embodiment, the sensor(s) are programmed to gather information at a particular frequency during the trial mode and are programmed to be activated when prompted by the patient programmer or physician programmer during the chronic mode. In yet another embodiment, the sensor(s) are programmed to gather information at particular frequencies during the trial and chronic mode (either the same frequency or different frequencies) and can also be activated when prompted by the patient programmer or physician programmer during both the trial mode and the chronic mode. 
     Rather than immediately transmitting the information to the neurostimulator or the patient programmer, the sensor(s) may initially store the information internally for subsequent wireless transmission. Hence, in some embodiments, the information may be stored within the sensor(s), and later transmitted to the neurostimulator or the patient programmer. In this case, the neurostimulator or the patient programmer may interrogate the sensor(s) to obtain the stored information. 
     In one embodiment, the sensor(s) can include a sensor processor, a sensing element, memory, wireless telemetry interface, and a power source. The sensor(s) also may include an internal clock to track date and time of voiding events. The sensor(s) may have a capsule-like shape, and may be placed within the bladder or the urethra by endoscopic introduction via the urethra, or by hypodermic injection using a hypodermic needle. Alternatively, the sensor(s) may be surgically implanted. In the case of minimally invasive endoscopic introduction, the sensor(s) may be constructed in a manner similar to the sensors described in U.S. patent application Ser. No. 10/978,233, to Martin Gerber, filed Oct. 29, 2004, and entitled “Wireless Urinary Voiding Diary System,” which claims the benefit of U.S. provisional application no. 60/589,442, filed Jul. 20, 2004; or U.S. patent application Ser. No. 10/833,776, to Mark Christopherson and Warren Starkebaum, filed Apr. 28, 2004, entitled “Implantable Urinary Tract Monitor,” the entire content of each of which is incorporated herein by reference. 
     The sensing element may be selected for any of a variety of urodynamic testing applications, and may include appropriate signal processing circuitry such as amplifier, filter, driver, and analog-to-digital conversion circuitry for presentation of sensed information to sensor processor. For urodynamic testing, sensing element may take the form of a pressure, flow, velocity, volume, temperature, impedance, or contractile force sensor. For pressure measurements, for example, sensing element may include one or more diaphragm sensors, strain gauge sensors, capacitive sensors, piezoelectric sensors, or other sensors used in conventional catheter-based urodynamic testing to sense pressure. As a further example, for bladder emptying, sensing element may include a conductive sensor to sense the presence of urine within the lower region of the bladder. 
     For flow measurements, sensing element may comprise a pulsed Doppler ultrasonic sensor, or a laser Doppler flow sensor. Doppler shifting of the frequency of the reflected energy indicates the velocity of the fluid flow passing over a surface of sensing element. Consequently, in some embodiments, the sensor(s) may include circuitry, such as a quadrature phase detector, in order to enable the monitor to distinguish the direction of the flow of fluid in addition to its velocity. 
     As a further example, sensing element may include any one or more thermal-convection velocity sensors. A thermal-convection velocity sensor may include a heating element upstream of a thermistor to heat urine within the urethra such that flow rate may be measured according to the temperature of the heated fluid when it arrives at the thermistor. In other embodiments, flow rate may be determined from the output of a concentration or temperature sensor using Fick&#39;s techniques. 
     In some embodiments, the sensing element may include multiple sensors of a given type, as well as multiple types of sensors, e.g., pressure, flow, bladder emptying, or the like. Accordingly, the information obtained by the sensor(s) may then include different types of physiological parameters associated with a voiding event. Alternatively, multiple the sensor(s) may be deployed within the bladder or the urethra. In this case, each of the sensor(s) may be configured with a different type or set of sensing elements to collect a variety of different urodynamic parameters during a voiding event. 
     In some other embodiments, the sensing element may be chosen to sense a physiological state, such as an activity type, activity level, or posture of the patient. For example, the sensing element can include an accelerometer to detect an elevated activity level, or a decreased activity level. 
