Myoclonic dystonia.

We studied 14 patients who had a combination of idiopathic torsion dystonia and myoclonic jerks. In many cases, the same muscles were involved in both the myoclonus and the dystonia. This made observation of the crucial dystonic postures difficult and led to misdiagnosis of other dyskinesias. The jerks usually were brief (50 to 200 msec) and occurred irregularly, often showing cocontraction in antagonist muscles. Frequently, they were superimposed upon sustained dystonic contractions in the same or distant muscles. We found no time-locked EEG event before the jerks. The myoclonus probably arises from a subcortical focus, and the visible jerks probably are part of the spectrum of involuntary movements that accompany torsion dystonia.