Abstract:
A method for determining a change in position of a neurostimulation lead relative to a stimulation target tissue of a patient is provided. The method comprises implanting a first proximity sensor on a surface of the patient, implanting a second proximity sensor on the surface of the patient, measuring a change in a distance between the first and second proximity sensors, and inferring the change in position of the lead relative to the stimulation target tissue from the measured change in distance. The method further comprises inferring an increase in a distance between the lead and the stimulation target tissue when the distance between the first and second proximity sensors increases. The method also comprises conveying electrical stimulation energy to therapeutically stimulate the stimulation target tissue, and modulating a stimulation parameter in response to the measured change in distance.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to tissue stimulation systems, and more particularly, to a system and method for measuring lead shifts in a patient implanted with a tissue stimulation system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients. Furthermore, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Specifically, Occipital Nerve Stimulation (ONS), in which leads are implanted in the tissue over the occipital nerves, has shown promise as a treatment for various headaches, including migraine headaches, cluster headaches, and cervicogenic headaches. 
         [0003]    Each of these implantable neurostimulation systems typically includes an electrode lead, having one or more electrodes, implanted at the desired stimulation site and an implantable pulse generator (IPG) implanted remotely from the stimulation site, but coupled either directly to the electrode lead or indirectly to the electrode lead via a lead extension. Thus, electrical pulses can be delivered from the IPG to the electrode lead to stimulate the tissue and provide the desired efficacious therapy to the patient. 
         [0004]    Significantly, precise positioning of the leads proximal to the targets of stimulation is critical to the success of the therapy. If the leads shift position, the stimulation target tissue may no longer be appropriately stimulated. For example, when electrical stimulation devices, such as occipital nerve stimulators, are implanted in a patient, the leads and the IPG are anchored to the tissue. However, during postural changes and patient movement, the leads (and/or other system components) may shift. Notably, in lead shifting, as opposed to lead migration, the leads return to their previous position after the patient returns to a neutral/resting position. 
         [0005]    During lead shifting, the patient feels a change in stimulation sensations and possibly no stimulation at all. Changes in stimulation sensation may be drastic, causing jolting or a piercing sensation. When lead shifting eliminates paresthesia, i.e. the tingling sensation that replaces pain during successful treatment, the patient may revert back to their pain state, or, in the case of ONS, generate a migraine headache. 
         [0006]    Changes in stimulation may be explained by the lead moving back and forth over the targeted nerve, while removal of stimulation therapy may be explained by the lead shifting off of the nerve. The entire lead may not shift off of the nerve. Instead, one stimulating electrode may shift off of the nerve or shift too far away from the nerve for effective stimulation. For instance, when an ONS patient&#39;s head is rotated, implanted stimulating electrodes may no longer be properly positioned over the targeted occipital nerves. 
         [0007]    Changes in stimulation with posture change can also be caused by changes in the thickness of the tissue between the lead and the targeted nerve. The thickness of this intervening tissue may decrease when postural changes, such as neck movements, stretch the tissue or increase when postural changes bunch up the tissue. This change in thickness of the intervening tissue, in turn, moves the lead closer to or farther from the targeted nerve, resulting in changes in stimulation. 
         [0008]    Lead shifting can be overcome by reprogramming the tissue stimulation system based on the new position of the leads to restore therapy. However, determining the presence and degree of lead shifting based paresthesia is imprecise. Also, attempting to reprogram the leads based on paresthesia locations is challenging. 
         [0009]    Alternatively, a determination of the position of implanted leads can be made using X-ray or fluoroscopy. Disadvantageously, X-ray and fluoroscopy require expensive equipment, significant time, and appropriate medical facilities, most of which are not readily available. Moreover, if the leads shift after a fluoroscopic image is taken, this image may no longer be valid, thereby resulting in poor patient outcomes due to inappropriate or unexpected stimulation effects. 
         [0010]    There, thus, remains a need for an improved method and system for compensating for lead shifting during patient movement. 
       SUMMARY OF THE INVENTION 
       [0011]    In accordance with a first aspect of the present inventions, a method for determining a change in position of a neurostimulation lead relative to a stimulation target tissue of a patient is provided. The method comprises implanting a first proximity sensor on a surface of the patient, implanting a second proximity sensor on the surface of the patient, measuring a change in a distance between the first and second proximity sensors, and inferring the change in position of the lead relative to the stimulation target tissue from the measured change in distance. The method further comprises inferring an increase in a distance between the lead and the stimulation target tissue when the distance between the first and second proximity sensors increases. The method also comprises conveying electrical stimulation energy to therapeutically stimulate the stimulation target tissue, and modulating a stimulation parameter in response to the measured change in distance. 
         [0012]    The method further comprises automatically modulating the stimulation parameter in response to the inferred change in position. In one optional method, modulating a stimulation parameter in response to the inferred change in position comprises increasing an amplitude of a stimulation current when the distance between the first and second proximity sensors increases. In another optional method, modulating a stimulation parameter comprises adjusting an amplitude of the stimulus applied to selected electrodes during the conveyance of the stimulation energy. The method also comprises conveying electrical stimulation energy via a combination of electrodes to therapeutically stimulate the stimulation target tissue via electrodes. In still another optional method, the step of modulating a stimulation parameter comprises changing the combination of the electrodes. 