     As further shown in  FIG. 1 , implantable neurostimulation system  20  also may include a clinician programmer  34  and a patient programmer  36 . Clinician programmer  34  may be a handheld computing device that permits a clinician to program neurostimulation therapy for patient  12 , e.g., using input keys and a display. For example, using clinician programmer  34 , the clinician may specify neurostimulation parameters for use in delivery of neurostimulation therapy. 
     Clinician programmer  34  supports radio frequency telemetry with neurostimulator  24  to download neurostimulation parameters and, optionally, upload operational or physiological data stored by neurostimulator  24 . In this manner, the clinician may periodically interrogate neurostimulator  24  to evaluate efficacy and, if necessary, modify the stimulation parameters. Once the trial period has concluded and has been deemed effective, programmer  34  may have the ability to renew the operational “subscription” of neurostimulator  24  with neurostimulation parameters for chronic stimulation therapy. 
     Like clinician programmer  34 , patient programmer  36  may be a handheld computing device. Patient programmer  36  may also include a display and input keys to allow patient  12  to interact with patient programmer  36  and implantable neurostimulator  24 . In this manner, patient programmer  36  provides patient  12  with an interface for control of neurostimulation therapy by neurostimulator  24 . 
     For example, patient  12  may use patient programmer  36  to start, stop or adjust neurostimulation therapy during the trial or chronic mode. In particular, patient programmer  36  may permit patient  12  to adjust stimulation parameters such as duration, amplitude, pulse width and pulse rate within an adjustment range specified by the clinician via clinician programmer  34 . In one embodiment of the invention, different options may be provided to the patient during the trial period and the chronic period. 
     Neurostimulator  24 , clinician programmer  34  and patient programmer  36  may communicate via wireless communication, as shown in  FIG. 1 . Clinician programmer  34  and patient programmer  36  may, for example, communicate via wireless communication with neurostimulator  12  using RF telemetry techniques known in the art. Clinician programmer  34  and patient programmer  36  also may communicate with each other using any of a variety of local wireless communication techniques, such as RF communication according to the 802.11 or Bluetooth specification sets, or other standard or proprietary telemetry protocols. 
     The trial mode may be initiated by an enable signal transmitted by neurostimulator  24  from clinician programmer  34  or patient programmer  36 . Likewise, the trial period may be terminated by a disable command transmitted by clinician programmer  34  or patient programmer  36 , e.g., in response to a physician or patient command or upon expiration of a trial period timer. 
     Alternatively, the trial mode may be operative only so long as neurostimulator  24  continues to receive a periodic enable signal. In this case, if the enable signal is not received within a scheduled interval, neurostimulator  24  disables the trial mode. As a further alternative, once enabled, neurostimulator  24  may track the progress of the trial period using an internal clock or clock derived from an external source, and unilaterally disable the trial mode when the trial period has lapsed. 
     Upon disablement, neurostimulator  24  may remain idle until it is explanted, in the case of an unsuccessful trial, or until a clinician transmits an authorization command via the clinician programmer  34  or patient programmer  36  to enter the chronic mode, in the case of a successful trial. In the chronic mode, neurostimulator  24  is reactivated and enabled for continued delivery of neurostimulation therapy. In this case, neurostimulator  24  may receive simply the authorization command to invoke a set of parameters or a program stored in the neurostimulator to support the chronic mode. Alternatively, neurostimulator  24  may receive a new parameter or program via the clinician programmer  34  or patient programmer  36 , or be entirely reprogrammed. In each case, there is no need for another surgical procedure. Instead, the provisioning of neurostimulator  24  is accomplished by wireless telemetry. 
       FIG. 2  is a block diagram illustrating various components of an implantable neurostimulator  24 A. As shown in  FIG. 2 , device  12  delivers neurostimulation therapy via electrodes  37 A,  37 B,  37 C,  37 D of lead  10  (collectively “electrodes”). Electrodes  37  may be ring electrodes arranged on an axial lead or pad electrodes arranged in an array on a paddle lead. The configuration, type and number of electrodes  37  illustrated in  FIG. 2  are merely exemplary. Electrodes  37  are electrically coupled to a therapy delivery circuit  36  via lead  10 . 
