Spinal and cortical evoked potentials following stimulation of the posterior tibial nerve in the diagnosis and localization of spinal cord diseases.

Evoked potentials from unilateral stimulation of the posterior tibial nerve at the ankle were recorded over the spinous processes L5, L1, C2 and Cz' in 30 normal subjects (Table Ia, b and c), 11 patients with multiple sclerosis (Table II) and 8 patients with a proven space-occupying spinal cord lesion (Table III). Delayed sensory conduction of both absolute latencies and side to side differences of P40 was seen in 91% of MS patients. Additional recording of spinal evoked potentials over L5, L1 and C2 did not significantly increase the percentage of abnormal responses. The spinal cord evoked responses therefore have their diagnostic importance in localizing the demyelinating process to the spinal or supraspinal section of the sensory pathway and in excluding peripherally delayed impulse conduction. The absolute latencies were within normal limits or only slightly delayed in the spinal tumor group. The interpeak latencies between the lumbar and the cervical or cortical responses showed less variability as compared to the absolute ones and revealed slight delays in some cases. Diagnostically more important seems the amplitude quotient between the cortical and the lumbar evoked potentials (P40/S response) which was below the normal range in two thirds of the tumor group patients.