         [0013]    In accordance with a second aspect of the present inventions, a system for determining a change in position of a neurostimulation lead relative to a stimulation target tissue of a patient is provided. The system comprises a first proximity sensor configured to be placed on a surface of the patient, a second proximity sensor configured to be placed on the surface of the patient, monitoring circuitry configured to measure a change in a distance between the first and second proximity sensors, and processing circuitry configured to infer the change in position of the lead relative to the stimulation target tissue from the measured change in distance. In one embodiment, the processing circuitry is configured to infer an increase in a distance between the lead and the stimulation target tissue when the distance between the first and second proximity sensors increases. 
         [0014]    The system also includes an implantable pulse generator configured to be coupled to the lead and to convey electrical stimulation energy to therapeutically stimulate the stimulation target tissue, and control circuitry configured to modulate a stimulation parameter in response to the inferred change in position. In another embodiment, the control circuitry is configured to increase an amplitude of a stimulation current when the distance between the first and second proximity sensors increases. In yet another embodiment, the control circuitry is configured to automatically modulate the stimulation parameter in response to the measured change in distance. In still another embodiment, the control circuitry is configured to adjust an amplitude of the stimulus applied to selected electrodes during the conveyance of the stimulation energy in response to the inferred change in position. The system further comprises a plurality of electrodes configured to be coupled to the lead and to convey electrical stimulation energy to therapeutically stimulate the stimulation target tissue via a combination of electrodes, whereby the control circuitry is configured to change the combination of the electrodes in response to the inferred change in position. 
         [0015]    In accordance with a third aspect of the present inventions, a method for determining a change in position of a neurostimulation lead relative to a stimulation target tissue of a patient is provided. The method comprises implanting the lead having a first proximity sensor disposed thereon into the patient, implanting a second proximity sensor on a surface of the patient, measuring a change in alignment between the first and second proximity sensors, and inferring the change in position of the lead relative to the stimulation target tissue from the measured change in alignment. The method further comprises inferring an increase in a distance between the lead and the stimulation target tissue when the first and second proximity sensors become misaligned. The method also comprises conveying electrical stimulation energy to therapeutically stimulate the stimulation target tissue, and modulating a stimulation parameter in response to the inferred change in position. The method also comprising conveying electrical stimulation energy via a combination of electrodes to therapeutically stimulate the stimulation target tissue via electrodes. Modulating a stimulation parameter can be performed in the same manner described above. 
         [0016]    In accordance with a fourth aspect of the present inventions, a system for determining a change in position of a neurostimulation lead relative to a stimulation target tissue of a patient is provided. The system comprises a lead configured to be placed into the patient, a first proximity sensor disposed on the lead, a second proximity sensor configured to be placed on a surface of the patient, monitoring circuitry configured to measure a change in alignment between the first and second proximity sensors, and processing circuitry configured to infer the change in position of the lead relative to the stimulation target tissue from the measured change in alignment. In one embodiment, the processing circuitry is configured to infer an increase in a distance between the lead and the stimulation target tissue when the first and second proximity sensors become misaligned. The system further comprises an implantable pulse generator configured to be coupled to the lead and to convey electrical stimulation energy to therapeutically stimulate the stimulation target tissue, and control circuitry configured to modulate a stimulation parameter in response to the inferred change in position. In another embodiment, the control circuitry is configured to increase an amplitude of a stimulation current when the first and second proximity sensors become misaligned. The system also comprises a plurality of electrodes configured to be coupled to the lead and to convey electrical stimulation energy to therapeutically stimulate the stimulation target tissue via a combination of electrodes. The system modulates stimulation parameters in the same manner described above. 
         [0017]    In accordance with a fifth aspect of the present inventions, a method for determining a change in position of a neurostimulation lead relative to a stimulation target tissue of a patient is provided. The method comprises implanting the lead having a voltage sensor disposed thereon into the patient, implanting a plurality of magnets on a surface of the patient, measuring a change in voltage at the voltage sensor in response to movement of the magnets relative to the voltage sensor, and inferring the change in position of the lead relative to the stimulation target tissue from the measured change in voltage. The method further comprises conveying electrical stimulation energy to therapeutically stimulate the stimulation target tissue, and modulating a stimulation parameter in response to the inferred change in position. The method also comprises inferring a distance between the neurostimulation lead and the stimulation target tissue. The method further comprises conveying electrical stimulation energy via a combination of electrodes to therapeutically stimulate the stimulation target tissue via electrodes. Modulating a stimulation parameter can be performed in the same manner described above. 
         [0018]    In accordance with a sixth aspect of the present inventions, a system for determining a change in position of a neurostimulation lead relative to a stimulation target tissue of a patient is provided. The system comprises a lead configured to be placed into the patient, a voltage sensor disposed on the lead, a plurality of magnets configured to be placed on a surface of the patient, monitoring circuitry configured to measure a change in voltage at the voltage sensor in response to movement of the magnets relative to the voltage sensor, and processing circuitry configured to infer the change in position of the lead relative to the stimulation target tissue from the measured change in voltage. The system further comprises an implantable pulse generator configured to be coupled to the lead and to convey electrical stimulation energy to therapeutically stimulate the stimulation target tissue, and control circuitry configured to modulate a stimulation parameter in response to the inferred change in position. The system also comprises a plurality of electrodes configured to be coupled to the lead and to convey electrical stimulation energy to therapeutically stimulate the stimulation target tissue via a combination of electrodes. The system modulates stimulation parameters in the same manner described above. 
