Origin of far-field subcortical evoked potentials to posterior tibial and median nerve stimulation. A comparative study.

Posterior tibial nerve (PTN) evoked potentials (EPs) at the lumbar-low thoracic level have waveforms similar to median nerve (MN) EPs at the cervical level. They consist of a short-duration negativity (N18 and N10, respectively), which reflects the afferent volley before it enters the spinal canal, and a longer-duration, later negativity (N20 and N12, respectively), which consists mainly of slow frequencies and most probably is a postsynaptic dorsal cord potential. At the neck-scalp derivation the MN EP consists of two near-field negativities (N10 and N12) that are recorded from the neck electrode and two far-field positivities (P11 and P13) that are recorded from the scalp electrode. The neck-scalp response to PTN stimulation consists of a near field potential N24 that is followed by a far-field potential P27.