Adult-onset myoclonus ataxia associated with the mitochondrial m.8993T>C "NARP" mutation.

Mitochondrial dysfunction is an important cause of neurological disease. Mitochondrial disease can present at any age, and the clinical features are extremely varied. The FIG. 1. Neurophysiological exam of the boy at 3 years of age, performed during a myoclonic attack involving the left upper limb: Polymyographic recording (A and B) shows profuse continuous positive rhythmic (8-Hz) myoclonus composed of short duration (range, 40-48 ms) repetitive bursts (arrows), which were synchronous in all the recorded muscles. Negative myoclonus (duration, 270 ms) also occurred intermittently (asterisks), inducing transient loss of tonus. EEG jerk-locked-back-averaging (C) shows a 3-mV transient over the right rolandic central (C4) area preceding myoclonus by 17.8 ms, which, taken together with polymyography, highly suggests the cortical origin of the myoclonus (averaging of 3 consecutive series of 100 positive jerks). Comparison of T2-weighted MRIs of the patient’s brain at 3 (D) and 7 (E) years of age demonstrates progressive atrophy and cortical hypersignal of the cerebellar hemispheres. A1-A2, ear-linked reference; ECR, extensor carpi radialis; FCR, flexor carpi radialis; Acc, accelerometer. L E T T E R S : N E W O B S E R V A T I O N S