         [0019]    In accordance with a seventh aspect of the present inventions, a method for determining a change in position of a neurostimulation lead relative to a stimulation target tissue of a patient is provided. The method comprises measuring a change in capacitance at the lead in response to a change in thickness of tissue adjacent the lead, and inferring the change in position of the lead relative to the stimulation target tissue from the measured change in capacitance. The method further comprises implanting a capacitor within the patient, where the capacitor comprises a plurality of capacitor plates that measure the change in capacitance, and a compressible dielectric material disposed between the capacitor plates, wherein the capacitor plates move closer to each other when the dielectric material is compressed and the capacitor plates move farther from each other when the dielectric material is expanded. 
         [0020]    One optional method comprises inferring an increase in a distance between the lead and the stimulation target tissue when the capacitance decreases. Another optional method, comprises inferring a decrease in a distance between the lead and the stimulation target tissue when the capacitance increases. The method also comprises conveying electrical stimulation energy to therapeutically stimulate the stimulation target tissue, and modulating a stimulation parameter in response to the measured change in capacitance. Modulating a stimulation parameter can be performed in the same manner described above. 
         [0021]    In accordance with an eighth aspect of the present inventions, a system for determining a change in position of a neurostimulation lead relative to a stimulation target tissue of a patient is provided. The system comprises an implantable capacitor, monitoring circuitry configured to measure a change in capacitance at the capacitor, and processing circuitry configured to infer the change in position of the lead relative to the stimulation target tissue from the measured change in capacitance. The capacitor comprises a plurality of capacitor plates that measure the change in capacitance, and a compressible dielectric material disposed between the capacitor plates, where the capacitor plates move closer to each other when the dielectric material is compressed and the capacitor plates move farther from each other when the dielectric material is expanded. In one embodiment, the processing circuitry is configured to infer an increase in a distance between the lead and the stimulation target tissue when the capacitance decreases. In another embodiment, the processing circuitry is configured to infer a decrease in a distance between the lead and the stimulation target tissue when the capacitance increases. 
         [0022]    The system further comprises an implantable pulse generator configured to be coupled to the lead and to convey electrical stimulation energy to therapeutically stimulate the stimulation target tissue, and control circuitry configured to modulate a stimulation parameter in response to the inferred change in position. The system also comprises a plurality of electrodes configured to be coupled to the lead and to convey electrical stimulation energy to therapeutically stimulate the stimulation target tissue via a combination of electrodes. Modulating a stimulation parameter can be performed in the same manner described above. 
         [0023]    In accordance with a ninth aspect of the present inventions, a method for determining a change in position of a neurostimulation lead relative to a stimulation target tissue of a patient is provided. The method comprises measuring a change in temperature at the lead in response to a change in thickness of tissue adjacent the lead, and inferring the change in position of the lead relative to the stimulation target tissue from the measured change in temperature. One optional method comprises inferring an increase in a distance between the lead and the stimulation target tissue when the temperature decreases. Another optional method comprises inferring a decrease in a distance between the lead and the stimulation target tissue when the temperature increases. The method further comprises conveying electrical stimulation energy to therapeutically stimulate the stimulation target tissue, and modulating a stimulation parameter in response to the measured change in temperature. Modulating a stimulation parameter can be performed in the same manner described above. 
         [0024]    In accordance with a tenth aspect of the present inventions, a system for determining a change in position of a neurostimulation lead relative to a stimulation target tissue of a patient is provided. The system comprises an implantable temperature sensor, monitoring circuitry configured to measure a change in temperature at the lead in response to a change in thickness of tissue adjacent the lead, and processing circuitry configured to infer the change in position of the lead relative to the stimulation target tissue from the measured change in temperature. In one embodiment, the processing circuitry is configured to infer an increase in a distance between the lead and the stimulation target tissue when the temperature decreases. In another embodiment, the processing circuitry is configured to infer a decrease in a distance between the lead and the stimulation target tissue when the temperature increases. The system further comprises an implantable pulse generator configured to be coupled to the lead and to convey electrical stimulation energy to therapeutically stimulate the stimulation target tissue, and control circuitry configured to modulate a stimulation parameter in response to the measured change in temperature. The system modulates stimulation parameters in the same manner described above. 
         [0025]    In accordance with an eleventh aspect of the present inventions, a method for determining a change in position of a neurostimulation lead relative to an occipital nerve is provided. The method comprises measuring a change in a parameter adjacent the lead, and inferring the change in position of the lead relative to the occipital nerve from the measured change in the parameter. The method further comprises inferring a change in a distance between the lead and the occipital nerve from the measured change in the parameter, conveying electrical stimulation energy to therapeutically stimulate the occipital nerve, and modulating a stimulation parameter in response to the inferred change in position. Modulating a stimulation parameter can be performed in the same manner described above. 
