Abstract:
Apparatus for multi-channel transverse epidural spinal cord stimulation uses a multi-channel pulse generator driving a plurality of electrodes mounted near the distal end of a lead. These electrodes are mounted in one or more lines, generally perpendicular to the lead axis, and have a planar surface along one surface of the lead. The lead is implanted adjacent to spinal cord dura mater with the electrodes transverse and facing the spinal cord. Pulses generated by the pulse generator for each channel are normally simultaneous, of equal amplitude and of equal duration, however the pulse generator is arranged such that pulses for each channel can selectably alternate in time, can selectably be of unequal amplitude, or both. The changes in pulse timing and magnitude permit shifting the electrical stimulation field and the resulting paresthesia pattern after installation to accommodate improper lead placement or postoperative dislocation and to minimize unwanted motor responses.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to apparatus and method for electrically stimulating a spinal cord. More specifically, this invention relates to an apparatus and method for changing the intensity and location of resulting spinal cord stimulation by changing the pulse parameters of at least two separate voltage or current controlled sources applied to in line electrodes transverse to the spinal cord axis. 
     2. Description of the Prior Art 
     In epidural spinal cord stimulation (ESCS) two major practical problems reduce the efficacy of this therapy. One is the difficulty of directing the stimulation induced paresthesia to the desired skin areas and the other is the problem of motor responses to the stimulation, which reduces the amplitude range of the stimulation. It is generally agreed that in ESCS, for chronic pain, paresthesia should cover the whole pain region. With present stimulation methods and equipment only highly skilled and experienced surgeons are able to position the lead in such a way that the desired overlap is reached and desired results are obtained over time. It is difficult to focus the stimulation on the desired region during surgery and, with single channel approaches, impossible to refocus it afterwards, even though some small readjustments can be made by selecting a different contact combination, pulse rate, pulse width or voltage. 
     Especially the possibility of refocusing paresthesia after surgery would be highly desirable because, even if during surgery paresthesia covers the pain area perfectly, the required paresthesia pattern often changes later. This may be caused by such things as lead migration or histological changes, such as the growth of connective tissue around the electrode. The problem of lead placement has been addressed by U.S. Pat. No. 5,121,754 by the use of a lead with a deformable distal shape. 
     Using mathematical modeling we have discovered that the superposition of potential fields due to simultaneous stimulation by multiple pulse generators and connected electrodes will result in a significant change in the size and shape of the stimulated spinal cord area. This means that post-operative changes in stimulation fields can be obtained by selective parametric changes in the pulse generator outputs. Such changes in the stimulated spinal cord area will not only improve pain suppression but unwanted motor responses will be minimized or eliminated as well. These changes in stimulated area are impossible to obtain using a single channel stimulation. 
     U.S. Pat. No. 4,379,462 provides multiple electrodes but does not address the problem of post operative field changes and does not provide superimposed fields due to multiple channel stimulation. 
     U.S. Pat. No. 3,646,940 provides electrical means for locally stimulating masses of electrically excitable tissue using several pulse generators which are electrically connected to multiple electrodes at distant sites. The problem addressed includes bladder evacuation where an electrical pulse will contract the bladder but simultaneously contract the sphincter thus inhibiting evacuation. This problem is overcome by the use of a second time shifted electrical pulse to inhibit the sphincter response. This approach using separate bipolar electrodes to stimulate a nerve at multiple sites can not address the problem of the field superposition necessary to shift a stimulation field with respect to the spinal cord. This is because the stimulation sites according to this teaching are so far apart that the potential fields do not overlap, and thus will not give another field by linear superposition even if pulses are applied simultaneously to the two bipolar electrodes. Moreover, the precise and stable positioning of bipolar electrodes relative to each other necessary to establish desired and known field superposition is not obtainable by surgical implantation of separate electrode pairs. Therefore, this patent does not address the use of varying superimposed fields to vary the population of recruited nerve fibers. 
     The problems of directing stimulation induced paresthesia to desired skin areas, of unwanted motor responses to stimulation, of correcting for lead migration or incorrect positioning during surgery, and of making significant postoperative field changes have not been solved by existing apparatus and methods. 
