System and method of rapid, comfortable parameter switching in spinal cord stimulation

[0001] The present application claims the benefit of U.S. Provisional Application Ser. No. 60/311,856, filed Aug. 13, 2001, which application is incorporated herein by reference.

[0002] The present invention relates to Spinal Cord Stimulation (SCS) systems and more particularly to a system and method for rapidly switching between SCS system stimulation parameter sets. An SCS system treats chronic pain by providing electrical stimulation pulses through the electrodes of an electrode array, which electrode array is placed epidurally next to a patient's spinal cord. The stimulation parameter set specifies both the characteristics of the stimulation pulses provided through the electrode array, and the electrodes used to provide the stimulation pulses. An effective stimulation parameter set for a specific patient may be determined from the response of the patient to various stimulation parameters sets. However, there may be a very large number of stimulation parameter sets, and evaluating all possible sets is very time consuming, and may be impractical.

[0003] Spinal cord stimulation is a well accepted clinical method for reducing pain in certain populations of patients. An SCS system typically includes an Implantable Pulse Generator (IPG), electrodes, electrode lead, electrode lead extension, and at least one stimulation parameter set. The electrodes are implanted along the dura of the spinal cord, and the IPG generates electrical pulses which are delivered, through the electrodes, to the fibers of the spinal cord, according to the stimulation parameter set in use. Individual electrode contacts (the “electrodes”) are arranged in a desired pattern and spacing in order to create an electrode array. Individual wires within one or more electrode leads connect with each electrode in the array. The electrode leads exit the spinal column and may attach to one or more electrode lead extensions. The electrode lead extensions, or the leads, are typically tunneled around the torso of the patient to a subcutaneous pocket where the IPG is implanted.

[0004] Spinal cord stimulators and other stimulation systems are known in the art. For example, an implantable electronic stimulator is disclosed in U.S. Pat. No. 3,646,940 issued Mar. 7, 1972 for “Implantable Electronic Stimulator Electrode and Method” that provides timed sequenced electrical pulses to a plurality of electrodes. As another example, U.S. Pat. No. 3,724,467 issued Apr. 3, 1973 for “Electrode Implant For The Neuro-Stimulation of The Spinal Cord,” teaches an electrode implant for the neuro-stimulation of the spinal cord. A relatively thin and flexible strip of physiologically inert plastic is provided as a carrier on which a plurality of electrodes are formed. The electrodes are connected by leads to an RF receiver, which is also implanted.

[0005] In U.S. Pat. No. 3,822,708, issued Jul. 9, 1974 for “Electrical Spinal Cord Stimulating Device and Method for Management of Pain,” another type of electrical spinal cord stimulation device is taught. The device disclosed in the '708 patent has five aligned electrodes which are positioned longitudinally on the spinal cord. Electrical pulses applied to the electrodes block perceived intractable pain, while allowing passage of other sensations. A patient operated switch allows the patient to adjust the stimulation parameters.

[0006] Most of the electrode arrays used with known SCS systems employ between 4 and 16 electrodes. At least two electrodes are selectively programmed to act as anodes and cathodes, creating a bipolar stimulating group, or at least one electrode is selected to cooperate with the IPG case acting as a ground, creating a monopolar stimulation group. The number of stimulation groups available, combined with the ability of integrated circuits to generate a variety of complex stimulation pulses and pulse trains, presents a multiplicity of candidate stimulation parameter sets to the clinician. When an SCS system is implanted, a fitting procedure is performed to select at least one stimulation parameter set for use by the particular patient. The stimulation parameter set is selected for both treatment efficacy, and to minimize power consumption. It is desirable to try as many different stimulation parameter sets as possible during a fitting session in order to increase the likelihood of finding a near optimal set.

[0007] A known fitting method is to manually test one parameter set, and then to manually deactivate the stimulation, select a new stimulation parameter set, and then to slowly increase the stimulation level from a zero energy setting until the patient can perceive and assess the effect of the new stimulation parameter set. The testing of each stimulation parameter set is initiated at a zero energy setting. While this is a relatively safe procedure and avoids startling the patient during fitting, it is a slow procedure which prevents large numbers of combinations of parameters from being tested in a reasonable period of time. An alternative approach is to simply switch to a new stimulation parameter set at a stimulation energy setting similar to a level previously exercised. However, even if the new stimulation parameter set provides good results, switching parameter sets abruptly may provide an unpleasant sensation to the patient.

