Ramping of Neural Dosing for Comprehensive Spinal Cord Stimulation Therapy

Abstract

Methods and systems for providing sub-perception spinal cord stimulation are described. In some examples, the stimulation current is shared among three or more anodes and three or more cathodes to provide virtual poles that are configured to cover a relatively large area of the patient's neural tissue that contains the “sweet spot” for treating the patient's pain. Covering a relatively large area mitigates the need to perform time-intensive sweet spot searching. In some examples, one or more stimulation parameters are varied while the stimulation is being provided.

Claims

1. A method for providing sub-perception electrical stimulation to a patient's spinal cord, the method comprising: using one or more electrodes implanted within the patient's spinal column to provide a plurality of electrical pulses to the patient's spinal cord, wherein of each of the pulses are below the patient's perception threshold, wherein each pulse comprises an amplitude and a pulse width, and wherein the amplitudes of the plurality of pulses are ramped from a first amplitude value to second amplitude value at a rate of no more than 3 mA/s over a first duration.

2. The method of claim 1, wherein the first amplitude value is less than the second amplitude value.

3. The method of claim 2, wherein the second amplitude value is 80% or less than an amplitude value that causes paresthesia in the patient.

4. The method of claim 1, wherein the first amplitude value is greater than the second amplitude value.

5. The method of claim 1, further comprising providing no stimulation for a second duration and then repeating the steps of claim 1.

6. The method of claim 5, wherein the second duration is at least one second.

7. The method of claim 1, wherein the pulse widths of the plurality of pulses vary over the first duration.

8. The method of claim 7, wherein the amplitudes of the plurality of pulses increase and the pulse widths of the plurality of pulses decrease over the first duration.

9. The method of claim 7, wherein the amplitudes of the plurality of pulses decrease and the pulse widths of the plurality of pulses increase over the first duration.

10. The method of claim 1, wherein each of the electrical pulses are biphasic pulses.

11. The method of claim 1, wherein the one or more of the electrodes comprise at least three anodes and at least three cathodes.

12. The method of claim 11, wherein the anodes are configured as a first three or more adjacent electrodes and the cathodes are each configured as a second set of three or more adjacent electrodes.

13. The method of claim 12, wherein: the anodes each share an anodic current fractionalized among each of the first three or more adjacent electrodes, and the cathodes each share a cathodic current fractionalized among each of the second three or more adjacent electrodes.

14. The method of claim 13, wherein the anodic current is fractionalized equally among the first three or more adjacent electrodes and the cathodic current is fractionalized equally among the second three or more adjacent electrodes.

15. The method of claim 13, wherein: the first set of adjacent electrodes comprises one or more rostral anodes, one or more middle anodes, and one or more caudal anodes, the rostral anodes and the caudal anodes each share more of the anodic current than the middle anodes, the second set of adjacent electrodes comprises one or more rostral cathodes, one or more middle cathodes and one or more caudal cathodes, and the rostral cathodes and the caudal cathodes each share more cathodic current than the middle cathodes.

16. A system, comprising: a spinal cord stimulator comprising: stimulation circuitry programmed to generate a plurality of electrical stimulation pulses at a plurality of electrodes, wherein each of the pulses have a shape comprising an amplitude and a pulse width, wherein each of the pulses are configured to be below the patient's perception threshold, and wherein the amplitudes of the plurality of pulses are ramped from a first amplitude value to second amplitude value at a rate of no more than 3 mA/s.

17. The system of claim 16, wherein the pulse widths of the plurality of pulses vary over the first duration.

18. The system of claim 17, wherein the amplitudes of the plurality of pulses increase and the pulse widths of the plurality of pulses decrease over the first duration.

19. The system of claim 17, wherein the amplitudes of the plurality of pulses decrease and the pulse widths of the plurality of pulses increase over the first duration.

20. The system of claim 16, wherein the one or more of the electrodes comprise at least three anodes and at least three cathodes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1 shows an Implantable Pulse Generator (IPG) useable for Spinal Cord Stimulation (SC S), in accordance with the prior art.

