Abstract:
The disclosure describes an optical fiber pressure sensor to measure sphincter pressure which may be incorporated into a therapeutic sphincter control system. The system senses sphincter pressure and sends the information to a stimulator that is capable of stimulation therapy to control sphincter contractility, thus reducing unwanted urinary incontinence. Measuring sphincter pressure is accomplished through the use of an optical fiber connected to flexible tube section placed through the sphincter, where properties of the emitted light are changed proportional to the pressure on the tube section. The light is returned to a light detector to measure light properties and create an electrical signal representative of the pressure on the tube section. The signal may then be sent by wireless telemetry to an implanted stimulator or external programmer.

Description:
TECHNICAL FIELD 
     The invention relates to implantable medical devices and, more particularly, implantable sensors. 
     BACKGROUND 
     Urinary incontinence, or an inability to control urinary function, is a common problem afflicting people of all ages, genders, and races. Various muscles, nerves, organs and conduits within the urinary tract cooperate to collect, store and release urine. A variety of disorders may compromise urinary tract performance and contribute to incontinence. Many of the disorders may be associated with aging, injury or illness. 
     In some cases, urinary incontinence can be attributed to improper sphincter function, either in the internal urinary sphincter or external urinary sphincter. For example, aging can often result in weakened sphincter muscles, which causes incontinence. Some patients also may suffer from nerve disorders that prevent proper triggering and operation of the bladder or sphincter muscles. Nerves running though the pelvic floor stimulate contractility in the sphincter. A breakdown in communication between the nervous system and the urinary sphincter can result in urinary incontinence. 
     Electrical stimulation of nerves in the pelvic floor may provide an effective therapy for a variety of disorders, including urinary incontinence. For example, an implantable neurostimulator may be provided to deliver electrical stimulation to the sacral nerve to induce sphincter constriction and thereby close or maintain closure of the urethra at the bladder neck. An appropriate course of neurostimulation therapy may be aided by a sensor that monitors physiological conditions with the urinary tract. In some cases, an implantable stimulation device may deliver stimulation therapy based on the level or state of a sensed physiological condition. 
     SUMMARY 
     The invention is directed to an implantable optical pressure sensor for sensing urinary sphincter pressure, as well as a neurostimulation system and method that make use of such a sensor for alleviation of urinary incontinence. The sensor includes an optical fiber and a flexible tube section. In some embodiments, the flexible tube section may contain a reflective, flexible diaphragm. The tube section is deployed within the bladder neck to transduce urinary sphincter pressure as a function of pressure exerted on the tube by the urinary sphincter. The optical fiber transmits light to the diaphragm, which reflects light back into the optical fiber. The diaphragm deflects under pressure exerted on the flexible tube by the urinary sphincter. As a result, optical properties of the light reflected by the diaphragm change, indicating a change in urinary sphincter pressure. 
     Inadequate sphincter pressure may result in involuntary bladder voiding, i.e., incontinence. The optical pressure sensor may provide short- or long-term monitoring of urinary sphincter pressure, e.g., for analysis by a clinician. Alternatively, the optical pressure sensor may form part of a closed-loop neurostimulation system. For example, neurostimulation therapy can be applied to increase sphincter pressure, and thereby prevent involuntary urine leakage. In particular, an implantable neurostimulator may be responsive to urinary sphincter pressure signals generated by the optical pressure sensor, as described herein, to provide closed loop neurostimulation therapy to alleviate incontinence. 
     In one embodiment, the invention provides an implantable electrical stimulation system comprising an implantable pressure sensor including an optical fiber, an emitter that transmits light via the optical fiber, a detector that detects reflected light via the optical fiber, circuitry that generates pressure information based on the detected light, and a fixation mechanism that positions the optical fiber proximate a sphincter within a patient, and an implantable stimulator that delivers electrical stimulation to the patient based on the pressure information. 
     In another embodiment, the invention provides a method comprising transmitting light via an optical fiber positioned proximate a sphincter within a patient, detecting reflected light via the optical fiber, and generating pressure information based on the detected light. 
     In an additional embodiment, the invention provides an implantable pressure sensor comprising an optical fiber, an emitter that transmits light via the optical fiber, a detector that detects reflected light via the optical fiber, circuitry that generates pressure information based on the detected light, and a fixation mechanism that positions the optical fiber proximate a sphincter within a patient. 
     In a further embodiment, the invention provides an implantable pressure sensor comprising a sensor housing, an optical fiber extending from the sensor housing, a flexible tube section coupled to the optical fiber, a reflective, flexible diaphragm within the flexible tube section, an emitter that transmits light via the optical fiber to the diaphragm, a detector that detects reflected light from the diaphragm the optical fiber, circuitry that generates pressure information based on the detected light, and a fixation mechanism that positions the optical fiber proximate a sphincter within a patient, wherein the diaphragm deflects in response to exertion of pressure against the flexible tube section by the sphincter. 
     Although the invention may be especially applicable to sensing urinary sphincter pressure, the invention alternatively may be applied more generally to other sphincters within the patient, such as the lower esophageal sphincter (LES) or pyloric sphincter. In addition, in those instances, the invention may be adapted to support electrical stimulation of other body organs, such as the stomach or intestines, e.g., for treatment of obesity or gastric mobility disorders. 
     In various embodiments, the invention may provide one or more advantages. For example, the use of a thin, flexible optical pressure sensor permits pressure to be sensed within the narrow, constricted passage proximate the urinary sphincter. In this manner, pressure can be sensed without significantly obstructing or altering the physiological function or the urinary sphincter. 
     The optical pressure sensor may be coupled to a larger sensor housing that resides within the bladder and houses sensor electronics for emitting and detecting light to measure the pressure on the tube. The optical pressure sensor permits pressure information to be obtained on a continuous or periodic basis as the patient goes about a daily routine. In addition, the flexible nature of the tube permits the sensor to be implanted in a variety of locations, and to be constructed in variety of shapes and sizes. 
     The optical pressure sensor may transmit sensed pressure information to an implantable stimulator to permit dynamic control of the therapy delivered by the stimulator on a closed-loop basis. For example, the stimulator may adjust stimulation parameters, such as amplitude, pulse width or pulse rate, in response to the sensed pressure. In this manner, the stimulator can provide enhanced efficacy and prevent involuntary leakage. In addition, or alternatively, adjustment may involve on and off cycling of the stimulation in response to pressure levels indicative of a particular bladder fill stage. For example, stimulation may be turned off until the pressure level exceeds a threshold indicative of a particular fill stage of the bladder. Also, with closed-loop stimulation, the stimulator may generate stimulation parameter adjustments that more effectively target the function of the urinary sphincter muscle, thereby enhancing stimulation efficacy. In some patients, more effective stimulation via the sacral nerve may actually serve to strengthen the sphincter muscle, restoring proper operation. 
