[Pathophysiology of involuntary movements--dystonia and myoclonus. Symptomatological view]

Among involuntary movements, dystonia is defined as abnormal posturing produced by slow sustained muscle contractions. On the contrary, myoclonus is characterized by sudden shock-like contractions of a muscle or a group of muscles. The electromyogram (EMG) in dystonia shows continuous activity lasting 5 seconds or more. The muscles usually co-contract in the antagonists. In myoclonus the muscle bursts on EMG last usually between 10 and 50 ms. In some cases the bursts last longer, but they are 200 ms at most. Thus, the characteristics of myoclonus is quite different from those of dystonia. There are, however, unusual combination of dystonia and myoclonus. Myoclonic dystonia, in which myoclonic jerks are so severe that crucial dystonic posturing may be ignored, has been reported. Essential tremor, writing tremor and writer's cramp (writing dystonia) and myoclonic writer's cramp are sometimes seen in one family in various combination. It is suggested that there may be pathophysiological relationship between dystonia and myoclonus, although these two movement disorders have different clinical characteristics.