A five-year-old, developmentally-normal girl presented with involuntary, intermittent trembling of the chin noted since early infancy (Web Video 1 and Fig. 1). There were no triggering-factors. These movements could not be initiated or suppressed voluntarily. These were absent during sleep. No abnormal movement of jaw, face, head, or any other body part were noted. There was no functional or social impairment. Physical examination, neuroimaging and electro-encephalography (EEG) was unremarkable. Father of the child had similar chin- trembling; the intensity and duration of which had decreased with age.

Fig. 1 Spontaneous, involuntary, intermittent movements of the mentalis muscle; chin myoclonus. (see video at website)

Chin-trembling or geniospasm is a rare movement disorder, caused by continuous or intermittent tremulous activity of the mentalis muscle. It may be familial (autosomal-dominant) or sporadic. It is characterized by paroxysmal, rhythmic, up-and-down movements of the chin and/or lower-lip with episodes lasting from seconds to hours. Episodes may occur spontaneously or be precipitated by stress, concentration, and emotion. These typically become apparent in infancy or in early-life, and the episodes tend to reduce in frequency/intensity with advancing age. Impairments include social embarrass-ment, and interference with speech, feeding or sleep.

Electrophysiological studies have suggested these to be a form of subcortical myoclonus, and Hereditary chin myoclonus has been proposed to be a better term to describe these movements. Treatment is usually not required; benzodiazepines, haloperidol, phenytoin and botulinum toxin have been tried in desperate situations with limited success.