     SUMMARY OF THE INVENTION 
     The apparatus of this invention provides a number of superimposed current generated electrical fields for epidural spinal cord stimulation. The apparatus uses a multi-channel neurological pulse generator which provides independently controlled voltage or current pulses. A lead connected to the pulse generator has electrodes at the distal end corresponding to the number of channels. The lead is implanted a few mm apart from the spinal cord with the electrode array transverse and facing the spinal cord. The pulses given by the stimulator channels are selectably simultaneous or alternate in time, are selectably equal or different in amplitude, or both. These capabilities permit shifting the electrical field after implantation to optimize the paresthesia effects or to eliminate unwanted motor responses. This use of multiple, superimposed potential fields, generated by transverse combinations of electrodes, results in different and variable stimulated spinal cord areas as compared to a single field, and thus provides a better controllable paresthesia effect. The various means provided for shifting and changing the stimulated spinal area postoperatively, whether used individually or collectively, permit tailoring the stimulated area to a particular individual&#39;s spinal cord site. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a patient with an implanted neurological stimulation system employing the present invention. 
     FIG. 2 is a cross sectional view of the spinal cord showing implantation of an insulated lead of the present invention. 
     FIG. 3 is a simplified geometric model of the cross section of the midcervical portion of the spinal cord used for computer modeling. 
     FIG. 4A is a schematic drawing of three in-line electrodes and their connections to two pulse generators. 
     FIG. 4B is a schematic drawing of a stimulating cathodal electrode and a distant anodal electrode and their connections to one pulse generator used in monopolar stimulation. 
     FIG. 5 shows simultaneous pulses from two pulse generators. 
     FIG. 6 shows alternating pulses from two pulse generators. 
     FIG. 7 shows the resulting electrical potential field when a single pulse train of FIG. 5, generated by the circuit of FIG. 4B, is applied to the model, with the field being represented by isopotential lines. 
     FIG. 8 shows the resulting potential field when two simultaneous pulse trains of equal amplitude, generated by the circuit of FIG. 4A, are applied to the model. 
     FIG. 9 shows the recruited area related to the potential field shown in FIG. 7 using the single pulse train circuit of FIG. 4B. 
     FIG. 10 shows the recruited area related to the potential field of FIG. 8 with two simultaneous pulse trains of equal amplitude when using the circuit of FIG. 4A. 
     FIG. 11 shows the resulting potential field when the amplitude of the pulse train, generated by V2 of FIG. 4A, is set equal to zero, with electrodes 58 and 60 having the same negative voltage and both acting as cathodes. 
     FIG. 12 shows the recruited area related to the potential field of FIG. 11, with the pulse train generated between electrodes 58 and 60 having a zero amplitude such that the electrodes have the same negative voltage. 
     FIG. 13 shows the resulting potential field when two simultaneous pulse trains of equal amplitude are applied to the model with the center electrode offset 1.0 mm. from the spinal cord midline. 
     FIG. 14 shows the recruited area related to the potential field of FIG. 13 having two simultaneous pulse trains with the same amplitude and with the center electrode offset 1.0 mm from the spinal cord midline. 
     FIG. 15 shows the resulting potential field when two simultaneous pulse trains are applied to the model with the pulse amplitude between electrodes 56 and 58, V1, lower than the pulse amplitude between electrodes 58 and 60, V2, and the center electrode being offset 1.0 mm from the spinal cord midline. 
     FIG. 16 shows the recruited area related to the potential field of FIG. 15 having two simultaneous pulse trains with different amplitudes, and with the center electrode offset 1.0 mm from midline of the spinal cord. 
     FIG. 17 shows the recruited area when two simultaneous pulse trains of equal amplitude are applied to the model, with the center electrode centered at the spinal cord midline. 
     FIG. 18 shows the recruited area when the alternating pulse trains of equal amplitude from FIG. 6 are applied to the model. 
     FIG. 19 shows a schematic of the pulse generator driving a first embodiment of the lead. 
     FIG. 20 shows a schematic of the pulse generator driving a second embodiment of the lead. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic view of a patient 10 having an implant of a neurological stimulation system employing the present invention to stimulate spinal cord 12 of the patient. The preferred system employs implantable pulse generator 14 to produce a number of independent stimulation pulses which are sent to spinal cord 12 by insulated lead 16 and coupled to the spinal cord by electrodes located at point 18. 
