Facial nerve palsy and hemifacial spasm.

Facial nerve lesions are usually benign conditions even though patients may present with emotional distress. Facial palsy usually resolves in 3-6 weeks, but if axonal degeneration takes place, it is likely that the patient will end up with a postparalytic facial syndrome featuring synkinesis, myokymic discharges, and hemifacial mass contractions after abnormal reinnervation. Essential hemifacial spasm is one form of facial hyperactivity that must be distinguished from synkinesis after facial palsy and also from other forms of facial dyskinesias. In this condition, there can be ectopic discharges, ephaptic transmission, and lateral spread of excitation among nerve fibers, giving rise to involuntary muscle twitching and spasms. Electrodiagnostic assessment is of relevance for the diagnosis and prognosis of peripheral facial palsy and hemifacial spasm. In this chapter the most relevant clinical and electrodiagnostic aspects of the two disorders are reviewed, with emphasis on the various stages of facial palsy after axonal degeneration, the pathophysiological mechanisms underlying the various features of hemifacial spasm, and the cues for differential diagnosis between the two entities.