         [0026]    In accordance with a twelfth aspect of the present inventions, a system for determining a change in position of a lead of an occipital nerve stimulation device relative to an occipital nerve is provided. The system comprises a sensor configured to measure a change in a parameter adjacent the lead, and processing circuitry configured to infer the change in position of the lead relative to the occipital nerve from the measured change in the parameter. The system further comprises an implantable pulse generator configured to be coupled to the lead and to convey electrical stimulation energy to therapeutically stimulate the occipital nerve, and control circuitry configured to modulate a stimulation parameter in response to the inferred change in position. The system modulates stimulation parameters in the same manner described above. 
         [0027]    In accordance with a thirteenth aspect of the present inventions, a method for determining a change in position of a neurostimulation lead relative to a stimulation target tissue of a patient is provided. The method comprises implanting the lead with a sensor disposed thereon into the patient, conveying electrical energy from the lead into the stimulation target tissue of the patient over a period of time, measuring data from the sensor, whereby the data is modulated in response to changes in a position of the lead relative to the stimulation target tissue, analyzing the time-varying data, and tracking the changes in the position of the lead relative to the stimulation target tissue during the time period based on the analyzed time-varying data. In one optional method, the electrical energy conveyed from the lead provides therapy to the patient. In another optional method the data is one or both of capacitance data or temperature data. In still another optional method, the step of analyzing the time-varying data comprises determining a magnitude of the time-varying data. 
         [0028]    Yet another optional method further comprises implanting a second sensor disposed on a surface of the patient, where the data is a distance between the sensor and the second sensor. Another optional method further comprises implanting a plurality of magnets on a surface of the patient, where the data is voltage data. 
         [0029]    The method further comprises modulating a stimulation parameter in response to the changes in the position of the lead relative to the stimulation target tissue. Modulating a stimulation parameter can be performed in the same manner described above. In an alternative method, the implanted lead does not have a sensor disposed thereon and the step of implanting a second sensor is replaced with the step of implanting two proximity sensors disposed on a surface of the patient. 
         [0030]    In accordance with a fourteenth aspect of the present inventions, a tissue stimulation system is provided. The system comprises a sensor disposed on an implantable lead, an implantable electrical stimulation device configured for being coupled to the lead, the electrical stimulation device configured for conveying electrical energy from the lead into a stimulation target tissue of a patient over a period of time and measuring data from the sensor, where the data is modulated in response to changes in a position of the lead relative to the stimulation target tissue, and a processing device configured for analyzing the time-varying data, and tracking the changes in the position of the lead relative to the stimulation target tissue during the time period based on the analyzed time-varying data. In one embodiment, the electrical energy conveyed from the lead provides therapy to the patient. In another embodiment, the data is one or both of capacitance date or temperature data. In still another embodiment, the processing device is the stimulation device. In yet another embodiment, the processing device is an external programmer configured for communicating with the stimulation device. 
         [0031]    In one embodiment, the time-varying data analysis comprises determining a magnitude of the time-varying data. In another embodiment, the system further comprises a second sensor disposed on a surface of the patient, where the data is a distance between the sensor and the second sensor. In still another embodiment, the system further comprises a plurality of magnets disposed on a surface of the patient, where the data is voltage data. In yet another embodiment, the stimulation device is configured to modulate a stimulation parameter in response to the changes in the position of the lead relative to the stimulation target tissue. The system modulates stimulation parameters in the same manner described above. In an alternative embodiment, two proximity sensors disposed on a surface of the patient replace the sensor disposed on the implantable lead and the second sensor. 
         [0032]    Other and further aspects and features of the invention will be evident from reading the following detailed description of the preferred embodiments, which are intended to illustrate, not limit, the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The drawings illustrate the design and utility of preferred embodiments of the present invention, in which similar elements are referred to by common reference numerals. In order to better appreciate how the above-recited and other advantages and objects of the present inventions are obtained, a more particular description of the present inventions briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated in the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
           [0034]      FIG. 1  is plan view of one embodiment of a occipital nerve stimulation (ONS) system arranged in accordance with the present inventions; 
           [0035]      FIG. 2  is a profile view of an implantable pulse generator (IPG) used in the ONS system of  FIG. 1 ; 
           [0036]      FIG. 3  is a schematic view of the ONS system of  FIG. 1  in use with a patient; 
           [0037]      FIG. 4  is a block diagram of the internal components of the IPG of  FIG. 2 ; 
           [0038]      FIG. 5  is a schematic view of two proximity sensors arranged in accordance with the present inventions in use with a patient and connected to an external programmer, with the distal ends of two electrode leads shown in phantom; 
           [0039]      FIG. 6  is a detailed posterior view of an external proximity sensor and an internal proximity sensor of an ONS system arranged in accordance with the present inventions in use with a patient—the electrode lead on the left side of the figure is shown in phantom, and the patient&#39;s scalp on the right side of the figure has been removed for clarity; 
           [0040]      FIG. 7  is a detailed posterior view of a groups of magnets and two voltage sensors of an ONS system arranged in accordance with the present inventions in use with a patient—the electrode lead on the left side of the figure is shown in phantom, and the patient&#39;s scalp on the right side of the figure has been removed for clarity; 
           [0041]      FIG. 8  is a detailed posterior view of two electrode leads and two capacitive sensors of an ONS system arranged in accordance with the present inventions in use with a patient, whose scalp has been rendered transparent for clarity—the distal portion of the electrode lead on the left side of the figure, except for the capacitive sensors, has also been removed for clarity; 
           [0042]      FIG. 9  is a detailed cross sectional view through a capacitive sensor embedded in an electrode lead of an ONS system arranged in accordance with the present inventions in use with a patient; and 
           [0043]      FIG. 10  is a detailed posterior view of two electrode leads and two temperature sensors of an ONS system arranged in accordance with the present inventions in use with a patient, whose scalp has been rendered transparent for clarity—the distal portion of the electrode lead on the left side of the figure, except for the temperature sensors, has also been removed for clarity. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0044]    The description that follows relates to an occipital nerve stimulation (ONS) system. However, it is to be understood that the while the invention lends itself well to applications in ONS, the invention, in its broadest aspects, may not be so limited. Rather, the invention may be used with any type of implantable electrical circuitry used to stimulate tissue. For example, the present invention may be used as part of a spinal cord stimulation (SCS) system, pacemaker, a defibrillator, a cochlear stimulator, a retinal stimulator, a stimulator configured to produce coordinated limb movement, a cortical and deep brain stimulator, peripheral nerve stimulator, or in any other neural stimulator configured to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. 
