SURFACE AND EPIDURAL LUMBOSACRAL SPINAL-CORD EVOKED-POTENTIALS IN CHRONIC SPINAL-CORD INJURY

Nine patients were examined in the chronic stage of spinal cord injury (12 to 56 months postinjury). Surface lumbosacral spinal cord evoked potentials (LSEPs) were obtained using electrodes placed over the S1, L2, L4, and T12 vertebral levels, referenced to a T6 surface electrode . Epidural LSEPs were obtained using a multielectrode lead placed perc utaneously into the epidural space for evaluation of the efficacy of s pinal cord stimulation for modification of pain and spasticity. The LS EPs resulting from supramaximal stimulation of the tibial nerve at the popliteal fossa were composed of propagating and stationary action po tential components. Based on the surface LSEP amplitudes and latencies established in healthy subjects, the data was divided into normal (le ss than 2 SD), marginal (between 2 and 2.5 SD), and abnormal (greater than 2.5 SD) categories. Comparison of surface and epidural LSEPs at t he T12 vertebral level for the normal group (n = 6, 4 incomplete and 2 complete) revealed a mean epidural/surface amplitude ratio of 9.44 an d a latency for the major negative component of 15.2 +/- 0.6 ms for th e epidural versus 14.8 +/- 0.6 ms for the surface LSEP. In cases where the lead was progressively removed and LSEPs recorded (n = 4) the pro pagating components rapidly attenuated and increased in duration while the stationary components attenuated but did not change in duration. The LSEPs for the marginal group (n = 2, 1 incomplete and 1 complete) showed similar epidural/surface amplitude ratios. In the abnormal case (n = 1, complete) surface LSEPs were absent but epidural LSEPs were p resent but with stationary and propagating components of low amplitude . This study demonstrates the ability of the epidural LSEP to provide more information than the surface LSEP of the functional condition of the lumbosacral spinal cord, particularly regarding the character of t he propagating action potentials and in cases when the surface LSEPs a ppear to be of very low amplitude or absent.