CORTICAL REFLEX MYOCLONUS IN RETT-SYNDROME

Rett syndrome (RS) is one of the most frequent causes of mental retard ation in females. As there are no known biochemical, genetic, or morph ological markers, diagnosis is based on clinical phenotype including s evere dementia, autism, truncal ataxia/apraxia, loss of purposeful han d movements, breathing abnormalities, stereotypies, seizures, and extr apyramidal signs, Myoclonus, although reported in some series, has nev er been characterized. We studied 10 RS patients, age 3 to 20 years, a nd observed myoclonus in 9. Severity of myoclonus did not correlate wi th that of the other symptoms or with age. Multifocal, arrhythmic, and asynchronous jerks mainly involved distal limbs. Electromyographic bu rsts lasted 48 +/- 12 msec Burst-locked electroencephalographic averag ing generated a contralateral centroparietal premyoclonus transient pr eceding the burst by 34 +/- 7.2 msec Motor evoked potentials showed no rmal latencies, indicating integrity of the corticospinal pathway. Som atosensory evoked potentials were enlarged. The C-reflex was hyperexci table and markedly prolonged (62 +/- 4.3 msec), mainly due to increase in cortical relay time (28.4 +/- 4.5 msec). We conclude that RS patie nts show a distinctive pattern of cortical reflex myoclonus with prolo nged intracortical delay of the long-loop reflex.