Surface and epidural lumbosacral spinal cord evoked potentials in chronic spinal cord injury.

Abstract

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 percutaneously into the epidural space for evaluation of the efficacy of spinal cord stimulation for modification of pain and spasticity. The LSEPs resulting from supramaximal stimulation of the tibial nerve at the popliteal fossa were composed of propagating and stationary action potential components. Based on the surface LSEP amplitudes and latencies established in healthy subjects, the data was divided into normal (less 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 the T12 vertebral level for the normal group (n = 6, 4 incomplete and 2 complete) revealed a mean epidural/surface amplitude ratio of 9.44 and a latency for the major negative component of 15.2 +/- 0.6 ms for the 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 propagating 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 present 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 the propagating action potentials and in cases when the surface LSEPs appear to be of very low amplitude or absent.