[0034] FIG. 2 shows an example of stimulation pulses producible by the IPG, in accordance with the prior art.

[0035] FIG. 3 shows use of an External Trial Stimulator (ETS) useable to provide stimulation before implantation of an IPG, in accordance with the prior art.

[0036] FIG. 4 shows various external devices capable of communicating with and programming stimulation in an IPG and ETS, in accordance with the prior art.

[0037] FIG. 5 shows a Graphical User Interface (GUI) of a clinician programmer external device for setting or adjusting stimulation parameters, in accordance with the prior art.

[0038] FIGS. 6A and 6B show sweet spot searching to determine effective electrodes for a patient using a movable sub-perception bipole.

[0039] FIG. 7 shows a virtual bipole for treating an area of neural tissue believed to contain the center of a patient's pain.

[0040] FIG. 8 shows stimulation circuitry useable in the IPG or ETS capable of providing Multiple Independent Current Control to independently set the current at each of the electrodes.

[0041] FIG. 9 shows a virtual bipole for treating an area of neural tissue believed to contain the center of a patient's pain.

[0042] FIGS. 10A and 10B show examples of sub-perception stimulation waveforms wherein the amplitudes ramp between a first value and a second value.

[0043] FIG. 11 shows an example of a stimulation waveform wherein both the amplitude and the pulse width varies.

DETAILED DESCRIPTION

[0044] While Spinal Cord Stimulation (SCS) therapy can be an effective means of alleviating a patient's pain, such stimulation can also cause paresthesia. Paresthesia is a sensation such as tingling, prickling, heat, cold, etc. that can accompany SCS therapy. Generally, the effects of paresthesia are mild, or at least are not overly concerning to a patient. Moreover, paresthesia is generally a reasonable tradeoff for a patient whose chronic pain has now been brought under control by SCS therapy. Some patients even find paresthesia comfortable and soothing.

[0045] Nonetheless, at least for some patients, SCS therapy would ideally provide complete pain relief without paresthesia—what is often referred to as “sub-perception” or sub-threshold therapy that a patient cannot feel. Effective sub-perception therapy may provide pain relief without paresthesia by issuing stimulation pulses at higher frequencies. Unfortunately, such higher-frequency stimulation may require more power, which tends to drain the battery 14 of the IPG 10. See, e.g., U.S. Patent Application Publication 2016/0367822. If an IPG's battery 14 is a primary cell and not rechargeable, high-frequency stimulation means that the IPG 10 will need to be replaced more quickly. Alternatively, if an IPG battery 14 is rechargeable, the IPG 10 will need to be charged more frequently, or for longer periods of time. Either way, the patient is inconvenienced.

[0046] In an SCS application, it is desirable to determine a stimulation program that will be effective for each patient. A significant part of determining an effective stimulation program is to determine a “sweet spot” for stimulation in each patient, i.e., to select which electrodes should be active (E) and with what polarities (P) and relative amplitudes (X %) to recruit and thus treat a neural site at which pain originates in a patient. Selecting electrodes proximate to this neural site of pain can be difficult to determine, and experimentation is typically undertaken to select the best combination of electrodes to provide a patient's therapy.

[0047] As described in U.S. patent application Ser. No. 16/419,879, filed May 22, 2019, which is hereby expressly incorporated by reference, selecting electrodes for a given patient can be even more difficult when sub-perception therapy is used, because the patient does not feel the stimulation, and therefore it can be difficult for the patient to feel whether the stimulation is “covering” his pain and therefore whether selected electrodes are effective. Further, sub-perception stimulation therapy may require a “wash in” period before it can become effective. A wash in period can take up to a day or more, and therefore sub-perception stimulation may not be immediately effective, making electrode selection more difficult.

[0048] FIGS. 6A and 6B briefly explain the '879 application's technique for a sweet spot search, i.e., how electrodes can be selected that are proximate to a site of neural pain 298 in a patient, when sub-perception stimulation is used. The technique of FIGS. 6A and 6B is particularly useful in a trial setting after a patient is first implanted with an electrode array 15 (or 15′), i.e., after receiving their IPG or ETS.