         [0045]    Turning first to  FIGS. 1 and 2 , an exemplary ONS system  10  generally includes first and second implantable neurostimulation leads  12  ( 12   a  and  12   b ), one or more lead movement sensors  14  (only one shown in  FIG. 1 ), an implantable pulse generator (IPG)  16 , and an external (non-implanted) programmer  18 . In the illustrated embodiment, the leads  12  are percutaneous leads and, to that end, both of the leads comprise a plurality of in-line electrodes  20  carried on a flexible body  22 . Alternatively, the leads  12  may be replaced with paddle electrode leads. In the illustrated embodiment, the first lead  12   a  has four electrodes  20  (labeled E 1 -E 4 ), and the second lead  12   b  includes four electrodes  20  (labeled E 5 -E 8 ). The actual number of leads and electrodes will, of course, vary according to the intended application. 
         [0046]    The IPG  16  is capable of directing electrical stimulation energy to each of the electrodes  20 . To that end, the electrodes  20  of the first lead  12   a  are electrically connected to the IPG  16  by respective wires  24   a  that extend through, or are embedded in, the associated flexible lead body  22 . Similarly, the electrodes  20  of the second lead  12   b  are electrically connected to the IPG  16  by respective wires  24   b.  The wires  24   a,    24   b  are connected to the IPG  16  by way of an interface  28 . The interface  28  may be any suitable device that allows the leads  12  to be removably or permanently electrically connected to the IPG  16 . Such an interface may, for example, be an electro-mechanical connector arrangement including lead connectors  30   a,    30   b  within the IPG  16  that are configured to mate with corresponding connectors (only connector  32   a  is shown) on the corresponding leads  12 . Alternatively, the leads  12  can share a single connector that mates with a corresponding connector on the IPG  16 . Exemplary connector arrangements are disclosed in U.S. Pat. Nos. 6,609,029 and 6,741,892, which are incorporated herein by reference. The IPG  16  includes an outer case  34  formed from an electrically conductive, biocompatible material, such as titanium and, in some instances, will function as an electrode. The case  34  forms a hermetically sealed compartment wherein the electronic and other components (described in further detail below) are protected from the body tissue and fluids. 
         [0047]    The IPG  16  is typically programmed, or controlled, through the use of the external programmer  18 . The external programmer  18  is coupled to the IPG  16  through a suitable communications link (represented by the arrow  36 ) that passes through the patient&#39;s skin  38 . Suitable links include, but are not limited to radio frequency (RF) links, inductive links, optical links, and magnetic links. The programmer  18  or other external device may also be used to couple power into the IPG  16  for the purpose of operating the IPG  16  or replenishing a power source, such as a rechargeable battery, within the IPG  16 . Once the IPG  16  has been programmed, and its power source has been charged or otherwise replenished, the IPG  16  may function as programmed without the external programmer  18  being present. 
         [0048]    With respect to the stimulus patterns provided during operation of the ONS system  10 , electrodes  20  that are selected to transmit or receive stimulation energy are referred to herein as “activated,” while electrodes  20  that are not selected to transmit or receive stimulation energy are referred to herein as “non-activated.” Electrical stimulation will occur between two (or more) electrodes, one of which may be the IPG case  34 , so that the electrical current associated with the stimulus has a path from the energy source contained within the IPG case  34  to the tissue and a return path from the tissue to the energy source contained within the case  34 . Stimulation energy may be transmitted to the tissue in a monopolar or multipolar (e.g., bipolar, tripolar, etc.) fashion. 
         [0049]    Monopolar stimulation occurs when a selected one of the lead electrodes  20  is activated along with the case  34 , so that stimulation energy is transmitted between the selected electrode  20  and case  34 . Bipolar stimulation occurs when two of the lead electrodes  20  are activated as anode and cathode, so that stimulation energy is transmitted between the selected electrodes  20 . For example, electrode E 3  on the first lead  12   a  may be activated as an anode at the same time that electrode E 7  on the second lead  12   b  is activated as a cathode. Tripolar stimulation occurs when three of the lead electrodes  20  are activated, two as anodes and the remaining one as a cathode, or two as cathodes and the remaining one as an anode. For example, electrodes E 2  and E 3  on the first lead  12   a  may be activated as anodes at the same time that electrode E 6  on the second lead  12   b  is activated as a cathode. 
