Facial Nerve Evoked Potentials in the Cat

Abstract

Electrophysiologic facial nerve testing usually involves stimulation of the peripheral nerve in order to make some indirect inference about nerve integrity at a more proximal site of lesion. In an attempt to develop a test of facial nerve function by use of across-the-lesion testing, the cat facial nerve was stimulated percutaneously at the stylomastold foramen while retrograde activity through the temporal bone and cranium was monitored with scalp electrodes. A biphasic evoked potential could be identified within 3 milliseconds of stimulation with the use of a signal-averaging computer. This potential remained when the animal was paralyzed and disappeared when the facial nerve was cut proximal to the stimulation site. A potential of similar latency and duration but larger amplitude was recorded from the subarachnold space. Mapping studies indicated its origin to be a dipole located between ipsilateral mastoid and parietal recording sites that corresponded to the region of the intracranial facial nerve. Optimal stimulation and recording techniques for subsequent studies of human beings are discussed.