     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 THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an implantable stimulation system, incorporating urinary sphincter pressure sensor, for alleviation of urinary incontinence. 
         FIG. 2  is an enlarged schematic diagram illustrating an implantable pressure sensor with an optical tube extending through the urinary sphincter of a patient. 
         FIG. 3  is an enlarged, cross-sectional side view of the implantable pressure sensor of  FIGS. 1 and 2 . 
         FIG. 4  is a schematic diagram illustrating placement of an implantable pressure sensor with an optical tube extending through the internal urinary sphincter of a patient. 
         FIG. 5  is functional block diagram illustrating various components of an exemplary implantable pressure sensor. 
         FIG. 6  is a functional block diagram illustrating various components of an implantable stimulator. 
         FIG. 7  is a schematic diagram illustrating cystoscopic deployment of an implantable pressure sensor via the urethra. 
         FIG. 8  is a schematic diagram illustrating retraction of a deployment device upon fixation of a pressure sensor within a patient&#39;s urinary tract. 
         FIG. 9  is a cross-sectional side view of a deployment device during deployment and fixation of a pressure sensor. 
         FIG. 10  is a cross-sectional bottom view of the deployment device of  FIG. 10  before attachment of the pressure sensor. 
         FIG. 11  is a flow diagram illustrating a technique for delivery of stimulation therapy based on closed loop feedback from an implantable pressure sensor. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic diagram illustrating an implantable stimulation system  10  for alleviation of urinary incontinence. As shown in  FIG. 1 , system  10  includes an implantable optical pressure sensor  12 , implantable stimulator  14  and external programmer  16  shown in conjunction with a patient  18 . Pressure sensor  12  senses a pressure level exerted by urinary sphincter  22  on urethra  20  proximate the neck  23  of bladder  24 , and transmits pressure information based on the sensed pressure level to at least one of stimulator  14  and programmer  16  by wireless telemetry. Stimulator  14  or programmer  16  may record the information, generate adjustments to electrical stimulation parameters applied by the stimulator, or both. 
       FIG. 2  is an enlarged schematic diagram illustrating implantable optical pressure sensor  12 . As shown in  FIGS. 1 and 2 , pressure sensor  12  includes a sensor housing  26 , an optical fiber  28 , and a flexible tube section  30 . Flexible tube section  30  is positioned for engagement with urinary sphincter  22 , and is sealed from the environment. Tube section  30  contains a reflective, flexible diaphragm that deflects in response to pressure changes within the tube section. Tube section  30  may be filled with air or other optically transmissive media. Optical fiber  28  transmits light to the diaphragm and receives reflected light from the diaphragm. When the diaphragm deflects, the properties of the reflected light change, indicating a change in pressure within the flexible tube and, in turn, a change in the pressure of urinary sphincter  22 . 
     Sensor housing  26  contains a light emitter that transmits light through optical fiber  28  and a light detector that detects the reflected light received from the optical fiber, as will be described in further detail. The light emitter and detector are positioned adjacent to a proximal end of optical fiber  28 . If a single optical fiber is used for both transmission of light and reception of reflected light, an optical coupling element may be provided in sensor housing  26  to couple the emitter and detector to the optical fiber  28 . In other embodiments, separate optical fibers can be used for transmission or reception. In either case, the light detector generates an output signal that varies according to the properties of the reflected light. Sensor housing  26  further includes electronics to generate pressure information based on the output signal, and telemetry circuitry for wireless transmission of the information to stimulator  14 , programmer  16  or both. 
     As further shown in  FIGS. 1 and 2 , sensor housing  26  may reside within bladder  24 . Sensor housing  26  may be temporarily or permanently attached to an inner wall  27  of bladder  24 , such has the mucosal lining, as will be described. Alternatively, housing  26  may be implanted sub-mucosally. Optical fiber  28  extends away from sensor housing  26  and through an inner lumen defined by the bladder neck proximate urinary sphincter  22 . In this manner, flexible tube section  30  is positioned to directly sense the pressure level exerted by urinary sphincter  22 . Yet, optical fiber  28  and tube section  30  may be sufficiently thin to avoid significant obstruction of urethra  20  or disruption of the function of urinary sphincter. 
     As a further alternative, housing  26  may reside outside bladder  24 , in which case optical fiber  28  and tube section  30  may extend into bladder  24  and through urinary sphincter  22  through a hole formed in the bladder. In this case, housing  26  may be surgically or laparoscopically implanted within the abdomen. Fiber  28  and tube section  30  may be surgically or laparoscopically guided through a hole in the wall of bladder  24 . A cystoscope may be used to grab tube section  30  and pull it downward through urinary sphincter  22  and urethra  20 . In some embodiments, housing  26  and its contents may be integrated with stimulator  14 , in which case optical fiber  28  and tube section  30  extends from the stimulator housing and into bladder  24 , much like leads carrying stimulation or sense electrodes 
     With further reference to  FIG. 1 , implantable stimulator  14  includes an electrical lead  15  (partially shown in  FIG. 1 ) carrying one or more electrodes that are placed at a nerve site within the pelvic floor. For example, the electrodes may be positioned to stimulate the sacral nerve and thereby innervate urinary sphincter  22 . In particular, electrical stimulation may be applied to cause urinary sphincter  22  to increase closing pressure to avoid involuntary leakage from bladder  24 . Alternatively, if voluntary voiding is desired by patient  18 , electrical stimulation may be suspended or reduced to reduce the closing pressure exerted by urinary sphincter  22  on urethra  20  at the bladder neck. 
     For spinal cord injury patients who cannot perceive a sensation of bladder fullness, sphincter pressure sensed by pressure sensor  12  may be transmitted to external programmer  16 , with or without an accompanying stimulator  14 , to advise the patient when urinary sphincter pressure is high, indicating bladder fullness. In this case, the advice may be in the form of a audible, visual or vibratory stimulus. In response to the advice, the spinal cord injury patient is able to catheterize the urethra  20  and bladder  24  to voluntarily relieve urine. 