         [0050]    The lead movement sensors  14  are electrically connected to the IPG  16  via signal wires  26 , which in some embodiments, run completely inside of the electrode leads  12 . As will be described in further detail below, the sensors  14  may be used to determine when a lead  12  has shifted relative to a target nerve, and in this case, an occipital nerve. Although the lead movement sensors  14  are shown to be separate from the leads  12  in  FIG. 1 , the sensors  14  may be located on the leads  12  depending upon the technique used to sense the lead shift. In some embodiments, the sensors  14  are ring sensors (see  FIG. 3 ) that are mounted onto the leads  12 . In still other embodiments, the sensors  14  are incorporated into the lead electrodes  20 , and can be used to measure parameters, such as impedance, when the lead electrode  20  is not being used to transmit stimulation energy. 
         [0051]    As shown in  FIG. 3 , the neurostimulation leads  12  are implanted subcutaneously near the intermastoid line using a percutaneous needle or other convention technique, so as to be in close proximity to the occipital nerves  42 . Once in place, the electrodes  20  may be used to supply stimulation energy to the occipital nerves  42 . The preferred placement of the leads  12  is such, that the electrodes  20  are adjacent, i.e., resting upon, the occipital nerve area to be stimulated. Due to the lack of space near the location where the leads  12  are implanted, the IPG  16  is generally implanted in a surgically-made pocket either in the abdomen or above the buttocks. The IPG  16  may, of course, also be implanted in other locations of the patient&#39;s body. A lead extension  44  may facilitate locating the IPG  16  away from the exit point of the leads  12 . 
         [0052]    Turning next to  FIG. 4 , the main internal components of the IPG  16  will now be described. The IPG  16  includes analog output circuitry  50  capable of individually generating electrical stimulation pulses via capacitors C 1 -C 8  at the electrodes  20  (E 1 -E 8 ) of specified amplitude under control of control logic  52  over data bus  54 . The duration of the electrical stimulation (i.e., the width of the stimulation pulses), is controlled by the timer logic circuitry  56 . The analog output circuitry  50  may either comprise independently controlled current sources for providing stimulation pulses of a specified and known amperage to or from the electrodes  20 , or independently controlled voltage sources for providing stimulation pulses of a specified and known voltage at the electrodes  20 . The operation of this analog output circuitry, including alternative embodiments of suitable output circuitry for performing the same function of generating stimulation pulses of a prescribed amplitude and width, is described more fully in U.S. Pat. Nos. 6,516,227 and 6,993,384, which are expressly incorporated herein by reference. 
         [0053]    The IPG  16  also comprises monitoring circuitry  58  for monitoring the status of various nodes or other points  60  throughout the IPG  16 , e.g., power supply voltages, temperature, battery voltage, and the like. Significantly, the monitoring circuitry  58  is also configured for monitoring, via the signal wires  26 , the status of lead movement sensors  14  used to determine when a lead  12  has shifted relative to the occipital nerves  42 . The IPG  16  further comprises processing circuitry in the form of a microcontroller (μC)  62  that controls the control logic  52  over data bus  64 , and obtains status data from the monitoring circuitry  58  via data bus  66 . The IPG  16  additionally controls the timer logic  56 . The IPG  16  further comprises memory  68  and oscillator and clock circuit  70  coupled to the microcontroller  62 . The microcontroller  62 , in combination with the memory  68  and oscillator and clock circuit  70 , thus comprise a microprocessor system that carries out a program function in accordance with a suitable program stored in the memory  68 . Alternatively, for some applications, the function provided by the microprocessor system may be carried out by a suitable state machine. 
         [0054]    Thus, the microcontroller  62  generates the necessary control and status signals, which allow the microcontroller  62  to control the operation of the IPG  16  in accordance with a selected operating program and stimulation parameters. In controlling the operation of the IPG  16 , the microcontroller  62  is able to individually generate stimulus pulses at the electrodes  20  using the analog output circuitry  50 , in combination with the control logic  52  and timer logic  56 , thereby allowing each electrode  20  to be paired or grouped with other electrodes  20 , including the monopolar case electrode, to control the polarity, amplitude, rate, pulse width and channel through which the current stimulus pulses are provided. The microcontroller  62  facilitates the storage of parameter data monitored by the monitoring circuitry  58  within memory  68 , and also provides any computational capability needed to analyze such parameter data and/or generate lead shift information. 
         [0055]    As briefly discussed above, the monitoring circuitry  58  is configured for monitoring when a lead  12  has shifted relative to the occipital nerves  42 . In particular, any movement of lead  12  is communicated from the lead movement sensors  14  via signal wires  26  to the monitoring circuitry  58 . In turn, the monitoring circuitry  58  communicates the change in distance via data bus  66  to the microcontroller  62 , which uses the change in distance to determine whether either of the leads  12  has shifted relative to the occipital nerves  42 . If the microcontroller  62  determines that a lead  12  has shifted, it sends a command via control logic over data bus  64  to control logic  52 , which modulates stimulation parameters to compensate for the lead shift. Modulation of stimulation parameters includes varying the amplitude of a stimulation current and/or the combination of electrodes  20  through which electrical stimulation energy is conveyed to the occipital nerves  42 . Other stimulation parameters that may be modulated may be, e.g., pulse width and pulse frequency. This cycle of measurement, analysis, and modulation is repeated to maintain optimal stimulation of the occipital nerves  42 . 