     Therapy delivery circuit  36  may, for example, include an implantable pulse generator coupled to a power supply  38  that generates stimulation energy from power delivered by a battery  40 . The implantable pulse generator within therapy delivery circuit  36  delivers electrical pulses to patient  12  via at least some of electrodes  37  under the control of a processor  42 . In one example, therapy delivery circuit  36  may deliver neurostimulation pulses with parameters selected to have values effective in controlling or managing symptoms of urinary incontinence, such as involuntary leakage. An exemplary range of neurostimulation stimulation pulse parameters likely to be effective in treating incontinence, e.g., when applied to the sacral or pudendal nerves, are as follows: 
     1. Frequency: from approximately 0.5 Hz to 500 Hz, in one embodiment from approximately 10 Hz to 250 Hz, and in yet another embodiment from approximately 10 Hz to 25 Hz. 
     2. Amplitude: from approximately 0.1 volts to 50 volts, in one embodiment from approximately 0.5 volts to 20 volts, and in yet another embodiment from approximately 1 volt to 10 volts. 
     3. Pulse Width: from about 10 microseconds to 5000 microseconds, in one embodiment from approximately 100 microseconds to 1000 microseconds, and in yet another embodiment from approximately 180 microseconds to 450 microseconds. 
     Processor  42  controls the implantable pulse generator within therapy delivery circuit  36  to deliver neurostimulation therapy according to selected stimulation parameters. Specifically, processor  42  controls therapy delivery circuit  36  to deliver electrical pulses with selected amplitudes, pulse widths, and rates specified by the programs. In addition, processor  42  also controls therapy delivery circuit  36  to deliver the neurostimulation pulses via selected subsets of electrodes  37  with selected polarities. 
     Processor  42  may control therapy delivery circuit  36  to deliver each pulse according to a different program, thereby interleaving programs to simultaneously treat different symptoms or provide a combined therapeutic effect. For example, in addition to treatment of urinary incontinence, neurostimulator  24 A may be configured to deliver neurostimulation therapy to treat pain. Processor  42  may include a microprocessor, a controller, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), an field-programmable gate array (FPGA), discrete logic circuitry, or the like. 
     Neurostimulator  24 A also includes a memory  46 . In some embodiments, memory  46  stores multiple sets of stimulation parameters that are available to be selected by patient  12  or a clinician for delivery of neurostimulation therapy. For example, memory  46  may store stimulation parameters transmitted by clinician programmer  34 . Memory  46  also stores program instructions that, when executed by processor  42 , cause device  12  to deliver neurostimulation therapy. 
     In the example of  FIG. 2 , memory  46  may store one or more trial programs  47  and one or more chronic programs  49  that control delivery of stimulation pulses for a trial mode and a chronic mode, respectively. In this case, an external device such as clinician programmer  34  or patient programmer  36  may send a signal to cause neurostimulator  24  to select either trial program  47  or chronic program  49 , which are already loaded into memory  46 . In other embodiments, trial and chronic programs may be selectively loaded into memory  46 , e.g., by programming from clinician programmer  34  or patient programmer  36 , to cause neurostimulator  24  to enter either the trial mode or the chronic mode. 
     Memory  46  may include any volatile, non-volatile, magnetic, optical, or electrical media, such as a RAM, ROM, CD-ROM, hard disk, removable magnetic disk, memory cards or sticks, NVRAM, EEPROM, flash memory, and the like. Accordingly, the invention also contemplates computer-readable media storing instructions to cause processor  42  to provide the functionality described herein. 
     A telemetry circuit  44  supports wireless communication between device  12 , clinician programmer  34 , patient programmer  36 , or some combination thereof. In addition, in some embodiments, neurostimulator  24 A may optionally include a timer  48  to be used during the trial mode. Timer  48  may serve to time the duration of the trial period, in some embodiments. For example, upon initiation of the trial period, timer  48  starts running to track the elapsed time in the trial period relative to a maximum trial period time. In some embodiments, when timer  48  expires, processor  42  responds by disabling therapy delivery circuit  36  to terminate the trial mode. In other words, processor  42  stops the trial period by stopping delivery of neurostimulation therapy to patient  12  by trial neurostimulator  24 A. 
