Two new families with hereditary essential chin myoclonus: clinical features, neurophysiological findings and treatment.

Hereditary essential chin myoclonus (HECM) is a rare movement disorder characterised by involuntary and irregular twitches of the chin. It can be considered a benign, focal variant of hereditary essential myoclonus. We studied two families that had several members suffering from HECM. Both families showed an autosomal dominant transmission with incomplete penetrance. Extensive neurophysiological studies were carried out on three patients. The results confirmed the myoclonic nature of phenotypical appearance and allowed identification of the subcortical origin of the myoclonus. The neurophysiological mechanisms underlying HECM are still unknown. It is supposed that pontine reticular formation, not controlled by superior centers, may be responsible of an abnormal activation of motor neurons innervating mentalis muscle. We treated three patients with periodic injections of botulinum toxin; a complete disappearance of chin myoclonus improved their quality of life.