     Implantable pulse generator 14 preferably is an ITREL IIR implantable pulse generator available from Medtronic, Inc. with provisions for multiple pulse outputs which are selectably either simultaneous or with one shifted in time with respect to the other, and which are selectably of independently varying amplitudes. This preferred system employs programmer 20 which is coupled via conductor 22 to radio frequency antenna 24. This permits attending medical personnel to select the various pulse output options after implant using radio frequency communications. While the preferred system employs fully implanted elements, systems employing partially implanted generators and radio-frequency coupling may also practice the present invention. 
     FIG. 2 is a cross sectional view of spine 12 showing implantation of the distal end of insulated lead 16 at point 18 within epidural space 26. Also shown is the subdural space 28 filled with cerebrospinal fluid (cfs), vertebral body 30, vertebral arch 31, and dura mater 32. 
     The following models were developed to compute the effects of multiple superimposed field stimulation of the spinal cord particularly related to the problems of paresthesia coverage and unwanted motor responses. The results obtained show that using multiple field stimulation it is possible to change the paresthesia pattern from symmetrical to asymmetrical or vice versa to correct for changes in paresthesia pattern due to postoperative lead displacement, and also to reduce the activation of dorsal root fibers in favor of dorsal column fibers to reduce the occurrence of motor responses. After the explanation of the models, the invention incorporating the information provided by the models will be described. 
     Two complementary models provide the theoretical basis for the instant invention. One model is a three dimensional volume conductor model of the spinal cord and its surroundings which incorporates the major macro anatomical structures with the corresponding electrical conductivities and the stimulating electrodes. The second model represents the electrical properties of the largest myelinated dorsal root and dorsal column nerve fibers. These models are extensively described by J. J. Struijk in his Doctor of Philosophy thesis at the University of Twente, the Netherlands &#34;Immediate Effects of Spinal Cord Stimulation&#34;, and in four publications in peer review journals (IEEE Trans on Biomed Engin, IEEE Trans on Rehab Engin). 
     In order to assess the direct effects of stimulation on the nerve fibers a two step procedure was followed. First, the potential field in the volume conductor model was calculated. Second, this field was applied to the nerve fiber model to determine which fibers are excited by the stimulation. The results of these calculations, shown in later figures as isopotential lines and nerve fiber recruitment areas in the dorsal columns of the spinal cord, provide the effects of changing various stimulation parameters. 
     Three dimensional volume conductor models of the spine 12 were developed using a simplified model of a transverse section of the midcervical spinal cord as shown in FIG. 3. A similar model of the midthoracic region was also studied. FIG. 3 shows the spinal cord composed of gray matter 40, white mater 42, cerebrospinal fluid (csf) 44, epidural space 46, vertebral bone 48, surrounding tissues represented by layer 50, and a thin layer of dura mater 54. This figure also shows electrode contact insulation 52 and electrical contacts 56, 58 and 60 for two channel stimulation. The electrodes 56, 58 and 60 are positioned in the dorsal epidural space 46 next to the dura mater 54. 
     The electrical conductivities for these various elements are given in the following table A. The thickness of the dorsal csf layer was measured from magnetic resonance imaging (MRI) scans obtained from twenty six subjects. In the midcervical and the midthoracic models the average thicknesses of the dorsal csf layers, 2.4 mm and 5.6 mm respectively, were used. This MRI study by Holsheimer et al. appears in Amer J. Neuroradiol. 
     The three dimensional volume conductor model was made up of discrete elements using a rectangular grid with variable grid spacings. The length of the model was 60 mm. The number of grid points was 57 times 57 times 57 which is equal to 185,193. A finite difference method was used to apply the governing Laplace equation to discrete elements. The resulting set of linear equations was solved using a Red-Black Gauss-Seidel iteration with variable overrelaxation. 
     A fiber model for the dorsal column fibers was based upon D.R. McNeal&#39;s &#34;Analysis of a model for excitation of myelinated nerve&#34;, IEEE Transactions Biom. Eng., Vol. 23, pp. 329-337, 1976. In the model used here collaterals entering the dorsal and ventral horns (grey matter) of the spinal cord model are attached to every second node of Ranvier of a 21-noded fiber. The diameters of these collaterals were one third of the diameter of the corresponding dorsal column fiber which was 10 micrometer. For the dorsal root fibers a cable model with a curved trajectory was used, the proximal end being connected to a dorsal column fiber model. The dorsal root fiber model had a diameter of 10 micrometer. In order to assess the direct effects of stimulation on the nerve fibers, the potential field in the volume conductor model is calculated and then the potential field is applied to the nerve fiber models to determine which fibers are excited by the stimulation. 