     In this manner, implantable neurostimulator  24 A can be configured to prevent patient  12  or a physician from prolonging the trial period beyond a prescribed period of time. Rather, implantable neurostimulator  24 A has a finite period of operation, determined by a maximum trial period that may be specified by the manufacturer or in a programmable manner by a clinician. In chronic mode, timer  48  may be used in some embodiments to serve as a method to ensure clinician evaluation of stimulation throughout chronic therapy. In other words, the chronic mode may be periodically disabled, e.g., with advance warning to the patient through patient programmer  36 , to compel a clinical visit by the patient for periodic evaluation. Timer  48  may be implemented in hardware using a real-time clock, in software by processor  42 , or a combination thereof. Accordingly, timer  48  is illustrated as a separate component in  FIG. 2  merely for exemplary purposes. 
     Battery  40  of implantable neurostimulator  24 A may be selected based on the stimulation therapy needed. In some cases, battery  40  may be a conventional lithium battery normally used in typical stimulation devices. The size and shape of battery  40  may be different to make implantable stimulator  24  smaller to fit in certain places on the patient or to allow a plurality of stimulation or sensing leads to be attached. In some embodiments, other battery technologies such as Nickel-metal-hydride or NiCad may be used in battery  40 . 
       FIG. 3  is a block diagram illustrating various components of another implantable neurostimulator  24 B. Neurostimulator  24 B conforms substantially to neurostimulator  24 A, but incorporates a transcutaneous receiver coil  50  instead of a battery  40 . Transcutaneous receiver coil  50  is implanted with neurostimulator  24 B and transduces electromagnetic energy received from an external coil into power for power supply  38 . In this embodiment, neurostimulator  24 B does not rely on battery resources as a power supply. Instead, neurostimulator  24 B receives power via transcutaneous receiver coil  50  and controls the duration of the trial period and any subsequent chronic period in response to timer  48 . 
       FIG. 4  is a block diagram illustrating various components of a patient programmer  36 A for use with one embodiment of the implantable neurostimulation system  20  of  FIG. 1 . As shown in  FIG. 4 , patient programmer  36 A includes a processor  52 , a user interface  54 , and a telemetry circuit  56  for communication with telemetry circuit  44  of either of neurostimulators  24 A,  24 B, or clinician programmer  34 . Patient programmer  36 A also includes memory  58  to store parameters, settings and instructions, and optionally a timer  60  for use during the trial mode, the chronic mode, or both. Timer  60  may be implemented in hardware or software, and may operate as a programmable feature of processor  52 . 
     Patient programmer  36 A responds to user input entered via user interface  54  to adjust stimulation parameters, settings, and the like. The scope of adjustments permitted by patient programmer  36 A may vary according to whether neurostimulator  24  is in trial mode or chronic mode. In addition, patient programmer  36 A may interrogate neurostimulator  24  to obtain parameters, settings, and other operational data. For example, patient programmer  36 A may be used to initially program neurostimulator  24  for the trial period, and to upload parameters, settings, and other operational data from neurostimulator  24  upon expiration of the trial period. During the course of the trial period or chronic period, neurostimulator  24  may store a variety of information concerning adjustments made by the user, usage profiles and the like. 
     Notably, patient programmer  36 A is physically decoupled from neurostimulator  24  in the sense that neurostimulator  24  is implanted and the patient programmer is external to patient  12 . While patient programmer  36 A may be used during trial mode, chronic mode, or both, some embodiments of patient programmer  36 A may include separate patient programmers for use during trial mode and chronic mode. This distinction may make it possible to save cost on the trial programmer, which may be less durable and contain fewer features than the full chronic programmer. Accordingly, patient programmer  36 A may upload applicable parameters, settings, and operational information from neurostimulator  24  at the end of the trial period, and then transfer that information directly to chronic programmer, providing significant convenience to the physician and patient. 