[0049] In the example shown, it is assumed that a pain site 298 is likely within a tissue region 299. Such region 299 may be deduced by a clinician based on the patient symptoms, e.g., by understanding which electrodes are proximate to certain vertebrae (not shown), such as within the T7-T10 interspace, as just one example. Of course, the region 299 may be any portion of the spine. In the example shown, region 299 is bounded by electrodes E2, E7, E15, and E10, meaning that electrodes outside of this region (e.g., E1, E8, E9, E16) are unlikely to have an effect on the patient's symptoms. Therefore, these electrodes may not be selected during the sweet spot search depicted in FIG. 6A, as explained further below.

[0050] In FIG. 6A, a sub-perception bipole 297a is selected, in which one electrode (e.g., E2) is selected as an anode that will source a positive current (+A) to the patient's tissue, while another electrode (e.g., E3) is selected as a cathode that will sink a negative current (−A) from the tissue. This is similar to what was illustrated earlier with respect to FIG. 2, and biphasic stimulation pulses can be used employing active charge recovery. Because the bipole 297a provides sub-perception stimulation, the amplitude A used during the sweet spot search is titrated down until the patient no longer feels paresthesia. This sub-perception bipole 297a is provided to the patient for a duration, such as a few days, which allows the sub-perception bipole's potential effectiveness to “wash in,” and allows the patient to provide feedback concerning how well the bipole 297a is helping their symptoms. Such patient feedback can comprise a pain scale ranking. For example, the patient can rank their pain on a scale from 1-10 using a Numerical Rating Scale (NRS) or the Visual Analogue Scale (VAS), with 1 denoting no or little pain and 10 denoting a worst pain imaginable. As discussed in the '539 application, such pain scale ranking can be entered into the patient's external controller 45.

[0051] After the bipole 297a is tested at this first location, a different combination of electrodes is chosen (anode electrode E3, cathode electrode E4), which moves the location of the bipole 297 in the patient's tissue. Again, the amplitude of the current A may need to be titrated to an appropriate sub-perception level. In the example shown, the bipole 297a is moved down one electrode lead, and up the other, as shown by path 296 in the hope of finding a combination of electrodes that covers the pain site 298. In the example of FIGS. 6A and 6B, given the pain site 298's proximity to electrodes E13 and E14, it might be expected that a bipole 297a at those electrodes, as reflected in FIG. 6B, will provide the best relief for the patient, as reflected by the patient's pain score rankings. The particular stimulation parameters chosen when forming bipole 297a can be selected at the GUI 64 of the clinician programmer 50 or other external device (such as a patient external controller 45) and wirelessly telemetered to the patient's IPG or ETS for execution. Note, as used herein, the term spinal cord stimulator may refer to both an IPG or an ETS.

[0052] While the sweet spot search of FIGS. 6A and 6B can be effective, it can also take a significantly long time when sub-perception stimulation is used. As noted, sub-perception stimulation is provided at each bipole 297 location for a number of days, and because a large number of bipole locations are chosen, the entire sweep spot search can take up to a month to complete.

[0053] The inventor has discovered that effective sub-perception stimulation can be achieved without conducting the cumbersome sweet spot search described with respect to FIGS. 6A and 6B. An aspect of this disclosure is directed to sub-perception stimulation using virtual poles that encompass a plurality of electrodes and that are large enough to cover the patient's pain site without extensive sweet spot searching. Virtual poles are discussed further in U.S. Pat. No. 10,881,859, issued Jan. 5, 2021, which is incorporated herein by reference in its entirety, and thus virtual poles are only briefly explained here. Forming virtual poles is assisted if the stimulation circuitry 28 or 44 used in the IPG or ETS is capable of independently setting the current at any of the electrodes—what is sometimes known as a Multiple Independent Current Control (MICC), which is explained further below with reference to FIG. 8.