         [0056]    The IPG  16  further comprises an alternating current (AC) receiving coil  72  for receiving programming data (e.g., the operating program and/or stimulation parameters) from the external programmer  18  in an appropriate modulated carrier signal, and charging and forward telemetry circuitry  74  for demodulating the carrier signal it receives through the AC receiving coil  72  to recover the programming data, which programming data is then stored within the memory  68 , or within other memory elements (not shown) distributed throughout the IPG  16 . 
         [0057]    The IPG  16  further comprises back telemetry circuitry  76  and an alternating current (AC) transmission coil  78  for sending informational data sensed through the monitoring circuitry  58  to the external programmer  18 . The back telemetry features of the IPG  16  also allow its status to be checked. For example, when the external programmer  18  initiates a programming session with the IPG  16 , the capacity of the battery is telemetered, so that the external programmer  18  can calculate the estimated time to recharge. Any changes made to the current stimulus parameters are confirmed through back telemetry, thereby assuring that such changes have been correctly received and implemented within the implant system. Moreover, upon interrogation by the external programmer  18 , all programmable settings stored within the IPG  16  may be uploaded to the external programmer  18 . The back telemetry features allow raw or processed parameter data and/or lead shifting information previously stored in the memory  68  to be downloaded from the IPG  16  to the external programmer  18 , which information can be used to track the shifting of leads. 
         [0058]    The IPG  16  further comprises a rechargeable power source  80  and power circuits  82  for providing the operating power to the IPG  16 . The rechargeable power source  80  may, e.g., comprise a lithium-ion or lithium-ion polymer battery. The rechargeable battery  80  provides an unregulated voltage to the power circuits  82 . The power circuits  82 , in turn, generate the various voltages  84 , some of which are regulated and some of which are not, as needed by the various circuits located within the IPG  16 . The rechargeable power source  80  is recharged using rectified AC power (or DC power converted from AC power through other means, e.g., efficient AC-to-DC converter circuits, also known as “inverter circuits”) received by the AC receiving coil  72 . To recharge the power source  80 , an external charger (not shown), which generates the AC magnetic field, is placed against, or otherwise adjacent, to the patient&#39;s skin over the implanted IPG  16 . The AC magnetic field emitted by the external charger induces AC currents in the AC receiving coil  72 . The charging and forward telemetry circuitry  74  rectifies the AC current to produce DC current, which is used to charge the power source  80 . While the AC receiving coil  72  is described as being used for both wirelessly receiving communications (e.g., programming and control data) and charging energy from the external device, it should be appreciated that the AC receiving coil  72  can be arranged as a dedicated charging coil, while another coil, such as coil  78 , can be used for bi-directional telemetry. 
         [0059]    Additional details concerning the above-described and other IPGs may be found in U.S. Pat. No. 6,516,227, U.S. Patent Publication No. 2003/0139781, and U.S. patent application Ser. No. 11/138,632, entitled “Low Power Loss Current Digital-to-Analog Converter Used in an Implantable Pulse Generator,” which are expressly incorporated herein by reference. It should be noted that rather than an IPG, the ONS system  10  may alternatively utilize an implantable receiver-stimulator (not shown) connected to leads  12 . In this case, the power source, e.g., a battery, for powering the implanted receiver, as well as control circuitry to command the receiver-stimulator, will be contained in an external controller inductively coupled to the receiver-stimulator via an electromagnetic link. Data/power signals are transcutaneously coupled from a cable-connected transmission coil placed over the implanted receiver-stimulator. The implanted receiver-stimulator receives the signal and generates the stimulation in accordance with the control signals. 
         [0060]    As briefly discussed above, the monitoring circuitry  58  is configured to monitor the status of the lead movement sensors  14 , so that the microcontroller  62  can determine whether a lead  12  has moved relative to the occipital nerve  42  in which it is designed to stimulate and compensate for such lead movement. 
         [0061]    In one embodiment, as shown in  FIG. 5 , the monitoring circuitry  58  is configured to monitor the distance between two proximity sensors  14  (a mobile sensor  14 ( 1 ) and an immobile sensor  14 ( 2 )) mounted on the surface of a patient (i.e., the sensors  14  are secured to the surface of the patient in such a way that the sensors  14  resist unintentional removal). Methods for attaching devices to the surface of a patient include bonding with biocompatible adhesives, taping, and suturing. While the sensors  14 ( 1 ),  14 ( 2 ) are mounted on the surface of the skin, the electrodes  12   a,    12   b  (shown in phantom) are implanted under the skin. The sensors  14 ( 1 ),  14 ( 2 ) are connected by signal wires  26  to an external programmer  18 , which communicates wirelessly with the IPG  16  as described above. 
