Hereditary Chin Trembling

Dear Editor

A 3-year-old-boy was presented with involuntary, intermittent trembling of the chin noted since his early infancy [Video 1]. The initiation or suppression of these movements was involuntarily, and they were lessening at sleep. No abnormal movement of jaw, face, head, or any other body part was noted. No functional or social impairment was reported. He was the first and the only child of non-consanguineous parents. Physical examination, neuroimaging, and electroencephalography were unremarkable. Intermittent bursts of motor units firing consistent with muscle spasm were determined in electromyography. Motor nerve conduction velocities and F-wave latencies were normal. Botulinum toxin (50 units) was injected into the mentalis muscles. Within 2 days, both mentalis muscles ultimately paralyzed and trembling stopped.

Chin trembling or geniospasm is an extremely rare neurologically benign 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 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 embarrassment 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. Regular botulinum toxin injections to the mentalis muscle have been effective in the treatment of hereditary geniospasm.[1,2]

We report an extremely rare idiopathic case occurring in a child. It is important to rule out other causes such as essential tremor, palatal tremor, and facial myokymia. Botulinum toxin injection directly to the muscle may help to alleviate the symptoms. The clinical and electrophysiological features may help in differentiation.

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