[0054] When a virtual bipole is used, the GUI 64 (FIG. 5) of the clinician programmer 50 (FIG. 4) can be used to define an anode pole (+) and a cathode pole (−) at positions that may not necessarily correspond to the position of the physical electrodes. The control circuitry 70 in the clinician programmer 50 can compute from these positions and from other tissue modeling information which physical electrodes will need to be selected and with what amplitudes to form the virtual anode and virtual cathode at the designated positions. As described earlier, amplitudes at selected electrodes may be expressed as a percentage X % of the total current amplitude A specified at the GUI 64 of the clinician programmer 50.

[0055] According to some embodiments, the clinician programmer 50 can be used to assign percentages of the anodic and cathodic currents to be shared by the various electrodes. For example, the user may use the GUI 64 to select particular electrodes and assign the percentages of the anodic or cathodic currents that those electrodes should share. The inventor has discovered that effective sub-perception stimulation can be achieved when current is shared among a relatively large number of electrodes so as to generate a virtual pole that covers the region of the patient's pain. FIG. 7 illustrates one example of this approach. Assume that the patient's pain site 298 is within a tissue region 299, as was described above with respect to FIGS. 6A and 6B. The exact pain site 298 is not known. As described above, a cumbersome sweet spot searching routine could be performed to identify a specific, focused bipole that covers that pain site. However, the inventor has discovered that using a plurality of anodes to share anodic current and a plurality of cathodes to share cathodic current in or near the tissue region 299 provides a virtual bipole 291 that covers the pain site 298 without having to use a sweet spot search. In FIG. 7, current is shared among each of electrodes E1-E16 near and within the tissue region 299 to make the virtual bipole 291. Specifically, anodic current is shared essentially equally among electrodes E1-E4 and E9-E12 to provide a virtual anode pole, and cathodic current is shared essentially equally among electrodes E5-E8 and E13-E16 to provide a virtual cathode pole. The virtual anode and cathode poles combine to provide the virtual bipole 291. Note that if biphasic pulses are used, then the polarities of the electrodes switch during the second phase of the pulse. Other electrode configurations for providing a large virtual bipole are possible and some other configurations will be described below. According to some embodiments, anodic current is shared or fractionalized among three or more electrodes to make a virtual anode and cathodic current is shared of fractionalized among three or more electrodes to make a virtual cathode. According to some embodiments, the three or more anodes may be adjacent with each other and the three or more cathodes may be adjacent with each other. According to some embodiments, the total current amplitude shared amongst the electrodes is below the perception threshold of the patient.

[0056] As mentioned above, stimulation circuitry capable of Multiple Independent Current Control (MICC) can be used to make the virtual bipoles, as described herein. Multiple Independent Current Control (MICC) is explained in one example with reference to FIG. 8, which shows the stimulation circuitry 28 (FIG. 1) or 44 (FIG. 3) in the IPG or ETS used to form prescribed stimulation at a patient's tissue. The stimulation circuitry 28 or 44 can control the current or charge at each electrode independently, and using GUI 64 (FIG. 5) allows the current or charge to be steered to different electrodes, which is useful for example when fractionalizing the total anodic and cathodic current among the electrodes, as shown in FIG. 7. The stimulation circuitry 28 or 44 includes one or more current sources 440.sub.i and one or more current sinks 442.sub.i. The sources and sinks 440.sub.i and 442.sub.i can comprise Digital-to-Analog converters (DACs), and may be referred to as PDACs 440.sub.i and NDACs 442.sub.i in accordance with the Positive (sourced, anodic) and Negative (sunk, cathodic) currents they respectively issue. In the example shown, a NDAC/PDAC 440.sub.i/442.sub.i pair is dedicated (hardwired) to a particular electrode node ei 39. Each electrode node ei 39 is preferably connected to an electrode Ei 16 via a DC-blocking capacitor Ci 38, which act as a safety measure to prevent DC current injection into the patient, as could occur for example if there is a circuit fault in the stimulation circuitry 28 or 44. PDACs 440.sub.i and NDACs 442.sub.i can also comprise voltage sources.