     Implantable stimulator  14  delivers stimulation therapy to the sacral nerve in order to keep the sphincter  22  constricted and keep contents of bladder  24  from leaking out through urethra  20 . At predetermined times or at patient controlled instances, the external programmer  16  may program stimulator  14  to interrupt the stimulation to allow the sphincter to relax, thus permitting voiding of bladder  24 . Upon completion of the voiding event, external programmer  16  may program stimulator  14  to resume stimulation therapy and thereby maintain closure of urinary sphincter  22 . 
     In addition, adjustment of stimulation parameters may be responsive to pressure information transmitted by implantable optical pressure sensor  12 . For example, external programmer  16  or implantable stimulator  14  may adjust stimulation parameters, such as amplitude, pulse width, and pulse rate, based on pressure information received from implantable sensor  12 . In this manner, implantable stimulator  14  adjusts stimulation to either increase or reduce urinary sphincter pressure based on the actual pressure level exerted by urinary sphincter  22 . 
     Pressure sensor  12  may transmit pressure information periodically, e.g., every few seconds, minutes or hours. In some embodiments, pressure sensor  12  may transmit pressure information when there is an abrupt change in sphincter pressure, e.g., a pressure change that exceeds a predetermined threshold. In addition to parameter adjustments, or alternatively, adjustment may involve on and off cycling of the stimulation in response to pressure levels indicative of a particular bladder fill stage. For example, stimulation may be turned off until the pressure level exceeds a threshold indicative of a particular fill stage of the bladder, at which time stimulation is turned on. Then, stimulation parameters may be further adjusted as the sensed pressure level changes. 
     External programmer  16  may be a small, battery-powered, portable device that accompanies the patient  18  throughout a daily routine. Programmer  16  may have a simple user interface, such as a button or keypad, and a display or lights. Patient  18  may initiate a voiding event, i.e., a voluntary voiding of bladder  24 , via the user interface. In some embodiments, the length of time for a voiding event may be determined by pressing and holding down a button for the duration of a voiding event, pressing a button a first time to initiate voiding and a second time when voiding is complete, or by a predetermined length of time permitted by programmer  16  or implantable stimulator  14 . In each case, programmer  16  causes implantable stimulator  14  to temporarily terminate stimulation so that voluntary voiding is possible. 
     In some embodiments, stimulator  14  may immediately recommence stimulation upon completion of a voiding event, and thereafter adjust stimulation parameters based on pressure information generated by implantable sensor  12 . Alternatively, stimulator  14  may terminate stimulation upon initiation of a voiding event, and recommence stimulation only after implantable pressure sensor  12  measures a decrease of pressure in the urethra  20  that corresponds to bladder  24  being empty. As a further alternative, following completion of the voiding event, stimulator  14  may wait to recommence stimulation until pressure sensor  12  detects generation of an inadequate pressure level by urinary sphincter  22 , which could result in involuntary leakage. In this case, stimulator  14  recommences stimulation to enhance urinary sphincter pressure. 
     Implantable stimulator  14  may be constructed with a biocompatible housing, such as titanium or stainless steel, or a polymeric material such as silicone or polyurethane, and surgically implanted at a site in patient  18  near the pelvis. The implantation site may be a subcutaneous location in the side of the lower abdomen or the side of the lower back. One or more electrical stimulation leads  15  are connected to implantable stimulator  14  and surgically or percutaneously tunneled to place one or more electrodes carried by a distal end of the lead at a desired nerve site, such as a sacral nerve site within the sacrum. 
     In the example of  FIGS. 1 and 2 , sensor housing  26  of implantable pressure sensor  12  is attached to the inner wall  27  of bladder  24  near bladder neck  23 . However, the attachment site for sensor housing  26  could be anywhere with access to urinary sphincter  22 . With a relatively long optical fiber  28 , for example, sensor housing  26  could be positioned at a greater distance from bladder neck  23 . Also, in some embodiments, sensor housing  26  could be attached within urethra  20 , e.g., downstream from urinary sphincter  22 , although attachment of the sensor housing within bladder  24  may be desirable to avoid obstruction of the urethra. 
       FIG. 3  is an enlarged, cross-sectional side view of the implantable pressure sensor  12  of  FIGS. 1 and 2 . As shown in  FIG. 3 , sensor housing  26  receives the proximal end of flexible optical fiber  28 . A sensing element  34  is mounted within sensor housing  26  to sense a urinary sphincter pressure level via optical fiber  28 . Sensing element  34  may be coupled to a circuit board  38  within sensor housing  26 , and includes an optical emitter  35  and a detector  37 . Optical emitter  35  may be a light emitting diode (LED). Detector  37  may be a photodiode. In the example of  FIG. 3 , optical fiber  28  includes two optical fibers, i.e., a transmit fiber  39  coupled to emitter  35  and a receive fiber  41  coupled to optical detector  41 . Each optical fiber  39 ,  41  extends into flexible tube section  30 . 
     Sensor housing  26  may be made from a biocompatible material such as titanium, stainless steel or nitinol, or a polymeric material such as silicone or polyurethane. Another material for fabrication of sensor housing  26  is a two-part epoxy. An example of a suitable epoxy is a two-part medical implant epoxy manufactured by Epoxy Technology, Inc., mixed in a ratio of 10 grams of resin to one gram of activator. In general, sensor housing  26  contains no external openings, with the exception of the opening to receive optical fiber  28 , thereby protecting sensing element  26  and circuit board  38  from the environment within bladder  24 . The proximal end of optical fiber  28  resides within sensor housing  26  while the distal end resides outside of the sensor housing. The opening in sensor housing  26  that receives the proximal end of optical fiber  28  may be sealed to prevent exposure of interior components. 
     The core and cladding of optical fiber  28  may be formed from any of a variety of conventional glass or polymeric materials. In addition, single mode or multi-mode fibers may be selected. In some embodiments, a protective, a flexible sheath (not shown) may be formed over optical fiber  28 . The flexibility of optical fiber  28  permits it to bend and conform to contours within bladder neck  23 , facilitating placement of flexible tube section  30  within urethra  20  proximate urinary sphincter  22 . 
     Flexible tube section  30  may be formed from any of a variety of flexible, biocompatible materials such as polyurethane or silicone. The material should be sufficiently flexible to permit deform in response to pressure exerted on urethra  20  by urinary sphincter  22  at bladder neck  23 . Flexible tube section  30  preferably is sealed to define a compartment. so that deformation produces volumetric changes and pressure changes within the compartment. Accordingly, flexible tube section  30  may have a closed distal end and a sealed proximal end that is sealed about fiber  28 . The compartment may contain a gaseous medium such as air. During operation, urinary sphincter  22  exerts pressure inward against the outer wall of urethra  20 . In turn, the inner wall of urethra  20  exerts pressure inward against the outer wall of flexible tube section  30 , causing the wall of the tube section to deform and compress inward. In some embodiments, flexible tube section  30  may be coated to avoid calcification. 
