Neural mechanisms of hemifacial spasm.

摘要

Hemifacial spasm (HFS) is a disorder characterized by involuntary facial muscle spasms that begin in one eyelid and progress to the ipsilateral lower facial muscles. Other HFS symptoms include synkinesis, lateral spread, and increased blink excitability. Synkinesis is co-contraction of facial muscles other than the eyelid during a blink. Lateral spread occurs when stimulation of one facial nerve branch elicits a long latency response in a muscle innervated by a different branch. The primary cause of HFS is unilateral pulsatile arterial compression of the facial nerve at the root entry zone. Facial nerve compression can demyelinate and crush motor axons, producing mild facial weakness and consequent cornea irritation. The pulsatile compression also anti- and orthodromically stimulates nerve fibers, causing inappropriate motoneuron and muscle activation. The contribution of these multiple effects in the development of HFS symptoms is not well understood. My investigations elucidate the contribution of each of these mechanisms to the development of HFS.; I ascertained the contribution of facial weakness to HFS symptoms by identifying facial motoneuron excitability asymmetries of normal humans. The studies demonstrated that the nervous system adaptively maintains an excitability difference between the left and right facial motornuclei. I also investigated the effect of cornea irritation on lid movements. These studies revealed that cornea irritation converts the blink circuit into a central pattern generator that allows the production of multiple blinks to a stimulus. The spasms of lid closure in HFS might be an exaggeration of this condition. Finally, I determined the contribution of chronic stimulation of the facial nerve in the development of HFS in rodents. Chronic stimulation produced synkinesis and lateral spread symptoms as occur in HFS. These symptoms may have resulted from trigeminal sensory modifications in response to chronic stimulation of multiple facial nerve branches. These experiments demonstrate that the neural basis for HFS is more complicated than earlier models proposed. Multiple factors of puslatile arterial compression contribute to the development of unilateral facial muscle spasms.