     Patient programmer  36 A may operate with a rechargeable or replaceable battery (not shown). In the example of  FIG. 4 , it is assumed that neurostimulator  24  includes its own battery. In other embodiments, as described herein, patient programmer  36 A may deliver power to neurostimulator  24  transcutaneously. In addition, rather than download operational parameters and settings to neurostimulator  24  for substantially independent operation by the neurostimulator, patient programmer  36 A may dynamically control the operation of the neurostimulator by continuous communication with the neurostimulator via telemetry circuit  44 . 
     In addition, in some embodiments, patient programmer  36 A, rather than neurostimulator  24 , may control the end of the trial period. For example, processor  52  may be responsive to expiration of a finite period of time, as indicated by timer  60 . In this case, processor  52  transmits a signal to implanted neurostimulator  24  instructing the neurostimulator to cease operation. After the clinician analyzes the efficacy and safety of the stimulation therapy during the trial period, patient programmer  36 A may be used to renew a subscription for neurostimulator  24  and enable it to function in chronic mode for an unlimited time period, subject to longevity of battery resources in battery-powered systems. In some embodiments, the chronic mode may necessitate a fee or other action or commitment before the subscription to support chronic mode is activated. 
     Alternatively, processor  52  may simply terminate communication with neurostimulator  24  at the end of the trial period, in which case the neurostimulator terminates operation. Accordingly, termination of the trial period may be initiated within neurostimulator  24  or within patient programmer  36 A, and may be accomplished in a variety of ways. 
       FIG. 5  is a block diagram illustrating various components of another patient programmer  36 B for use with the implantable neurostimulation system  20  of  FIG. 1 . Patient programmer  36 B conforms substantially to patient programmer  36 A of  FIG. 4 . However, patient programmer  36 B further includes a transcutaneous transmitter coil  62  to deliver electromagnetic energy to transcutaneous receiver coil  50  of neurostimulator  24 B for transformation into operating power. 
     Accordingly, patient programmer  36 B is worn by the patient. Patient programmer  36 B, or at least transcutaneous transmitter coil  62 , is positioned adjacent neurostimulator  24  to provide effective electromagnetic coupling between the transcutaneous transmitter coil and transcutaneous receiver coil  50 . In the example of  FIG. 5 , termination of the trial mode can be made by simply terminating the supply of power from transcutaneous transmitter coil  62  and transcutaneous receiver coil  50 . In chronic mode, electromagnetic coupling should not be terminated. However, if this occurs, the renewed subscription is not invalidated. Once neurostimulator  24  receives power again, chronic mode may continue once again without interruption. 
       FIG. 6  is a diagram of an implantable trial neurostimulator  24 A with a small battery  40  as a power source. In the example of  FIG. 6 , battery  40  is depicted as a coin cell battery, although other battery configurations may be used. In general, battery  40  may have a power capacity and longevity of several years, similar to batteries used in other commercially available neurostimulators. 
       FIG. 6  also depicts patient programmer  36 A. As shown in  FIG. 6 , patient programmer  24 A may include buttons  66 ,  68  to increase and decrease stimulation settings, respectively. In addition, patient programmer  36 A includes a display  64 , and navigational buttons  70  to permit navigation and selection of control options presented via the display. Buttons  66 ,  68 ,  70  and display  64  form part of user interface  54 . In some cases, display  64  may present information advising patient  12  that expiration of the trial period is approaching. This information may help a patient receiving positive therapy by limiting the time between the end of the trial period and the start of the chronic period. 
       FIG. 7  is a diagram of an implantable neurostimulator  24 B with a transcutaneous receiver coil  50  for power delivery. In particular, patient programmer  36 B includes a transcutaneous receiver coil  50 , which either resides within or extends from a housing associated with neurostimulator  24 B. Transcutaneous receiver coil  50  receives electromagnetic energy from transcutaneous transmitter coil  62 , which may be integrated with patient programmer  36 B or extend from a cable  72 , as shown in  FIG. 7 . With the exception of transcutaneous transmitter coil  62 , patient programmer  36 B may otherwise conform to patient programmer  36 A of  FIG. 6 . 