[0008] What is therefore needed is a system or method to quickly switch between stimulation parameter sets without causing uncomfortable sensations to the patient.

[0009] The present invention addresses the above and other needs by providing a system or method for rapidly switching between stimulation parameter sets of a Spinal Cord Stimulation (SCS) system, which system or method increases the number of stimulation parameter sets which may be tested during a fitting procedure, or alternatively, reduces the time required for the fitting procedure. The switching system or method comprises the following steps, or means for achieving the following steps: selecting a new stimulation parameter set, setting the initial stimulation level at or below an estimated perception threshold of the patient for the new stimulation parameter set (the perception levels may be estimated based on previous stimulation results), increasing the stimulation level to determine a perceived optimal stimulation level, and/or a perceived maximum stimulation level (i.e., a discomfort level.)

[0010] In accordance with one aspect of the invention, there is provided a system or method for rapidly measuring the effectiveness of a large number of stimulation parameter sets. Each new stimulation parameter set is initiated at a stimulation level at, or just below, the estimated perception threshold for the stimulation parameter set, and then increased. As a result, the time required to increase (i.e, ramp up) from a zero energy setting, to the perception threshold, is eliminated.

[0011] It is a further feature of the invention to prevent discomfort resulting from initiating a new stimulation parameter set at a perceivable stimulation level. As a result of activating the new stimulation set at or below the perception threshold, the patient has no perception of stimulation upon activation.

[0012] It is an additional feature of the invention to facilitate the selection of a low energy level stimulation parameter set. The energy consumption of an SCS system is an important system parameter because high energy consumption may accelerate battery failure and create a requirement for battery replacement. By allowing the comparison of more stimulation parameter sets, a lower energy level stimulation parameter set is more likely to be tested and selected for inclusion in the SCS system.

[0025] Corresponding reference characters indicate corresponding components throughout the several views of the drawings.

[0026] The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims.

[0027] Spinal Cord Stimulation (SCS) systems treat chronic pain by providing electrical stimulation pulses through the electrodes of an electrode array, which electrode array is placed epidurally next to a patient's spinal cord. Elements comprising a typical SCS system 10 are shown in FIG. 1. An SCS system 10 typically comprises an Implantable Pulse Generator (IPG) 12, a lead extension 14, an electrode lead 16, and an electrode array 18 residing on a distal end of the lead 16. The IPG 12 generates stimulation pulses for implanted electrodes that make up the electrode array 18. A proximal end of the lead extension 14 is removably connected to the IPG 12 and a distal end of the lead extension 14 is removably connected to a proximal end of the electrode lead 16. The in-series combination of the lead extension 14 and electrode lead 16, carry the stimulation pulses from the IPG 12 to the electrode array 18.

[0028] The SCS system 10 described In FIG. 1 above is depicted implanted in a spinal column 21 in FIG. 2. The electrode array 18 is implanted at the site of nerves that are the target of stimulation, e.g., along the spinal cord. Due to the lack of space near the location where the electrode lead 16 exits the spinal column 21, the IPG 12 is generally implanted in the abdomen or above the buttocks. The lead extension 14 facilitates locating the IPG 12 away from the electrode lead exit point.

[0029] FIGS. 1 and 2 depict a typical SCS system, and other stimulation systems may utilize different components, utilize similar components organized differently, or be implanted differently. The present invention may be exercised with any stimulation system utilized to treat perceived pain, which system utilizes a fitting procedure to adjust the stimulation parameters to a patient. Therefore, the application of the present invention to any stimulation system is intended to come within the scope of the present invention.

[0030] An SCS system 10 may use a single in-line electrode array (one dimensional) or may use two or more in-line arrays 20 to create a two dimensional array of electrodes 22 as shown in FIG. 3A. An SCS system 10 may also use one or more paddle arrays 24 with two or more columns of electrodes 26 to create a two dimensional array as shown in FIG. 3B. Multiple in-line arrays 20 or a paddle array 24 may be connected to an IPG 12 as shown in FIG. 1.

