Electrical and magnetic stimulation techniques for the diagnosis of facial nerve palsy and hemifacial spasm

Electrophysiological tests are mainly performed for prognostic purposes in patients with facial nerve palsies. Most facial nerve lesions are located intracranially. Therefore, electrical stylomastoidal stimulation provides normal results until Wallerian degeneration has occurred. The technique of magnetic stimulation allows non-invasive examination of the entire facial nerve and contributes substantially to the early differential diagnosis of facial palsies. Bell's palsy patients typically show a unilateral local hypoexcitability of the facial nerve to canalicular magnetic stimulation. Normal canalicular excitability and/or subclinical involvement of the contralateral side are not compatible with Bell's palsy. In this case further examinations (e.g. lumbar puncture, magnet resonance imaging) have to be performed. Blink reflex studies also supply pathological results from the beginning of the palsy, however, do not allow location of the lesion. Therefore, blink reflex studies are not helpful in the differential diagnosis of facial nerve palsies. In patients with hemifacial spasm the blink reflex shows a synkinetic response in the mentalis muscle indicating lateral spread of impulses to other fibers in the facial nerve. Selective stimulation of facial nerve branches (e.g. zygomatic and mandibular branch) reveal pathognomonic findings in hemifacial spasm by bidirectional transmission of antidromic impulses between the two branches resulting in delayed (ephaptic) responses.