       FIG. 8  is a flow diagram illustrating implantation and use of an implantable neurostimulator system  20  in accordance with the invention. As shown in  FIG. 8 , a chronic stimulation lead  10  is first implanted ( 74 ), e.g., as shown in  FIG. 1 . In some embodiments, lead  10  could be chronic or temporary. With neurostimulator  24  implanted in the chronic implant site, however, it may be desirable to use the chronic lead and thereby avoid the need to withdraw a temporary lead and replace it with the chronic lead after a successful trial period. 
     Upon surgical creation of a subcutaneous pocket ( 76 ), the neurostimulator is placed in the subcutaneous pocket for implantation ( 78 ). The chronic lead is subcutaneously tunneled through the body of patient  12  to the subcutaneous pocket ( 80 ). As shown in  FIG. 1 , for example, the lead may extend from sacrum  16  to a subcutaneous pocket  31  in the upper left buttock area of patient  12 . 
     When lead  10  is tunneled to subcutaneous pocket  31 , the lead is connected to neurostimulator  24  either directly or via a lead extension and connector. Before connecting lead  10  to neurostimulator  24 , and tunneling the lead, a test stimulator may be connected to the lead to deliver neurostimulation for assistance in determining lead placement, lead depth and electrode selection. 
     Once the subcutaneous pocket  31  is sealed, neurostimulator  24  is activated for use in trial mode for the duration of the trial period ( 82 ). Patient  12  thereafter goes about his daily routine, and may control neurostimulator  24  via patient programmer  36 . When the trial period is complete ( 84 ), e.g., as evidenced by expiration of a timer within trial neurostimulator  24  or patient programmer  36 , the neurostimulator ceases operation. 
     Patient programmer  36 , or physician programmer  34 , then may upload from neurostimulator  24  a set of neurostimulator parameters, settings, operational information or the like either pre-programmed or accumulated during the course of the trial period ( 86 ). The information can be uploaded to patient programmer  36 . 
     If the results are unfavorable and stimulation is unsuccessful to treat the condition of the patient, neurostimulator  24  and lead  10  may be removed from the patient. If the results of the trial period are favorable, the physician may activate the chronic mode of neurostimulator  24  for operation in chronic mode ( 88 ). Following the reactivation of neurostimulator  24  in chronic mode, patient programmer  36  or physician programmer  34  may be used to download at least some updated parameters, settings, and other operational information to modify stimulation therapy based upon information received from the trial period ( 90 ). In this manner, patient programmer  36  or physician programmer  34  can be used to quickly and conveniently program the chronic mode based on the results of the trial period. This feature can greatly simplify programming for the chronic period following the trial period. 
     In other embodiments, instead of or, in addition to, authorizing transition from the trial mode to the chronic mode, authorization may serve to unlock additional features of the implanted stimulator. As examples, a clinician or administrator, or a manufacturer of the implanted stimulator, may provide an authorization to activate features such as a voiding diary, different algorithms, different stimulation patterns and the like. The authorization could be provided to any element within the neurostimulation system, such as a physician programmer, patient programmer or the implanted stimulator. The authorization may be an authorization key or code that is obtained from a clinician or a manufacturer of the neurostimulator. 
     In some cases, the authorization may be downloaded or otherwise obtained from a manufacturer for an additional fee, somewhat like a software license. The authorization may be obtained on a recurring basis, for a recurring fee, to permit extended use of the neurostimulator or particular features of the neurostimulator. For example, an initial payment and authorization may support use of the neurostimulator or particular features of the neurostimulator for a period of time, e.g., one year. After a year of use, the neurostimulator or features “expire” in the sense that they are deactivated. Upon payment of a renewal fee, the neurostimulator or features are reactivated, providing another “subscription” period for the user. 
     As a further variation, in the case of a rechargeable neurostimulator that is recharged at periodic intervals, a fee could be charged each time the battery in the neurostimulator is recharged. If the fee is not paid, the neurostimulator is inactivated. 
     In each of the renewable scenarios, features can be extended or unlocked on a selective basis, providing a pay as you go approach for patients and the ability to select appropriate features on an a la carte basis. 
     Many embodiments of the invention have been described. Various modifications may be made without departing from the scope of the claims. These and other embodiments are within the scope of the following claims.