     Inward deformation of flexible tube section  30  causes a mechanical deflection of the membrane mounted inside. As light is transmitted onto the membrane by optical fiber  39 , some of the reflected light received by optical fiber  41  is refracted to a varying degree based upon the deformation of the membrane. When the reflected light is detected by light detector  37 , the light detector generates an output signal that is influenced by the physical properties of the detected light. Circuitry within sensing element generates pressure information based on the reflected light detected by detector  37 . 
     The physical property may be simply an intensity of the received light, which is influenced by the degree of deflection of the membrane. In this case, an increase or decrease in the intensity of reflected light can be use to produce a urinary sphincter pressure level. Alternatively, physical property may be a wavelength of the reflected light, relative to a wavelength of the transmitted light. As the membrane deflects, changes in the wavelength of the reflected light can be used to produce a urinary sphincter pressure level. In other embodiments, the membrane may be formed with an interference pattern or grating that aids in wavelength differentiation between the reflected light and the transmitted light. Based upon the differences in amplitude, wavelength, or other optical properties, sensing element  34  generates a pressure signal that represents the pressure on flexible tube section  30 . Electronics on circuit board  38  generate pressure information based on the pressure signal. 
     Optical fiber  28  and flexible tube section  30  may be provided with different dimensions selected for patients having different anatomical dimensions. In particular, implantable pressure sensor  12  may be constructed with an optical fiber  28  and flexible tube section  30  having different lengths and diameters. Different tube lengths may be necessary given the distance between the attachment site of sensor housing  26  and urinary sphincter  22 , either to ensure that flexible tube section  30  reaches the sphincter or does not extend too far down urethra  20 . Multiple diameters may also be necessary to allow a dysfunctional sphincter  22  to close completely or to allow optical fiber  28  and flexible tube section  30  to be placed into a narrow urethra  20 . The dimensions may be fixed for a given pressure sensor  12 , as a complete assembly. Alternatively, fluid tubes of different sizes may be attached to a pressure sensor housing  26  by a physician prior to implantation. 
     In general, for male patients, optical fiber  28  and tube section  30  may have a combined length of less than approximately 9 cm and more preferably less than approximately 7 cm. For female patients, optical fiber  28  and tube section  30  may have a combined length f less than approximately 7 cm and more preferably less than approximately 5 cm. In some embodiments, optical fiber  28  and tube section  30  may have a combined length of approximately 0.5 cm to 3 cm. The length of optical fiber  28  and tube section  30  may vary according to the anatomy of the patient, and may vary between male, female and pediatric patients. In addition, tube  30  may have an outer diameter in a range of approximately 1 to 3 mm. The wall of tube  30  may be relatively thin to ensure sufficient deformation and conformability, yet thick enough to ensure structural integrity. As an example, the thickness of the wall of tube  30  may be in a range of approximately 0.1 mm to 0.3 mm. 
     Attaching implantable pressure sensor  12  to the mucosal lining of bladder  24  may be accomplished in a variety of ways, but preferably is completed in a manner that will not excessively injure bladder  24 . Preferably, attachment should cause limited inflammation no adverse physiological modification, such as tissue infection or a loss in structural integrity of bladder  24 . However, it is desirable that implantable pressure sensor  12  also be attached securely to the attachment site in order to provide an extended period of measurement without prematurely loosening or detaching from the intended location. 
     As an example, sensor housing  26  may contain a vacuum cavity  39  that permits a vacuum to be drawn by a vacuum channel  40 . The vacuum is created by a deployment device having a vacuum line in communication with vacuum channel  40 . The vacuum draws a portion  42  of the mucosal lining  44  of bladder  24  into vacuum cavity  39 . Once the portion  42  of mucosal lining  44  is captured within vacuum cavity  39 , a fastening pin  46  is driven into the captured tissue to attach sensor housing  26  within bladder  24 . Fastening pin  46  may be made from, for example, stainless steel, titanium, nitinol, or a high density polymer. The shaft of pin  46  may be smooth or rough, and the tip may be a sharp point to allow for easy penetration into tissue. Fastening pin  46  may be driven into housing  26  and the portion  42  of mucosal lining  44  under pressure, or upon actuation by a push rod, administered by a deployment device. 
     In some embodiments, fastening pin  46  may be manufactured from a degradable material that the breaks down over time, e.g. in the presence of urine, to release implantable pressure sensor  12  within a desired time period after attachment. In still another embodiment, implantable pressure sensor  12  may be attached without the use of a penetrating rod but with a spring-loaded clip to pinch trapped mucosal lining  44  within cavity  39 . A variety of other attachment mechanisms, such as pins, clips, barbs, sutures, helical screws, surgical adhesives, and the like may be used to attach sensor housing  26  to mucosal lining  44  of bladder  24 . 
       FIG. 4  is a schematic diagram illustrating placement of an implantable pressure sensor  12  with a flexible optical fiber  28  extending through the urinary sphincter  22  of a patient  18 .  FIG. 4  also illustrates flexible tube section  30  in greater detail. In the example of  FIG. 4 , optical fiber  28 , including transmit fiber  39  and receive fiber  41 , leaves bladder  24  through bladder neck  23  and passes through internal urinary sphincter  22  as it enters urethra  20 . In general, sphincter  22  is an annulus shaped muscle that surrounds the portion of urethra  20  below bladder neck  23  and constricts to make the urethral walls meet and thereby close urethra  20  to prevent involuntary urine leakage from bladder  24 . Upon constriction of sphincter  22 , the walls of urethra  20  close onto flexible tube section  30  of optical fiber  28  to increase the internal pressure of the tube section, which provides a measurement of the closing pressure of sphincter  22 . 
     As further shown in  FIG. 4 , flexible diaphragm  43  is mounted within flexible tube section  30  below optical fibers  39 ,  41 . Flexible diaphragm  43  includes an optically reflective surface on a side facing optical fibers  39 ,  41 . In this manner, light transmitted via optical fiber  39  is reflected by diaphragm  43  and received via optical fiber  41 . Flexible diaphragm may be substantially circular and bonded at its edges to an inner wall of flexible tube section  30 . For example, flexible diaphragm may be bonded to the inner wall of flexible tube section  30  by adhesives, ultrasonic welding, or other techniques. In some embodiments, tube section  30  may include an annular mounting ledge or other equivalent mounting structures to support at least an outer edge of the diaphragm  43 . Flexible diaphragm  43  may be formed from any of a variety of flexible materials. The materials may be reflective. Alternatively, a reflective coating may be formed on diaphragm  43 , e.g., by vapor deposition, sputtering, dip coating, roll coating or the like. 
