Subcortical P30 potential following tibial nerve stimulation: detection and normative data.

Stimulation of the tibial nerve evokes a P30 far-field potential over the scalp which, like the median nerve P14, probably originates in the vicinity of the cervico-medullary junction. Unlike the P14 potential, P30 recording has not been systematically performed in clinical practice, probably because of doubts about the generator of the potential and the possibility of consistently recording it on the scalp after the unilateral stimulation of the tibial nerve. In this study, we tested the reliability of the tibial nerve scalp far-field P30 potential in 34 normal subjects using different montages, of which the Fpz-Cv6 derivation gave the highest signal to noise ratio, making it possible to obtain a P30 potential peaking at 29.2 +/- 1.6 msec in all normal subjects. This suggests that this component should to be included in the routine recording of tibial nerve SEPs in order to evaluate the spinal and intracranial conduction of the somatosensory pathway separately.