Orthodromic near-field potentials of the intratemporal facial nerve

This study aimed to assess the electrophysiologic parameters of near-field action potentials recorded from the intratemporal facial nerve.

Assessment of facial nerve function is limited because of the inaccessibility of the majority of its course deep in the temporal bone. Indirect measurements have therefore evolved in which stimulation distal to most major pathology and indirect recordings from the tissues the nerve supplies offers some evidence as to facial function. Recording from the intratemporal portion of the facial nerve is possible. Antidromic nerve action potentials can be recorded from an extratympanic electrode after stimulation of peripheral facial nerve branches. This technique offers new alternatives to the assessment of the facial nerve.

The facial nerve was electrically stimulated close to the brain stem, and monopolar recordings were taken distally at several points along its intratemporal course. Averaging techniques produced a reliable and reproducible triphasic waveform that then was analyzed.

Neuromuscular blockade had no effect on amplitude of the waveform. Recording at different points along the nerve course showed an increasing nerve potential latency with increasing distance from the stimulus. The amplitude of the waveform decreased as the recording electrode was moved away from the nerve. These findings are in keeping with the electrophysiologic properties of a near-field nerve action potential. Conduction velocity along the facial nerve averaged 23.7 m/s and was found to vary along different nerve segments.

Facial nerve action potentials can be recorded from within the temporal bone despite its multidirectional course through different volume conductors. These action potentials exhibit the same characteristics as those arising from more accessible peripheral nerves.