     Because optical fiber  28  and flexible tube section  30  have circular cross-sections and a small diameter, a closed sphincter  22  will still be able to substantially seal urethra  20  around optical fiber  28 , flexible tube section  30 , or both. When sphincter  22  is relaxed, in some embodiments, implantable pressure sensor  12  may be used to measure the pressure of fluid in urethra  20 . The open sphincter  22  allows urine to be passed out of the urethra and patient  18 . Optical fiber  28  is under the same pressure as the urethra and can allow implantable pressure sensor  12  to measure this urethral pressure. This may allow monitoring of urinary dysfunctions due to pressure during voiding events and may also be used by implantable stimulator  14  to detect the end of a voiding event by measuring decrease of urethral pressure as an indication of reduced urine flow. 
     As shown in  FIG. 4 , the placement of optical fiber  28  and flexible tube section  30  does not significantly interfere with normal bladder function. Bladder function is unimpaired and fluid flow to urethra  20  can occur normally, as flexible tube section  30  allows enough room for urine to pass and exit bladder  24  via urethra  20 . Due to varying sizes and shapes of patient anatomy, optical fiber  28  and flexible tube section  30  may be manufactured in a variety of lengths and diameters. 
       FIG. 5  is functional block diagram illustrating various components of an exemplary implantable pressure sensor  12 . In the example of  FIG. 5 , implantable pressure sensor  12  includes a sensing element  34 , processor  48 , memory  50 , telemetry interface  52 , and power source  54 . Sensing element  34  transforms measured changes in emitted light from optical fiber  28  into electrical signals representative of closing pressure of urinary sphincter  22 . Again, optical fiber  28  may include a transmit fiber  39  and a receive fiber  41 , or a single fiber with an optical coupler for optical coupling to emitter  35  and detector  37 . The electrical signals may be amplified, filtered, and otherwise processed as appropriate by electronics within sensor  12 . In particular, sensor  12  may include circuitry to detect changes in light intensity or wavelength. In some embodiments, the signals may be converted to digital values and processed by processor  48  before being saved to memory  50  or sent to implantable stimulator  14  as pressure information via telemetry interface  52 . 
     Memory  50  stores instructions for execution by processor  48  and pressure information generated by sensing element  36 . Pressure data may then be sent to implantable stimulator  14  or external programmer  16  for long-term storage and retrieval by a user. Memory  50  may include separate memories for storing instructions and pressure information. In addition, processor  48  and memory  50  may implement loop recorder functionality in which processor  48  overwrites the oldest contents within the memory with new data as storage limits are met, thereby conserving memory space. 
     Processor  48  controls telemetry interface  52  to send pressure information to implantable stimulator  14  or programmer  16  on a continuous basis, at periodic intervals, or upon request from the implantable stimulator or programmer. Wireless telemetry may be accomplished by radio frequency (RF) communication or proximal inductive interaction of pressure sensor  12  with programmer  16 . 
     Power source  54  delivers operating power to the components of implantable pressure sensor  12 . Power source  54  may include a battery and a power generation circuit to produce the operating power. In some embodiments, the battery may be rechargeable to allow extended operation Recharging may be accomplished through proximal inductive interaction between an external charger and an inductive charging coil within sensor  12 . In some embodiments, power requirements may be small enough to allow sensor  12  to utilize patient motion and implement a kinetic energy-scavenging device to trickle charge a rechargeable battery. In other embodiments, traditional batteries may be used for a limited period of time. As a further alternative, an external inductive power supply could transcutaneously power sensor  12  whenever pressure measurements are needed or desired. 
       FIG. 6  is a functional block diagram illustrating various components of an implantable stimulator  14 . In the example of  FIG. 6 , stimulator  14  includes a processor  56 , memory  58 , stimulation pulse generator  60 , telemetry interface  62 , and power source  64 . Memory  58  stores instructions for execution by processor  56 , stimulation therapy data, and pressure information received from pressure sensor  12  via telemetry interface. Pressure information is received from pressure sensor  12  and may be recorded for long-term storage and retrieval by a user, or adjustment of stimulation parameters, such as amplitude, pulse width or pulse rate. Memory  58  may include separate memories for storing instructions, stimulation parameter sets, and pressure information. Processor  56  controls stimulation pulse generator  60  to deliver electrical stimulation therapy and telemetry interface  62  to send and receive information. 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: between approximately 0.5 Hz and 500 Hz, more preferably between approximately 5 Hz and 250 Hz, and still more preferably between approximately 10 Hz and 50 Hz. 
     2. Amplitude: between approximately 0.1 volts and 50 volts, more preferably between approximately 0.5 volts and 20 volts, and still more preferably between approximately 1 volt and 10 volts. 
     3. Pulse Width: between about 10 microseconds and 5000 microseconds, more preferably between approximately 100 microseconds and 1000 microseconds, and still more preferably between approximately 180 microseconds and 450 microseconds. 
     Based on pressure information received from sensor  12 , processor  56  interprets the information and determines whether any therapy parameter adjustments should be made. For example, processor  56  may compare the pressure level to one or more thresholds, and then take action to adjust stimulation parameters based on the pressure level. Information may be received from sensor  12  on a continuous basis, at periodic intervals, or upon request from stimulator  14  or external programmer  16 . Alternatively, or additionally, pressure sensor  12  may transmit pressure information when there is an abrupt change in the pressure level, e.g., at the onset of involuntary leakage. 
     In addition, processor  56  modifies parameter values stored in memory  58  in response to pressure information from sensor  12 , either independently or in response to programming changes from external programmer  16 . Stimulation pulse generator  60  provides electrical stimulation according to the stored parameter values via a lead  15  implanted proximate to a nerve, such as a sacral nerve. Processor  56  determines any parameter adjustments based on the pressure information obtained form sensor  12 , and loads the adjustments into memory  58  for use in delivery of stimulation. 
     As an example, if the pressure information indicates an inadequate sphincter closing pressure, processor  56  may increase the amplitude, pulse width or pulse rate of the electrical stimulation applied by stimulation pulse generator  60  to increase stimulation intensity, and thereby increase sphincter closing pressure. If sphincter closing pressure is adequate, processor  56  may implement a cycle of downward adjustments in stimulation intensity until sphincter closing pressure becomes inadequate, and then incrementally increase the stimulation upward until closing pressure is again adequate. In this way, processor  56  converges toward an optimum level of stimulation. Although processor  56  is described in this example as adjusting stimulation parameters, it is noted that the adjustments may be generated by external programmer  16 . 
