Electrodiagnosis of the cranial nerves.

Isolated facial weakness suggests either a contralateral hemispheric lesion or a disease of the facial nerve per se. The presence of sensory symptoms usually indicates a central facial weakness, which characteristically involves the lower part of the face. In contrast, the absence of sensory disturbances suggests a peripheral nerve lesion, some system diseases such as amyotrophic lateral sclerosis, or a stroke sparing the sensory cortex. Sporadic cases of Bell's palsy rank the first in incidence. Although its exact etiology remains unknown, accumulating evidence suggests reactivation of herpes simplex virus type I. A facial palsy that develops in patients with diabetes mellitus tends to show a more severe involvement with substantial denervation. Acoustic neuroma, strategically located at the cerebellopontine angle, may compress the facial nerve. Peripheral facial palsy may herald other symptoms of multiple sclerosis in young adults. Serial electrodiagnostic studies help delineate the course of the illness. The amplitude of the direct response elicited by stimulation of the facial nerve after the fourth to fifth day of onset serves as the best means predicting the eventual outcome of recovery. Blink reflex studies usually show an absent or delayed R1, implicating the central reflex arc, which includes the intrapontine portion of the facial nerve.