[Spinal and subcortical somatosensory evoked potentials after stimulation of the tibial nerve]

Evoked potentials in response to unilateral stimulation of the posterior tibial nerve at the ankle were recorded above the spinous processes L5, L1, C2, and at Cz' in 30 normal subjects. The "cauda-potential" recorded above L5 consists of two small negative peaks with a mean latency of 18 and 22.5 ms respectively, whereas the "cord-potential" recorded above L1 exhibited a peak latency of 21.2 ms and on average a three-times larger amplitude than the first of the two "cauda-potentials" (Fig. 1). Leads from the spinous process C2 revealed a sharp negative peak with a mean peak latency of 28.8 ms (N 30). Scalp recordings with a midfrontal (Fz-) reference inconsistently showed 1-2 small waves (P31, N33) prior to the primary cortical response (P40). Recordings with an ear- or non-cephalic reference consistently showed a large positive deflection (P30) which corresponded in latency with the simultaneously recorded cervical response (N30) and was followed by a distinct negative potential (N33) (Fig. 2a and b). Average latencies and amplitudes of the different spinal and subcortical evoked potentials (Tables 1 and 2), as well as the diagnostically more important interpeak-intervals, amplitude relations, and side-differences of latencies and amplitudes (Tables 3 and 4) were calculated. The diagnostic significance of these parameters will be shown in selected cases with spinal cord pathology.