     The adequacy of closing pressure is determined by reference to the pressure information obtained from sensor  12 . Sphincter pressure may change due to a variety of factors, such as an activity type, activity level or posture of the patient  18 . Hence, for a given set of stimulation parameters, the efficacy of stimulation may vary in terms of sphincter pressure, due to changes in the physiological condition of the patient. For this reason, the continuous or periodic availability of pressure information from implantable sensor  12  is highly desirable. 
     With this pressure information, stimulator  14  is able to respond to changes in sphincter pressure with dynamic adjustments in the stimulation parameters delivered to the patient  18 . In particular, processor  56  is able to adjustment parameters in order to cause constriction of sphincter  22  and thereby avoid involuntary leakage. In some cases, the adjustment may be nearly instantaneous, yet prevent leakage. As an example, if patient  18  laughs, coughs, or bends over, the resulted force on bladder  24  could overcome the closing pressure of urinary sphincter  22 . If pressure sensor  12  indicates an abrupt change in sphincter pressure, however, stimulator  14  can quickly respond by more vigorously stimulating the sacral nerves to increase sphincter closing pressure. 
     In general, if sphincter  22  is not constricting enough to effectively close urethra  20 , processor  56  may dynamically increase the level of therapy to be delivered. Conversely, if sphincter  22  is consistently achieving effective constriction, processor  56  may incrementally reduce stimulation, e.g., to conserve power resources. 
     As in the case of sensor  12 , wireless telemetry in stimulator  14  may be accomplished by radio frequency (RF) communication or proximal inductive interaction of pressure stimulator  14  with implantable pressure sensor  12  or external programmer  16 . Accordingly, telemetry interface  62  may be similar to telemetry interface  52 . Also, power source  64  of stimulator  14  may be constructed somewhat similarly to power source  54 . For example, power source  64  may be a rechargeable or non-rechargeable battery, or alternatively take the form of a transcutaneous inductive power interface. 
       FIG. 7  is a schematic diagram illustrating cystoscopic deployment of an implantable pressure sensor  12  via the urethra  20  using a deployment device  66 . Pressure sensor  12  may be surgically implanted. However, cystoscopic implantation via urethra is generally more desirable in terms of patient trauma, recovery time, and infection risk. In the example of  FIG. 7 , deployment device  66  includes a distal head  68 , a delivery sheath  69  and a control handle  70 . Deployment device  66  may be manufactured from disposable materials for single use applications or more durable materials for multiple applications capable of withstanding sterilization between patients. 
     As shown in  FIG. 7 , distal head  68  includes a cavity that retains sensor housing  26  of implantable pressure sensor  12  for delivery to a desired attachment site within bladder  24 . Sensor housing  26  may be held within cavity  72  by a friction fit, vacuum pressure, or a mechanical attachment. In each case, once distal head  68  reaches the attachment site, sensor housing  26  may be detached. Sheath  69  is attached to distal head  68  and is steerable to navigate urethra  20  and guide the distal head into position. In some embodiments, sheath  69  and distal head  68  may include cystoscopic viewing components to permit visualization of the attachment site. In other cases, external visualization techniques such as ultrasound may be used. Sheath  68  may include one or more steering mechanisms, such as wires, shape memory components, or the like, to permit the distal region adjacent distal head  68  to turn abruptly for access to the mucosal lining of bladder  24 . 
     A control handle  70  is attached to sheath  69  to aid the physician in manually maneuvering deployment device  66  throughout urethra  20  and bladder  24 . Control handle  70  may have a one or more controls that enable the physician to contort sheath  69  and allow for deployment device  66  to attach pressure sensor housing  26  to the mucosal lining of bladder  24  and then release the sensor housing to complete implantation. A vacuum source  74  supplies negative pressure to a vacuum line within sheath  69  to draw tissue into the vacuum cavity defined by sensor housing  66 . A positive pressure source  76  supplies positive pressure to a drive a fastening pin into the tissue captured in the vacuum cavity. 
     Deployment device  66  enters patient urethra  20  to deliver pressure sensor  12  and implant it within bladder  24 . First, the physician must guide distal head  68  through the opening of urethra  20  in patient  18 . Second, distal head  68  continues to glide up urethra  20  and past the relaxed internal sphincter  22 . Distal head  300  is then pushed through bladder neck  23  and into bladder  24 , for access to an appropriate site to attach pressure sensor  12 . Using actuators built into control handle  70 , sheath  69  is bent to angle distal head  68  into position. Again, sheath  69  may be steered using control wires, shape memory alloys or the like. 
     As pressure sensor  12  is guided into place against the mucosal wall  44  of bladder  24 , a physician actuates control handle  70  to attach sensor  12  to mucosal wall  44  and then release the attached sensor. Upon attachment, pressure sensor  12  is implanted within bladder  24  of patient  18  and deployment device  66  is free to exit the bladder. Exemplary methods for attachment and release of sensor  12 , including the use of both vacuum pressure and positive pressure, will be described in greater detail below. Although  FIG. 7  depicts cystoscopic deployment of pressure sensor  12 , surgical or laparoscopic implantation techniques alternatively may be used. 
       FIG. 8  is a schematic diagram illustrating retraction of deployment device  66  upon fixation of pressure sensor  12  within the urinary tract of patient  18 . Once the sensor  12  is released, optical fiber  28  remains attached to sensor housing  26 . During removal of deployment device  66 , optical fiber  28  and flexible tube section  30  maintain position within bladder neck  23  adjacent sphincter  22 . As deployment device  66  is removed, optical fiber  28  and flexible tube section  30  pass through a guide channel formed in the deployment device. The guide channel ensures that optical fiber  28  and flexible tube section  30  remain pinned between distal head  68  and the wall of bladder  24 . 
     As distal head  68  slides through sphincter  22  and urethra  20 , however, optical fiber  28  releases from deployment device  66  and is left in place within the urethra in the region proximate urinary sphincter  22 . Deployment device  66  may then be completely withdrawn past the external urinary sphincter and out of the remainder of urethra  20 . In the example of  FIG. 8 , optical fiber  28  is suspended by device housing  26 , which is attached to mucosal wall  44 , and is held in place by pressure exerted against the urethral wall by urinary sphincter  22 . In other embodiments, optical fiber  28  and flexible tube section  30  may be kept in place using other techniques such as actively fixing optical fiber  28  or tube section  30  to the side of urethra  20 , e.g., with sutures or other anchor mechanisms. 