     
                       TABLE A______________________________________CONDUCTIVITIES OF THE VOLUME CONDUCTORCOMPARTMENTS  S/sq. m.}______________________________________grey matter         0.23white matter (longitudinal)               0.60white matter (transverse)               0.083cerebrospinal fluid 1.70epidural space      0.040dura matter         0.030vertebral bone      0.040surrounding layer   0.004electrode insulation               0.001______________________________________ 
    
     These models were used to evaluate the differences between a stimulation field developed by pulses from a single pulse generator and a stimulation field developed by two separate pulse sources. The circuit of FIG. 4A was used for the two sources stimulation model having electrodes 56, 58 and 60 and V1 voltage source 64 and V2 voltage source 66. Electrode 58 has a median position while electrodes 56 and 60 have lateral positions with respect to the spinal cord. 
     The circuit of FIG. 4B was used for a single source monopolar stimulation model having voltage source 65 applied between electrode 59 and the outside of layer 50 of the spinal cord model of FIG. 3. The outside of layer 50 is used for the reference connection because the positive anode of V3 from voltage source 65 is assumed to connect to the case of the implantable pulse generator, which is distant from the spinal cord. The electrode areas used in the models were approximately 12 square millimeters in size because this size has been approved by the United States Federal Drug Administration. The contact separation is larger than the thickness of the dorsal csf layer to reduce the shunting effect of this well conducting layer. The contact separation is on the order of the distance between the dorsal root entry zone and the spinal cord midline. In FIG. 4A, anode contacts 56 and 60 are longer than cathode contact 58. This provides a shielding effect by the outer (anodal) electrodes even if the lead is somewhat rotated in the coronal plane, which is the case if the lead has not been implanted perfectly rostrocaudally. The shielding effect will diminish slightly if the outer anodal electrodes 56 and 60 are shorter than the cathodal electrode 58. 
     V1, V2 and V3 pulses, generated by voltage sources 64, 66 and 65 respectively of FIGS. 4A and 4B, have a pulse width of 210 microseconds. There are two modes of operation for the two voltage sources 64 and 66. Mode one, shown in FIG. 5, has simultaneous outputs of V1 and V2 Mode two, shown in FIG. 6, has the outputs V1 and V2 offset in time. There is also an independent amplitude control of voltage sources 64 and 66 to provide different V1 and V2 amplitudes. 
     FIG. 7 shows the resulting potential field represented by isopotential lines 68 when the pulse is applied to the model by a single cathode 59 and a distant anode 50 as shown in FIG. 4B. FIG. 8 shows the resulting isopotential lines 68 when two pulses with equal amplitudes are simultaneously applied to the model, according to the scheme of FIG. 4A. FIG. 9 shows the resulting recruited area 70 of dorsal column fibers with a diameter of 10 micrometer when a single cathode 59 is used with the same model as that used in FIG. 7. FIG. 10 shows the recruited area 70 for two simultaneous pulses of equal amplitude using the same model as in FIG. 8. These figures show that for stimulation with a transversely positioned tripole the negative potentials and the recruited area of dorsal column fibers are more restricted to the medial part of the dorsal columns than in monopolar stimulation. 
     The shape of the recruited area of dorsal column fibers does not differ significantly when mono-, bi-, tri-, or quadripolar stimulation with a conventional longitudinal SCS electrode array is given, as was shown by Holsheimer et al. using the same type of model (Stereotact Funct Neurosurg 1991, Vol. 56, pp. 220-233). Calculations also showed that dorsal root fibers need higher voltages for their activation when stimulating with a transversely positioned tripole, which will reduce the probability of motor responses significantly. 
     The use of simultaneous pulses from two unbalanced sources results in a controllable asymmetrical stimulation which is impossible to attain with single source stimulation. The resulting isopotential lines 68 obtained when V2 of FIG. 4A is set equal to zero, with electrodes 58 and 60 having the same potential, and applied to the model is shown in FIG. 11. This shows how to obtain asymmetrical stimulation by merely using unbalanced sources with multiple electrodes in a transverse plane, even though the electrode positions are perfectly symmetrical. FIG. 12 shows that a large shift in the recruited area 70 of dorsal column fibers is also obtained using these unbalanced sources. The example shown here is the most extreme one with V2 equal to zero volts. 