         [0062]    The mobile proximity sensor  14 ( 2 ) is mounted on the surface of the patient approximately overlying the occipital bone of the skull near the middles of the inferior and superior nuchal lines. The immobile proximity sensor  14 ( 2 ) is mounted on the surface of the patient approximately overlying the middles of the C3 and C4 cervical vertebrae. When the patient&#39;s neck is rotated, the mobile proximity sensor  14 ( 1 ) will rotate with the head away from the immobile proximity sensor  14 ( 2 ), and the distance between the two proximity sensors  14  will increase. Thus, movement of the mobile proximity sensor  14 ( 1 ) relative to immobile proximity sensor  14 ( 2 ) is measured when the patient changes posture and the patient&#39;s skin contorts. 
         [0063]    In general, increased distance between the proximity sensors  14  indicates an increased distance between the electrode leads  12  and the respective occipital nerves  42 , while decreased distance between the proximity sensors  14  indicates a decreased distance between the electrode leads  12  and the respective occipital nerves  42 . Ultimately, the correlation between the change in distance between the proximity sensors  14  and the change in distance between the electrode leads  12  and target tissue may depend on the particular application and location of stimulation. While two proximity sensors  14  are described with respect to the  FIG. 5  embodiment, in other embodiments, more than two proximity sensors  14  can be mounted on the surface of the patient to provide more information about the changing position of the electrode leads  12  relative to the occipital nerves  42 . 
         [0064]    In another embodiment, as shown in  FIG. 6 , the monitoring circuitry  58  is configured to monitor the distance between an external proximity sensor  14 ( 3 ) mounted on the surface of a patient overlying the distal tip of each electrode lead  12  and an internal proximity sensor  14 ( 4 ) attached to the distal tip of each electrode lead  12  implanted inside of the patient. The internal proximity sensor  14 ( 4 ) can be a ring sensor. Thus, movement of the external proximity sensor  14 ( 3 ) relative to internal proximity sensor  14 ( 4 ) is measured when the patient changes posture and the patient&#39;s skin contorts. While two proximity sensors  14 ( 3 ),  14 ( 4 ) are described in the  FIG. 6  embodiment, in other embodiments, more than two proximity sensors  14  (either mounted on the surface of the patient or attached to the electrode leads  12 ) can be used to provide more information about the changing position of the electrode leads  12  relative to the occipital nerves  42 . 
         [0065]    In yet another embodiment, as shown in  FIG. 7 , the monitoring circuitry  58  is configured to monitor voltages generated by voltage sensors  14 ( 5 ) attached to each electrode lead  12 . The voltages are generated at each sensor  14 ( 5 ) in response to movement of the sensor  14 ( 5 ) relative to a plurality of permanent magnets  112  mounted on the surface of the patient above the area in which the leads  12  are implanted. Notably, the voltage generated at the sensor  14 ( 5 ) is a result of the Hall effect, which creates a voltage across an electrical conductor resulting from current flow in the presence of a changing magnetic field. Thus, movement of each sensor  14 ( 5 ) relative to the magnets  112  are measured when the patient changes posture and the patient&#39;s skin contorts. 
         [0066]    In still another embodiment, as shown in  FIGS. 8 and 9 , the monitoring circuitry  58  is configured to monitor the capacitance of capacitive sensors  14 ( 6 ), which are each formed from two capacitive plates  118  separated by a compressible and/or flexible dielectric material  120  ( FIG. 9 ). The capacitive plates  118  may be flat or they may be curved to conform to the shape of the leads  12 . The capacitive sensors  14 ( 6 ) are embedded within the leads  12 . Alternatively, the capacitive sensors  14 ( 6 ) may be attached to the surfaces of the leads  12  closest to the respective occipital nerves  42 . Alternatively, the capacitor may be placed within the leads  12 . The capacitive sensors  14 ( 6 ) are configured to change capacitance when the patient changes posture and the thicknesses of the tissue  122  separating the electrode leads  12  from the respective occipital nerves  42  change. These changes in tissue thickness affect the distance between the two capacitive plates  118  between the electrode leads  12  and the respective occipital nerves  42 . Decreasing tissue thickness, which indicates decreased distance between the electrode leads  12  and the respective occipital nerves  42 , compresses the compressible dielectric material  120 , decreasing the distance between the capacitive plates  118  and resulting in increased capacitance. Increasing tissue thickness, which indicates increased distance between the electrode leads  12  and the respective occipital nerves  42 , releases pressure on the compressible dielectric material  120 , increasing the distance between the capacitive plates  118  and resulting in decreased capacitance. 
         [0067]    In another embodiment, as shown in  FIG. 10 , the monitoring circuitry  58  is configured to monitor the temperature at temperature sensors  14 ( 7 ) embedded within the leads  12 . Alternatively, the temperature sensors  14 ( 7 ) may be ring sensors attached to the surface of the leads  12 . The temperature sensors  14 ( 7 ) are configured to measure changes in temperature when the patient changes posture and the location of electrode leads  12  in the skin of the patient changes. Measured temperature increases as the temperature sensors  14 ( 7 ) and the electrode leads  12  move deeper into the skin and closer to the occipital nerves  42 . In contrast, measured temperature decreases as the temperature sensors  14 ( 7 ) and the electrode leads  12  move shallower into the skin and away from the occipital nerves  42 . 
         [0068]    Although particular embodiments of the present inventions have been shown and described, it will be understood that it is not intended to limit the present inventions to the preferred embodiments, and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present inventions. Thus, the present inventions are intended to cover alternatives, modifications, and equivalents, which may be included within the spirit and scope of the present inventions as defined by the claims.