     In a preferred embodiment, sheath  69  and distal head  68  may be disposable. Disposable devices that come into contact with patient  18  tissues and fluids greatly decrease the possibility of infection in implantable devices. Control handle  70  does not come into contact with body fluids of patient  18  and may be used for multiple patients. In another embodiment, the entire deployment device  66  may be manufactured out of robust materials intended for multiple uses. The device would then need to be sterilizable between uses. In still a further embodiment, the features of distal head  68  may be incorporated into pressure sensor  12 . In this configuration, pressure sensor  12  may be larger in size but would include the necessary elements for attachment within the device. After attachment, the entire sensor would detach from sheath  69 , making removal of deployment device  66  easier on patient  18 . 
     After the useful life of implantable pressure sensor  12  is complete or it is no longer needed within patient  18 , it can be removed from patient  18  in some manner. As an example, deployment device  66  may be reinserted into patient  18 , navigated into bladder  24 , and reattached to pressure sensor  12 . Deployment device  66  may then be withdrawn from the bladder  24  and urethra  20 , explanting sensor  12 , including housing  26  and optical fiber  28 , from patient  18 . In another embodiment, as mentioned with respect to  FIG. 3 , the attachment method of pressure sensor  12  to bladder  24  may involve degradable materials, such as a biodegradable fixation pin. After a certain period of time exposed to urine in the bladder  24 , the fixation material may structurally degrade and allow pressure sensor  12  to be released from the mucosal wall  44  of bladder  24 . In some embodiments, sensor  12  may be sized sufficiently small to follow urine out of the bladder, urethra, and body during a voiding event. In other embodiments, sensor housing  26  or tube section  30  may carry a suture-like loop that can be hooked by a catheter with a hooking element to withdraw the entire assembly from patient  18  via urethra  20 . In still further embodiments, such a loop may be long enough to extend out of the urethra, so that the loop can be grabbed with an external device or the human hand to pull the sensor  12  out of the patient. 
       FIG. 9  is a cross-sectional side view of distal head  68  of deployment device  66  during deployment and fixation of pressure sensor  12 . In the example of  FIG. 9 , distal head  68  a vacuum line  78  and a positive pressure line  80 . Vacuum line  78  is coupled to vacuum source  74  via a tube or lumen extending along the length of sheath  69 . Similarly, positive pressure line  80  is coupled to positive pressure source  76  via a tube or lumen extending along the length of sheath  69 . Vacuum line  78  is in fluid communication with vacuum cavity  39 , and permits the physician to draw a vacuum and thereby capture a portion  42  of mucosal lining  44  within the vacuum cavity. Positive pressure line  80  permits the physician to apply a pulse of high pressure fluid, such as a liquid or a gas, to drive fixation pin  46  into sensor housing  26  and through the portion  42  of mucosal lining  44 . Pin  46  thereby fixes sensor housing  26  to mucosal lining  44 . In some embodiments, a membrane mounted over an opening of positive pressure line  80  may be punctured by pin  46 . 
     Optical fiber  28  resides within a channel of sheath  69  prior to detachment or sensor  12  from distal head  68 . Once fixation pin  46  attaches sensor  12  to bladder  24 , vacuum line  78  is no longer needed. However, in some embodiments, vacuum line  78  may be used to detach pressure sensor  12  from distal head  68  of deployment device  66 . By terminating vacuum pressure, or briefly applying positive pressure through vacuum line  78 , for example, head  68  may separate from sensor  12  due to the force of the air pressure. In this manner, vacuum line  78  may aid in detachment of sensor  12  prior to withdrawal of deployment device  66 . 
     As described previously in  FIG. 3 , fixation pin  46  punctures mucosal lining  44  for fixation of sensor  12 . While the force of this fixation may vary with patient  18 , deployment device  66  provides adequate force for delivery of pin  46 . In an exemplary embodiment, positive pressure line  80  is completely sealed and filled with a biocompatible fluid, such as water, saline solution or air. Sealing the end of positive pressure line  80  is a head  82  on fixation pin  46 . Head  82  is generally able to move within positive pressure line  80  much like a piston. Force to push fixation pin  46  through the portion  42  of mucosal lining  44  captured in vacuum cavity  39  is created by application of a pulse of increased fluid pressure within positive pressure line  80 . For example, the physician may control positive pressure source  76  via control handle  70 . This simple delivery method may provide high levels of force, allow multiple curves and bends in articulating arm  306 , and enable a positive pressure line  80  of many shapes and sizes. 
     In an alternative embodiment, a flexible, but generally incompressible, wire may placed within positive pressure line  80  and used to force fixation pin  46  through the captured portion  42  of mucosal lining  44 . This wire presents compressive force from control handle  70  directly to the head  82  of fixation pin  46 . This method may eliminate any safety risk of pressurized fluids entering patient  18  or, in some embodiments, permit retraction of pin  46  after an unsuccessful fixation attempt. The flexible wire may be attached to pin  46  and pulled back to remove the pin from capture mucosal tissue  42 . The flexible wire may be sheared from fixation pin  46  for detachment purposes as distal head  68  releases sensor  12 . This detachment may be facilitated by a shearing element or simply low shear stress of the wire enables separation when distal head  68  slides past pin  46 . 
     In  FIG. 9 , deployment device  66  illustrates optical fiber  28  on the same end of housing  26  as sheath  69 , while the fixation structures are located in the opposite, or distal end of distal head  68 . In some embodiments, it may be necessary for pressure sensor  12  to be deployed with tube section  30  located at the distal end of head  68  and the fixation structures located near sheath  69 . In still other embodiments, the fixation structures and tube section  30  may be located on the same end of pressure sensor  12 . 
     In some embodiments, deployment device  66  may include a small endoscopic camera in the distal head  68 . The camera may enable the physician to better guide deployment device  66  through urethra  20 , past sphincter  22 , and to a desired attachment location of bladder  24  in less time with more accuracy. Images may be displayed using video fed to a display monitor. 
       FIG. 10  is a cross-sectional bottom view of the deployment device  66  of  FIG. 10  before attachment of pressure sensor  12 . As shown in  FIG. 10 , distal head  68  includes proximal tube channel  84  to accommodate optical fiber  28  during placement of sensor  12  and distal tube channel  86  to accommodate the flexible tube during retraction of deployment device  66 . In addition, sheath  69  includes a sheath channel  88  to accommodate optical fiber  28  and flexible tube section  30 . Channels  84 ,  86 ,  88  serve to retain tube section  30  during delivery of sensor  12  to an attachment site. Note that the channels are larger than the shown portion of optical fiber  28  to enable the passage of the larger perturbation section  30  of optical fiber  28 . In some embodiments, tube section  30  may be of similar diameter to optical fiber  28 . 
