Improvement of action myoclonus by an administration of 5-hydroxytryptophan and carbidopa in a child with muscular subsarcolemmal hyperactivity.

We report a 3-year-11-month-old boy who manifested action myoclonus only. Histochemical analysis of the quadriceps muscle revealed subsarcolemmal hyperactivity. The administration of 5-hydroxytryptophan and carbidopa dramatically improved the action myoclonus and reduced an amplitude of giant somatosensory evoked potentials. A nosological relation of this case with "essential myoclonus" and mitochondrial encephalomyopathy was discussed.