Cortical reflex myoclonus associated with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS): a case report.

A 9-year-old female MELAS patient with myoclonus is reported, with emphasis on the results of electrophysiological studies of the myoclonus. At age 5 years she experienced a stroke-like episode, and a diagnosis of MELAS was made at age 6 years on the basis of muscle biopsy findings. At age 9 years spontaneous and segmental myoclonus, predominantly affecting the upper extremities, developed because of complications. Electrophysiological examination, including of somatosensory-evoked potentials (SEPs) and averaged EMG for long loop reflexes, revealed so-called "giant SEP" and enhanced long loop reflexes reflecting cortical hyperexicitability. Jerk-locked averaging yielded no myoclonus related spikes, but myoclonus-contingent 4-5 Hz theta bursts appeared. These findings suggest that some types of MELAS may be associated with cortical types of myoclonus.