The source of far-field antidromic facial nerve potentials

Most electrodiagnostic tests of facial nerve function are based on recordings of the distal muscle action potentials evoked by a proximal electrical stimulus. In contrast, retrograde or antidromic nerve action potentials can be recorded by near-field (directly from the nerve) or far-field (scalp) techniques. Clinical and surgical applications of far-field antidromic recording recently have been suggested because of such potential benefits as obviating the need to await axonal degeneration distal to the stylomastoid foramen. Critical to far-field recording is an understanding of the source from which the distant response is derived. This study was undertaken to identify the generator sites of the far-field antidromic facial nerve response in dogs. The results suggest the following for the montage and stimulus protocol employed: 1) the far-field antidromic response is a volume-conducted nerve action potential generated primarily from the mastoid segment, 2) complete transection of the facial nerve at the cerebellopontine angle has little effect on the responses, and 3) there appears to be no significant supranuclear generator site.