[0031] The SCS system 10 is entirely implanted in a patient 28 in normal use, but may be connected to a programming computer 30 during fitting. A fitting suite used during initial implantation is shown in FIG. 4A. During the initial implantation an External Trial Stimulator (ETS) 32 is electrically connected by a transcutaneous cable 34 to the electrode lead 16. A second cable 36 connects the ETS 32 to the programming computer 30.

[0032] A second fitting suite shown in FIG. 4B is used following implantation of the IPG 12. The second suite includes a hand held programmer (or patient programmer) 38 which wirelessly communicates through the skin with the IPG 12. The programmer 38 also communicates with the programming computer 30 using a third cable 40, an RF link, an IR link, or some other communications technique.

[0033] The method of the present invention may be exercised in the context of the first fitting suite, the second fitting suite, or any other system adaptable to the fitting procedure. The exercise of the present invention with any fitting procedure using any equipment is intended to come within the scope of the present invention.

[0034] A plot of stimulation level (i.e., energy setting) trajectories for a first known (i.e., prior art) method of measuring the effectiveness of a multiplicity of stimulation parameter sets is depicted in FIG. 5. Each stimulation parameter set comprises a unique combination of electrodes, electrode polarity, relative amplitude, pulse width, and pulse rate. The perception threshold (the stimulation level where the patient starts to feel the effect of the stimulation) is plotted as a solid dot, the optimal stimulation level is plotted as a circle, and the discomfort threshold (the stimulation level where the stimulation itself results in discomfort apart from the pain being treated) is plotted as an “X”. The vertical axis is the stimulation level, and the horizontal axis corresponds to different candidate stimulation parameter sets. The stimulation levels are initiated at zero and increased slowly to the perception threshold (i.e., where the patient first feels the stimulation), and on to the optimum stimulation level (i.e., where the patient feels that optimal stimulation has been reached.) Additionally (not shown in FIG. 5), the stimulation level may be further increased to determine the discomfort threshold. As is evident from FIG. 5, the first known method wastes a significant amount of time reaching the perception threshold.

[0035] A plot of a second known (i.e., prior art) method is depicted in FIG. 6. The method of FIG. 6 activates stimulation at an estimate (plotted as an “e”) of the optimal stimulation level. The patient or clinician may vary the stimulation level about the estimated optimal level, and locate an optimal level as perceived by the patient. This method eliminates the time spent between the zero stimulation level and the perception threshold, but has a drawback in that initiating the stimulation at a perceivable level of stimulation may cause an unpleasant sensation. Such unpleasant sensation may prejudice the patient against the present stimulation parameter set and against future parameter set switching, and thereby bias the overall results.

[0036] A plot of stimulation levels resulting from exercise of a first method of the present invention is depicted in FIG. 7. The first method reduces the time required by known methods to converge to an optimal stimulation level by activating stimulation at a stimulation level at or just below an estimated perception threshold for the stimulation parameter set being tested (i.e., for a trial stimulation parameter set). The trial stimulation parameter set is selected from a set of candidate stimulation parameter sets comprising all of the stimulation parameter sets being considered for the patient. The trial stimulation parameter set is generally an untested candidate stimulation parameter set. After initiating stimulation, the stimulation level is then increased as shown in FIG. 7, and the patient indicates, for example, when a perceived optimal stimulation level is reached, thereby providing a measurement of the optimal stimulation level for the trial stimulation parameter set. The stimulation level may also be varied around the optimal stimulation level to ensure that the level first identified by the patient is truly the optimal level. The patient may also indicate when the perception threshold is reached, and/or when the discomfort threshold is reached.

[0037] After the optimal stimulation levels have been measured for all of the candidate stimulation parameter sets, the stimulation may be switched between two or more candidate stimulation parameter sets to determine which set provides the best results. In these cases, stimulation may be activated at or just below the stimulation threshold, and then the stimulation level may be increased manually or automatically to the optimal stimulation level measured previously for the candidate stimulation parameter set being exercised.

[0038] Because energy consumption is an important parameter of an implantable stimulation system, the energy consumptions of the candidate stimulation parameter sets may be recorded at the minimum effective stimulation level and/or optimal stimulation level of each candidate stimulation parameter set. Following the testing of all of the candidate stimulation parameter sets, a candidate stimulation parameter set providing effective stimulation with a low energy consumption may be selected as the stimulation parameter set to be used by the patient. Alternatively, one of a plurality of candidate stimulation parameter sets providing similar optimal stimulation results may be selected based on the energy consumption required for optimal stimulation.