     If the lead is not at the spinal midline due to lead migration, by lateral positioning during surgery, or to an asymmetrical position of the spinal cord in the spinal canal, it is still possible to obtain an almost symmetrical stimulation. FIG. 13 shows the resulting isopotential lines 68 and FIG. 14 shows recruited area 70 for an electrode offset of 1.0 mm from midline with V1 and V2 pulses simultaneous and of equal amplitude. The recruited area is asymmetrical even though the voltage sources are equal. 
     FIGS. 15 and 16 show the results with an electrode offset of 1.0 mm from midline and simultaneous inputs V1 and V2 of FIG. 4A set equal to 2.26 volts and 4.52 volts respectively to obtain an asymmetrical field. These figures show that the shape of potential field and recruited area are modified by this unbalanced input, resulting in an almost symmetrical recruited area 70 in the dorsal columns in FIG. 16. 
     FIG. 17 shows the recruited area 70 for equal amplitude simultaneous pulses applied to the model by a symmetrically positioned transverse electrode array, and FIG. 18 shows the recruited areas 70 for equal amplitude offset pulses of FIG. 6 applied to the model, which is the union of two asymmetrical recruited areas. 
     The results of this modeling indicate that areas of recruited spinal nerve fibers can be modified, when using more than one source for stimulation of the spinal cord versus Single source stimulation, in that a variety of parameters can now be changed to vary the stimulated area and intensity. These parameter changes can obviously be extended. For example, the effects of only two sources were investigated here, but these same parameters can also be changed if three, four or more independent sources were employed with analogous results. 
     The information developed using these models has been incorporated into this invention in two embodiments. FIG. 19 shows pulse generator 14 with positive going pulse outputs 72, 74, 76, and 78 with respect to ground reference 80. The outputs at 72, 74, 76, and 78 are each selectable in time as were V1 or V2 of FIG. 6, and each output can be changed in amplitude independent of the other outputs or can be electrically disconnected. Line 16 has electrodes 38 connected to these outputs with wire 80A connecting output 72 to electrode 38A, wire 80B connecting output 74 to electrode 38B, wire 80C connecting output 76 to electrode 38D, wire 80D connecting output 78 to electrode 38E, and wire 80E connecting ground reference 80 to electrode 38C. Electrodes 38 have different sizes with electrodes 38A, 38B, 38D and 38E, which are connected to the voltage outputs of pulse generator 14, wider than interspersed electrode 38C which is connected to ground reference 80. This provides the Improved shielding effect described previously. 
     With these connections and with the time and amplitude variables of pulse generator 14, a stimulation field will be set up between each electrode connected to a pulse generator output and the interleaved electrode connected to the pulse generator ground reference. The two modes of stimulation, shown in FIGS. 5 and 6, used in the modeling study are obtained by connecting pulse generator 14 to electrodes 38 as described above. If a smaller number of electrodes are used the unused outputs of pulse generator 14 are electrically disconnected. 
     FIG. 20 shows a second embodiment with pulse generator 14 having additional outputs with the same characteristics regarding the outputs, ground reference and capabilities as before, and with lead 17 having electrodes 39. In this second embodiment lead 17 has electrodes 39 connected to the outputs of pulse generator 14 differently, with wire 80A connecting output 72 to electrode 39A, wire 80B connecting output 74 to electrode 39C, wire 80C connecting output 76 to electrode 39D, wire 80D connecting output 78 to electrode 39F, wire 80G connecting output 82 to electrode 39G, and wire 80H connecting output 84 to electrode 39I. Wire 80E connecting electrode 39B to ground reference 80, wire 80F connecting electrode 39E to ground reference 80, and wire 80I connecting electrode 39H to ground reference 80, establish the ground connections. Electrode 39B is centered between the driven electrodes 39A and 39C. Similarly, the ground referenced electrode 39E is centered between the driven electrodes 39D and 39F, and electrode 39H is centered between electrodes 39G and 39I. 
     With this second embodiment, the stimulation can be applied at different spinal levels by using one out of three combinations 39A, B, and C; 39D, E, and F; or 39G, H, and I. Again, the unused outputs of pulse generator 14 are electrically disconnected. 
     This system provides the capability to change the depth and location of the stimulation by changing the amplitude or timing of one field with respect to another. The modeling of the fields described earlier shows that results are changed markedly by the use of multiple pulse generators connected to different electrodes positioned in a transverse plane with respect to the spinal cord. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be constructed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.