Lumbar spinal stenosis: analysis of pre- and postoperative somatosensory evoked potentials.

Cortical somatosensory evoked potential (CSEP) examinations were performed of 20 patients with lumbar spinal stenosis a day prior to surgery and 10 to 12 days after spinal decompression and bilateral lateral fusion. CSEPs were recorded to Cz-Fz (10-20 international EEG system) following stimulation of 32 tibial, peroneal and sural nerves and 16 saphenous nerves. A total of 110 nerves were examined. Using CSEP P1 latency as criteria for inclusion in the study, 21 tibial, 20 peroneal and 17 sural nerves were subjected to paired two-tailed t tests to determine whether the CSEP changes that occurred postoperatively were statistically significant (p less than 0.05). Postoperative P1 latencies of tibial, peroneal, and sural nerves changed significantly as well as N1 latencies and P1-N1 amplitudes of tibial and peroneal nerves. Nineteen patients improved clinically. It is postulated that pathologic narrowing of the spinal canal in spinal stenosis leads to nerve root compression and ischemia with resultant dysfunction primarily affecting large diameter myelinated fibers and that decompression procedure may adequately relieve the underlying pathologic processes. Improvement in CSEPs may be from increase in available numbers of functioning large diameter myelinated fibers, conversion to normal from a conduction block, and, perhaps, improved axoplasmic flow.