     Distal head  68  is rounded on both sides at the distal end to permit easier entry of deployment device into areas of patient  18 . Head  68  may also be lubricated before delivery to facilitate ease of navigation. On the proximal end of head  68 , proximal tube channel  84  runs through the head for unimpeded removal of optical fiber  28  and tube section  30  during detachment of pressure sensor  12 . This channel may be U-shaped, e.g. closed on  3  sides. In some embodiments, proximal tube channel  84  may be an enclosed hole in which optical fiber  28  resides and glides through upon deployment device  30  removal. 
     Sheath channel  88  is formed within sheath  69  to allow optical fiber  28  to stay in place during delivery of pressure sensor  12 . In this embodiment, optical fiber  28  is only partially retained within channel  88 . In some embodiments, sheath channel  88  may be deeper to allow optical fiber  28  to lie completely within sheath  69 , whereas others may include a completely enclosed channel out of which optical fiber  28  glides after attachment. 
     Distal channel  86  in distal end of head housing  68  is not used by optical fiber  28  before attachment. The purpose of this open channel is to allow optical fiber  28  and flexible tube section  30  to glide through it while head  68  is removed from bladder  24 . As head  68  slides back past pressure sensor  12 , optical fiber  28  and tube section  30  will slide through channel  86  and head housing  68  will keep optical fiber  28  and tube section  30  between the wall of bladder  24  and head  68  until head  68  has been removed beyond sphincter  22 . Optical fiber  28  and tube section  30  may then be ensured correct placing through sphincter  22 . 
     Some embodiments of optical fiber  28  and flexible tube section  30  include multiple length and diameter combinations which would lead to modifications in channels  84 ,  86  and  88 . These channels may be of different diameters or lengths to properly house optical fiber  28 , tube section  30 , or both. One embodiment may include flexible housing channels to accommodate a wide variety of dimensions. Further embodiments of deployment device  30  may contain modified channel locations in head housing  68 . These locations may be needed to place optical fiber  28  and flexible tube section  30  in different locations, particularly at different sphincter sites as in some embodiments. 
       FIG. 11  is a flow diagram illustrating a technique for delivery of stimulation therapy based on closed loop feedback from an implantable pressure sensor. In the example of  FIG. 11 , implantable stimulator  14  requires information from implantable pressure sensor  12  and external programmer  16 . The flow of events begins with implantable stimulator  14  communicating with implantable pressure sensor  12  and sending a command to sense the pressure of sphincter  22  ( 90 ). The pressure sensor  12  subsequently acquires a pressure measurement and delivers the data to implantable stimulator  14  ( 92 ). Upon receiving the pressure data, implantable stimulator  14  calibrates the data and compares it to a determined minimum pressure threshold ( 94 ). 
     If the measured pressure is higher than the threshold, the loop begins again. If the pressure is lower than the threshold, the flow continues to the next step of stimulation. Implantable stimulator  14  communicates with external programmer  16  to check if patient  18  has desired to void the contents of bladder  24  ( 96 ). If patient  18  has signaled a voiding event, stimulation is skipped and the process begins again. In the case of no voiding event desired, sphincter  22  is not providing adequate closing pressure and needs to be stimulated, or more vigorously stimulated. Implantable stimulator  14  next performs the necessary tasks to adjust a level of stimulation for stimulation pulse generator  60  ( 98 ). Stimulator  14  concludes the loop by delivering electric stimulation thereby to a nerve that innervates sphincter  22  ( 100 ). After stimulation therapy has commenced, the loop begins again to continue appropriate therapy to patient  18 . 
     In some embodiments, pressure sensor  12  may be used exclusively for monitoring pressure without providing feedback for stimulation therapy. In this case, the logic loop would be much simpler and only include collecting data and sending it to an external programmer ( 90  and  92 ). Pressure may be measured continuously, intermittently or at the request of external programmer  16 . These embodiments may be used for disease diagnosis or condition monitoring and may provide a patient to avoid frequent clinic visits and uncomfortable procedures. In some embodiments, the pressure measurements may form part of an automated voiding diary that records voluntary voiding events, involuntary voiding events, and urinary sphincter and urethral pressure levels prior to, contemporaneous with, of after such an event. 
     Although the invention may be especially applicable to sensing urinary sphincter pressure, the invention alternatively may be applied more generally to other sphincters within the patient, such as the lower esophageal sphincter (LES) or pyloric sphincter. In addition, in those instances, the invention may be adapted to support electrical stimulation of other body organs, such as the stomach or intestines, e.g., for treatment of obesity or gastric mobility disorders. Not only may stimulation of certain nerves allow for the proper closure of a sphincter, but nerve stimulation may be able to modify stomach contractions or intestinal contractions based upon pressure measurements at those sites. Pressure feedback from the implantable pressure sensor may be the most effective therapy for some patients, e.g., in the form of biofeedback that aids the patient in self-regulating bladder control. Also, the invention need not be limited to neurostimulation, and may be applied to stimulate other tissue, including muscle tissue. 
     Various embodiments of the described invention may include processors that are realized by microprocessors, Application-Specific Integrated Circuits (ASIC), Field-Programmable Gate Arrays (FPGA), or other equivalent integrated or discrete logic circuitry. The processor may also utilize several different types of data storage media to store computer-readable instructions for device operation. These memory and storage media types may include any form of computer-readable media such as magnetic or optical tape or disks, solid state volatile or non-volatile memory, including random access memory (RAM), read only memory (ROM), electronically programmable memory (EPROM or EEPROM), or flash memory. Each storage option may be chosen depending on the embodiment of the invention. While the implantable stimulator and implantable pressure sensor ordinarily will contain permanent memory, a patient or clinician programmer may contain a more portable removable memory type to enable easy data transfer for offline data analysis. 
     Many embodiments of the invention have been described. Various modifications may be made without departing from the scope of the claims. For example, although the invention has been generally described in conjunction with implantable neurostimulation devices, a flexible tube sensor may also be used with other implantable medical devices, such as electrical muscle stimulation devices, functional electrical stimulation (FES) devices, and implantable drug delivery devices, each of which may be configured to treat incontinence or other conditions or disorders. These and other embodiments are within the scope of the following claims.