Spinal cord stimulation (SCS) is a minimally invasive therapy used for the treatment of chronic neuropathic pain. SCS is a safe and effective alternative to medications such as opioids, and multiple randomized controlled studies have demonstrated efficacy for difficult-to-treat neuropathic conditions such as failed back surgery syndrome. Conventional SCS is believed mediate pain relief via activation of dorsal column Aβ fibers, resulting in variable effects on sensory and pain thresholds, and measurable alterations in higher order cortical processing. Although potentiation of inhibition, as suggested by Wall and Melzack's gate control theory, continues to be the leading explanatory model, other segmental and supraspinal mechanisms have been described. Novel, non-standard, stimulation waveforms such as high-frequency and burst have been shown in some studies to be clinically superior to conventional SCS, however their mechanisms of action remain to be determined. Additional studies are needed, both mechanistic and clinical, to better understand optimal stimulation strategies for different neuropathic conditions, improve patient selection and optimize efficacy.

Spinal Cord Stimulation: Clinical Efficacy and Potential Mechanisms.

Human lumbar spinal cord networks controlling stepping and standing can be activated through posterior root stimulation using implanted electrodes. A new stimulation method utilizing surface electrodes has been shown to excite lumbar posterior root fibers similarly as with implants, an unexpected finding considering the distance to these target neurons. In the present study we apply computer modeling to compare the depolarization of posterior root fibers by both stimulation techniques. We further examine the potential for additional direct activation of motoneurons within the anterior roots. Using an implant, action potentials are initiated in the posterior root fibers at their entry into the spinal cord or along the longitudinal portions of the fiber trajectories, depending on the cathode position. For transcutaneous stimulation low threshold sites of the same fibers are identified at their exits from the spinal canal in addition to their spinal cord entries. In these exit regions anterior root fibers can also be activated. The simulation results provide a biophysical explanation for the electrophysiological findings of lower limb muscle responses induced by posterior root stimulation. Efficient excitation of afferent spinal cord structures with a simple noninvasive method can become a promising modality in the rehabilitation of people with motor disorders.

Stimulation of the Human Lumbar Spinal Cord With Implanted and Surface Electrodes: A Computer Simulation Study

The effects of anisotropy and inhomogeneity of the electrical conductivity of extracellular tissue on excitation of nerve fibers by an extracellular point source electrode were determined by computer simulation. Analytical solutions to Poisson's equation were used to calculate potentials in anisotropic infinite homogeneous media and isotropic semi-infinite inhomogeneous media, and the net driving function was used to calculate excitation thresholds for nerve fibers. The slope and intercept of the current-distance curve in anisotropic media were power functions of the ratio and product of the orthogonal conductivities, respectively. Excitation thresholds in anisotropic media were also dependent on the orientation of the fibers, and in strongly anisotropic media (/spl sigma//sub z///spl sigma//sub xy/>4) there were reversals in the recruitment order between different diameter fibers and between fibers at different distances from the electrode. In source-free regions of inhomogeneous media (two regions of differing conductivity separated by a plane boundary), the current-distance relationship of fibers parallel to the interface was dependent only on the average conductivity, whereas in regions containing the source the current-distance relationship was dependent on the individual values of conductivity. Reversals in recruitment order between fibers at different distances from the electrode and between fibers of differing diameter were found in inhomogeneous media. The results of this simulation study demonstrate that the electrical properties of the extracellular medium can have a strong influence on the pattern of neuronal excitation generated by extracellular electric fields, and indicate the importance of tissue electrical properties in interpreting results of studies employing electrical stimulation applied in complex biological volume conductors.

Modeling the effects of electric fields on nerve fibers: influence of tissue electrical properties

The potential distributions produced in the spinal cord and surrounding tissues by dorsal epidural stimulation at the midcervical, midthoracic, and low thoracic levels were calculated with the use of a volume conductor model. Stimulus thresholds of myelinated dorsal column fibers and dorsal root fibers were calculated at each level in models in which the thickness of the dorsal cerebrospinal fluid (CSF) layer was varied. Calculated stimulus thresholds were compared with paresthesia thresholds obtained from measurements at the corresponding spinal levels in patients. The influences of the CSF layer thickness, the contact separation in bipolar stimulation and the laterality of the electrodes on the calculated thresholds were in general agreement with the clinical data. >

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Paresthesia thresholds in spinal cord stimulation: a comparison of theoretical results with clinical data

We have reviewed our experience with spinal cord stimulation in treating patients with the failed back surgery syndrome and have assessed patient and patient-selection characteristics as predictors of the long-term outcome. Neuroradiological investigations eliminated the possibility of a surgically treatable lesion and electromyogram assessed the chronic radicular suffering in correlation with the complaints and the clinical examination of the patient. Excellent pain relief (75% or more) during 1 week of trial stimulation and no major psychiatric or psychological pathology were criteria of selection. Seventy-eight patients underwent trial stimulation. Fourteen (18%) failed to obtain excellent pain relief during 1 week of stimulation and their electrodes were removed. The remaining 64 underwent an internalization of the system and they were followed by a clinical observation every 3 months and this for a mean follow-up period of 4 years (range: 1–7 years). Thirty-five patients (55%) continued to experience at least 50% of pain relief at the latest follow-up. Fifty-eight patients (90%) were able to reduce their medication, 39 patients (61%) reported a change in lifestyle, in that their ability to perform daily activities had improved significantly. Fifty-three patients (83%) continued to use their device at the latest follow-up.

Spinal cord stimulation in failed back surgery syndrome

The midsagittal plane of the human thorax is represented by a two-dimensional finite-element model. Solutions of steady-state potential fields of bipolar epidural stimulation are computer generated. Effects of biological tissue conductivity and geometry on the fields are studied. Interpretations in terms of current density show that variations of bone conductivity have little influence on the fields. For constant current stimulation, the presence of the tough dura mater, irrespective of its thickness, does not seem to be significant. In contrast, the crucial factor affecting the field is the width of subarachnoid space which contains the highly conductive cerebrospinal fluid.

W. K. Sin

Papers

Electrical stimulation of the spinal cord: a further analysis relating to anatomical factors and tissue properties.