[A case of stimulus-sensitive segmental spinal myoclonus].

A 37-year-old-man was admitted to our hospital because of periodic contraction of the right shoulder muscles of approximately one month's duration. He denied any significant history such as trauma or fever. General physical examination was unremarkable. Neurological examination was also unremarkable except for the periodic synchronous myoclonus involving the right sternocleidomastoid, trapezius, deltoid, rhomboid and serratus anterior muscles. X-ray fluoroscopy demonstrated a synchronized rhythmic contraction of the right diaphragm. The myoclonus was considered to be of spinal origin, because those muscles were innervated by the right fourth and fifth cervical spinal segments. Its frequency was approximately 1.2 Hz. The myoclonus disappeared during sleep. It was enhanced by a voluntary contraction of the corresponding muscles, but mental stress such as arithmetic induced no change in the myoclonus. CBC, serum laboratory data including viral antibody titer, and cerebrospinal fluid were all normal. Radiological examination including MRI showed no abnormal finding. EEG as well as somatosensory evoked potentials following median nerve stimulation were normal. Jerk-locked averaging triggered by the myoclonus of the right deltoid muscle showed no pre-myoclonus spike at the scalp electrodes. Clonazepam (1.5-6 mg/day) and phenytoin (200 mg/day) reduced the rhythmicity as well as frequency of the myoclonus, which then became stimulus-sensitive. The reflex myoclonus was induced by tapping anywhere on the body or by electric shock, but not by flash or sound. There was a refractory period of approximately 800 msec. Lack of the stimulus-sensitivity early in the clinical course could be due to this refractory period.(ABSTRACT TRUNCATED AT 250 WORDS)