[0057] Proper control of the PDACs 440.sub.i and NDACs 442.sub.i via GUI 64 allows any of the electrodes 16 and the case electrode Ec 12 to act as anodes or cathodes to create a current through a patient's tissue. Such control preferably comes in the form of digital signals Tip and Iin that set the anodic and cathodic current at each electrode Ei. If for example it is desired to set electrode E1 as an anode with a current of +3 mA, and to set electrodes E2 and E3 as cathodes with a current of −1.5 mA each, control signal I1p would be set to the digital equivalent of 3 mA to cause PDAC 440.sub.1 to produce +3 mA, and control signals I2n and I3n would be set to the digital equivalent of 1.5 mA to cause NDACs 442.sub.2 and 442.sub.3 to each produce −1.5 mA. Note that definition of these control signals can also occur using the programmed amplitude A and percentage X % set in the GUI 64. For example, A may be set to 3 mA, with E1 designated as an anode with X=100%, and with E2 and E3 designated at cathodes with X=50%. Alternatively, the control signals may not be set with a percentage, and instead the GUI 64 can simply prescribe the current that will appear at each electrode at any point in time.

[0058] In short, the GUI 64 may be used to independently set the current at each electrode, or to steer the current between different electrodes. This is particularly useful in forming virtual bipoles, which as explained earlier involve activation of more than two electrodes. MICC also allows more sophisticated electric fields to be formed in the patient's tissue.

[0059] Other stimulation circuitries 28 can also be used to implement MICC. In an example not shown, a switching matrix can intervene between the one or more PDACs 440.sub.i and the electrode nodes ei 39, and between the one or more NDACs 442.sub.i and the electrode nodes. Switching matrices allows one or more of the PDACs or one or more of the NDACs to be connected to one or more electrode nodes at a given time. Various examples of stimulation circuitries can be found in U.S. Pat. Nos. 6,181,969, 8,606,362, 8,620,436, 10,912,942, U.S. Patent Application Publication 2018/0071513, and 2018/0071520.

[0060] Much of the stimulation circuitry 28 or 44, including the PDACs 440.sub.i and NDACs 442.sub.i, the switch matrices (if present), and the electrode nodes ei 39 can be integrated on one or more Application Specific Integrated Circuits (ASICs), as described in U.S. Patent Application Publications 2012/0095529, 2012/0092031, and 2012/0095519. As explained in these references, ASIC(s) may also contain other circuitry useful in the IPG 10, such as telemetry circuitry (for interfacing off chip with the IPG's or ETS's telemetry antennas), circuitry for generating the compliance voltage VH that powers the stimulation circuitry, various measurement circuits, etc.

[0061] FIG. 9 illustrates another electrode configuration for providing a large virtual dipole for providing sub-perception stimulation therapy. The electrode leads 15 (or 15′) each comprise 16 electrodes in this illustration. For the sake of clarity, the electrodes are not individually labeled (e.g., by E1, E2, etc.). The numbers by each electrode denote the percentage of current each electrodes share. In the illustration, cathodic current is shared among a plurality of cathodes to provide a virtual cathode pole 902. According to some embodiments, the cathodes may comprise three or more electrodes that may be adjacent to each other. Anodic current is shared among a plurality of anodes to provide a virtual anode pole 904. According to some embodiments, the anodes may comprise three or more electrodes, which may be adjacent to each other. The virtual cathode pole and the virtual anode pole combine to form a virtual bipole that essentially covers the entire length of the leads in the illustrated example.

[0062] Notice in FIG. 9 that the virtual cathode pole 902 comprises rostral cathodes 902r, middle cathodes 902m, and caudal cathodes 902c. Likewise, the virtual anode pole 904 comprises rostral anodes 904r, middle anodes 904m, and caudal anodes 904c. In the illustrated embodiment, the rostral cathodes 902r and caudal cathodes 902c each share more cathodic current than do the middle cathodes 902m. Similarly, the rostral and caudal anodes (904r and 904c, respectively) share more of the anodic current than do the middle anodes 904m. Stated differently, the distribution of the current among the electrodes of each of the virtual poles is weighted m or heavily among the electrodes at the edges of the poles, compared to the electrodes in the middle of the poles. The inventor has found that such current distributions may provide more uniform electric fields in the patient's tissue.