[0039] A method for obtaining estimates of the perception thresholds by measuring a subset of perception thresholds, and then estimating (e.g., by interpolating) the perception thresholds for each untested stimulation parameter set, is described in U.S. Pat. No. 6,393,325 issued May 21, 2002 for “Directional Programming for Implantable Electrode Arrays.” The '325 patent is incorporated herein by reference.

[0040] A plot of stimulation levels resulting from exercise of a second embodiment of the present invention, wherein the discomfort threshold is being investigated, the stimulation level trajectories are as shown in FIG. 8. Stimulation is activated at or just below the perception threshold, as in FIG. 7, but instead of only searching for the optimal stimulation level, the stimulation level is increased until the patient indicates that the stimulation is resulting in discomfort independent of the pain being treated.

[0041] In the embodiments described in FIGS. 7 and 8, the methods may be modified by varying the stimulation level around the optimal stimulation level, and/or the discomfort threshold, to fine tune the measurement of the optimal stimulation level, and/or the discomfort threshold. The patient may directly vary the stimulation level, or a clinician may vary the stimulation level based on observations by the patient. Further, as the perception thresholds for additional stimulation parameter sets are measured, the methods of the '325 patent may be reapplied to refine the estimated perception thresholds for stimulation parameter sets yet to be tested.

[0042] A method for carrying out the present invention is depicted in FIG. 9 and may be described as including the following steps:

[0043] Testing a subset of candidate stimulation parameter sets;

[0044] Collecting perception level data for the tested candidate stimulation parameter sets;

[0045] Using the perception level data collected to estimate the perception level for the untested candidate stimulation parameter sets;

[0046] Selecting a trial stimulation parameter set from the untested stimulation parameter set;

[0047] Providing stimulation using the trial stimulation parameter set using the estimated perception threshold as the initial stimulation level;

[0048] Increasing the stimulation level;

[0049] Measuring the perception threshold, and/or the optimal stimulation level, and/or the discomfort threshold, and/or the energy consumption at the optimal stimulation level for the trial stimulation parameter set;

[0050] Selecting an untested candidate stimulation parameter set as a new trial stimulation parameter set, and repeating the above; and

[0051] After all of the candidate stimulation parameter sets have been tested, selecting the best candidate stimulation parameter set.

[0052] The method described in FIG. 9 may further include the following:

[0053] Varying the stimulation level to refine the perception threshold, and/or the optimal stimulation level, and/or the discomfort level;

[0054] Recording the perception threshold, and /or the optimal stimulation level, and/or the discomfort level, and/or the energy consumption at the optimal stimulation level; or

[0055] Holding the stimulation level at a fixed stimulation level to characterize the effect of stimulation at the fixed stimulation level for a short period of time.

[0056] The basis for selecting the best candidate stimulation parameter set may be one of the measured parameters, or a combination of the measured stimulation parameters. Also, more than one candidate stimulation parameter sets may be selected, wherein the patient may control which one of a set of stimulation parameter sets is executed at any given time.

[0057] Those skilled in the art will recognize that other applications of stimulation will benefit from the present invention, (e.g., stimulation to relieve migraine headaches, or peripheral nerve stimulation) and the use of the present invention with those other applications is intended to come within the scope of the present invention. Further, the present invention applies to measuring the effectiveness of any of the stimulation parameters, where activation above the perception threshold may cause unpleasant sensations, and is not intended to be limited to the variation of any specific parameter.

[0058] As described above, the present invention allows rapid manual or automatic switching between stimulation parameter sets, while avoiding unpleasant side effects that may result from activating stimulation at a perceivable stimulation level. The method applies to measurement of the minimum stimulation level required for effective stimulation, the optimal stimulation level, and the discomfort threshold for a stimulation parameter set. Further, the method may be applied to comparing the effectiveness of several stimulation parameter sets. A stimulation system, and in particular an SCS system 10, may thus include at least one stimulation parameter set selected from a multiplicity of candidate stimulation sets, wherein the stimulation system provides stimulation based on efficiently comparing the performance of all of the multiplicity of stimulation parameter sets.

[0059] While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.