Background
Hemifacial spasm is a segmental myoclonus of muscles innervated by the facial nerve. Hemifacial spasm presents in the fifth or sixth decade of life, almost always unilaterally, although bilateral involvement may occur rarely in severe cases. Hemifacial spasm generally begins with brief clonic movements of the orbicularis oculi and spreads over years to other facial muscles (corrugator, frontalis, orbicularis oris, platysma, zygomaticus). [1, 2]
Clonic movements progress to sustained tonic contractions of involved musculature. Chronic irritation of the facial nerve or nucleus, the near-universal cause of hemifacial spasm, may arise from numerous underlying conditions.
Facial musculature is subject to the same movement disorders as muscles of the limbs or trunk. Myoclonus, dystonia, and other movement disorders present with specific syndromes in the facial musculature. An understanding of the underlying mechanism leads to appropriate diagnostic evaluation and potential treatment.
The causes of hemifacial spasm include vascular compression, facial nerve compression by a mass, brainstem lesions such as stroke or multiple sclerosis plaques, and secondary causes such as trauma or Bell's palsy. [3]
Although specific treatments are available for many craniofacial movement disorders, botulinum toxin (BTX) chemodenervation has proven useful in many of these disorders, supplanting surgery and medical therapy.
Pathophysiology
Irritation of the facial nerve nucleus is believed to lead to hyperexcitability of the nucleus, while irritation of the proximal nerve segment may cause ephaptic transmission within the facial nerve. Either mechanism explains the rhythmic involuntary myoclonic contractions observed in hemifacial spasm. [4]
Compressive lesions (eg, tumor, arteriovenous malformation, Paget disease) and noncompressive lesions (eg, stroke, multiple sclerosis plaque, basilar meningitis) may present as hemifacial spasm. Most instances of hemifacial spasm previously thought to be idiopathic were probably caused by aberrant blood vessels (eg, distal branches of the anterior inferior cerebellar artery or vertebral artery) compressing the facial nerve within the cerebellopontine angle.
Epidemiology
Hemifacial spasm is observed in both men and women but is more common in middle-aged or older women. Studies also note that the condition is more prevalent among Asian populations. However, it generally affects all races without significant disparity. [5]
Idiopathic hemifacial spasm typically begins in the fifth or sixth decade of life. Onset of hemifacial spasm in patients younger than 40 years is unusual and often heralds an underlying neurologic illness (eg, multiple sclerosis). [5]
Prognosis
Hemifacial spasm rarely remits spontaneously. Current treatments, fortunately, prove highly effective. Patients should be counseled, however, that treatments are likely to continue indefinitely.