[0063] As described above, using large virtual bipoles that cover a large region where the patient's pain site is expected to be located may obviate the need to do extensive tedious sweet spot searching. This can significantly accelerate the fitting process for the clinician and the patient. Another aspect of the fitting process is determining appropriate stimulation parameters to treat the patient's pain and to avoid unwanted side effects. Such stimulation parameters include parameters such as stimulation amplitude, pulse width, and the like. A traditional fitting process typically involves trying many different stimulation parameters to find the best stimulation amplitude, pulse width, etc. for the patient. That can be difficult and time consuming, especially with sub-perception therapy, because of the lack of paresthesia and delayed patient feedback due to the lengthy wash-in period discussed above.

[0064] The traditional fitting procedure assumes that there is one optimal sweet spot and set of stimulation parameters (amplitude, pulse width, etc.) for treating the patient. The techniques described above obviate the need for finding the optimal sweet spot by using a large virtual bipole that effectively covers a large area in the region of the patient's pain center. The inventor has also found that the need to find the perfect amplitude, pulse width, etc., may be obviated in some instances by ramping the neural dosage of stimulation provided to a patient over a significantly long duration.

[0065] FIGS. 10A and 10B illustrate two embodiments of stimulation waveforms wherein the amplitude is ramped between a first amplitude A(1) and a second amplitude A(2) over a time duration dT. Note that only one “pole” of the stimulation is illustrated, which in this case is the anodic-first pole. A complementary cathodic-first waveform would typically be issued at one or more other electrodes to sink the current injected into the tissue by the illustrated waveform, as described above. Both A(1) and A(2) are below the patient's perception threshold. The clinician may determine the minimum stimulation amplitude at which the patient perceives the stimulation. The highest amplitude of the illustrated sub-perception waveform (A(2) in the illustration) may be set at about 80% of the patient's determined perception threshold amplitude value, for example.

[0066] Assume that the optimal stimulation amplitude is at some amplitude value between A(1) and A(2), but the actual value of the optimal amplitude is unknown. Rather than trying to identify a single optimum stimulation amplitude, ramping the amplitude slowly between A(1) and A(2) over a duration dT may provide effective pain-relieving stimulation. In FIG. 10A, the amplitude is ramped by increasing the amplitude with each pulse. In FIG. 10B, the amplitude is ramped by issuing a first plurality of pulses having a first amplitude, then a second plurality of pulses having a slightly higher amplitude, and repeating the process until the final amplitude A(2) is reached. In the embodiments illustrated in FIGS. 10A and 10B, the amplitudes are ramped from a lower amplitude value to a higher amplitude value (i.e., A(2)>A(1)). That is, the ramping rate dA/dT is positive. But other embodiments may involve ramping from a higher amplitude to a lower amplitude (i.e., A(2)<A(1)). In either case, the ramping function is typically a monotonic function.

[0067] The ramping of stimulation amplitude is described in the prior art. But typically, prior art modalities that involve the ramping of stimulation amplitude involve ramping the amplitude from an initial value to a final value over a duration that is on the order of milliseconds. By contrast, the paradigms described in this disclosure typically involve ramping the amplitude over a duration (dT) of a few seconds to tens of seconds. For example, the time duration dT may be about 4 seconds to about 20 seconds. Of course, the duration could be longer or shorter. Stated differently, the ramping rates (dA/dT) used in the methods described herein are typically much lower than those described in the prior art. Typically, IPGs such as those described herein, are capable of producing stimulation pulses on the order of about 12 mA. So, if the stimulation amplitude is ramped over the entire 12 mA range of the IPG over a duration of 4 seconds, then the ramp rate dA/dT is 3 mA/second. According to most embodiments described herein, the stimulation amplitude remains below the patient's perception threshold, which is likely well below 12 mA. Accordingly, the value of dA is typically well below the entire range provided by the IPG. For example, dA may more typically be about 6 mA or less. Likewise, the duration dT is typically longer, for example 3 seconds or longer. Ramping over a dT of 6 mA over a duration of 3 seconds yields a ramping rate (dA/dT) of 2 mA/second. Accordingly, embodiments of the disclosed methods involve ramping the amplitudes at a rate of less than 6 mA/second, or less than 4 mA/second, or less than 2 mA/second, or less than 1 mA/second. Once the ramping reaches the second (i.e., final) amplitude A(2), one more pulses of stimulation at the second amplitude may be delivered, followed by a duration of delivering no stimulation. The duration of delivering no stimulation may be on the order of one second, for example. The ramping sequence may begin again, using stimulation having the initial amplitude A(1). Alternatively, once the ramping reaches the final amplitude, the pattern may restart at the initial amplitude immediately without a period of delivering no stimulation. Still alternatively, once the ramping reaches the final amplitude, the amplitudes may ramp back down to the initial amplitude.

