If a peripheral facial paralysis has existed for some time and return of motor function has begun but is incomplete, a contracture with diffuse myokymic activity may appear. The palpebral fissure becomes narrowed, and the nasolabial fold deepens. Spasms of facial muscles may develop and persist indefinitely, being initiated by any facial movement. With the passage of time, the corner of the mouth and even the tip of the nose may become pulled to the affected side. Anomalous or aberrant regeneration of the seventh nerve fibers, following Bell’s palsy or other injury, may result in other curious disorders that represent limited types of synkineses. If regenerating fibers originally connected with the orbicularis oculi become connected with the orbicularis oris, closure of the lids may cause a retraction of the corner of the mouth; conversely, aberrant regeneration of fibers may cause synkinetic eyelid closure during contraction of the orbicularis oris. If visceromotor fibers originally innervating the salivary glands later come to innervate the lacrimal gland, anomalous tearing occurs whenever the patient salivates; this phenomenon is referred to as “crocodile tears.” A similar mechanism explains gustatory sweating of the cheek and upper lip following injury to parasympathetic innervation of the parotid (Frey syndrome). An unusual congenital synkinesis sometimes occurs between motor trigeminal fibers and branches of the facial nerve, producing a “jaw-winking” phenomenon (also called Wartenberg or inverse Marcus-Gunn sign), in which jaw movements, especially lateral movements (engaging the pterygoid muscle), cause an involuntary closure of the eyelid ipsilateral to the movement.
Hemifacial spasm is a disorder of painless, irregular clonic contractions of facial muscles on one side. This condition usually develops in the fifth and sixth decades, affects women more than men, and often proves to be caused by a compressive lesion of the facial nerve, usually by a tortuous branch of the basilar artery that lies on the ventral surface of the pons and forms a loop under the proximal seventh nerve. Less often the cause of compression is a fusiform basilar artery aneurysm or a vestibular schwannoma or meningioma. Multiple sclerosis is rare cause.
The spasm usually begins in the orbicularis oculi muscle and as the condition worsens, the contractions also occur in other muscles on that side of the face, including the platysma. Paroxysms may be induced or aggravated by voluntary and reflexive movements of the face.
The pathophysiology of the spasm is believed to be focal demyelination at the site of nerve root compression by the vessel. The demyelinated axon is presumed to activate adjacent nerve fibers by ephaptic transmission (“artificial” synapse of Granit et al). Another possible source of the spasm is spontaneous ectopic excitation arising in injured fibers.
Treatment Medical treatment can be effective in some patients with hemifacial spasm. Alexander and Moses noted that carbamazepine in doses of 600 to 1,200 mg/d controls the spasm in two-thirds of the patients. Baclofen or gabapentin can be tried if carbamazepine fails. Many patients, however, cannot tolerate these drugs, have only brief remissions, or fail to respond. Serial injections with botulinum toxin injected into the orbicularis oculi and other facial muscles can be highly effective to reduce or eliminate the involuntary contractions, without causing excessive weakness and we have usually resorted to this treatment early in the course of the disorder. Some patients have been injected repeatedly for more than 5 years without apparent adverse effects. Failing these conservative measures to treat the condition, surgery may be appropriate.