[0068] It should be noted that the currents of the waveforms illustrated in FIGS. 10A and 10B may be shared amongst multiple electrodes. For example, the currents can be shared among three or more electrodes to form large virtual poles to cover an area of the patient's tissue, as described above. It should also be noted that the waveforms illustrated in FIGS. 10A and 10B comprises symmetric biphasic pulses. However, different pulse shapes may be used. For example, monophasic pulses may be used, in which case, passive charge recovery may be used to recover charge from the patient's tissue. Asymmetric biphasic pulses may be used, for example, wherein the anodic and cathodic phases have different amplitudes and durations. The interval between the anodic and cathodic phases, i.e., the interphase interval (IPI), may be varied, as is known in the art.

[0069] Other parameters besides (or in addition to) the stimulation amplitude may be varied while delivering the stimulation. For example, the pulse width may be varied. It is believed that electrical stimulation generally recruits larger neural fibers more easily (or more quickly) than smaller fibers. Accordingly, stimulation having a relatively short pulse width is likely to recruit a higher percentage of larger fibers, whereas stimulation having a longer pulse width is likely to recruit more smaller fibers in addition to the larger fibers. Using traditional fitting paradigms, a clinician would endeavor to optimize the pulse width of the stimulation using a time-intensive trial-and-error process. The inventor has discovered that varying the stimulation pulse width while providing stimulation can be effective for providing the patient with beneficial therapy.

[0070] FIG. 11 illustrates a stimulation waveform wherein both the stimulation amplitude and the pulse width are varied. The amplitude of the pulses is varied (specifically, ramped) from an initial amplitude of A(1) to a final amplitude of A(2) over the duration dT. Note that in FIG. 11A(1) is greater than A(2). In other words, FIG. 11 illustrates an embodiment of ramping the amplitude using a negative rate (dA/dT), in contrast to the positive ramping rate illustrated in FIGS. 10A and 10B. The pulse width of the stimulation is varied from an initial pulse width of PW(1) to a final pulse width of PW(2). The pulse widths of the pulses are increased as the amplitudes are decreased. According to some embodiments, the amplitudes and pulse widths may each be varied so that the patient receives a constant dosage of current for each amplitude-pulse width combination. One example of a waveform wherein both the amplitude and the pulse widths of the pulses vary, is a waveform comprising a first plurality of pulses having amplitude of 6 mA and pulse widths of 100 μs, a second plurality of pulses having amplitudes of 3 mA and pulse widths of 200 μs, and a third plurality of pulses having amplitudes of 2 mA and pulse widths of 400 μs. Once the final plurality of pulses has been delivered, stimulation may be turned of for a duration and the sequence may be repeated. As discussed above, the currents of the illustrated waveforms may be shared among a plurality of anodes/cathodes to generate large virtual poles.

[0071] Various aspects of the disclosed techniques, including processes implementable in the IPG or ETS, or in external devices such as the clinician programmer or external controller to render and operate the GUI 64, can be formulated and stored as instructions in a computer-readable media associated with such devices, such as in a magnetic, optical, or solid state memory. The computer-readable media with such stored instructions may also comprise a device readable by the clinician programmer or external controller, such as in a memory stick or a removable disk, and may reside elsewhere. For example, the computer-readable media may be associated with a server or any other computer device, thus allowing instructions to be downloaded to the clinician programmer system or external controller or to the IPG or ETS, via the Internet for example.

[0072] Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. 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 invention. Thus, the present invention is intended to cover alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.