Self-paced voluntary movements are preceded by a premovement potential—the readiness potential or Bereitschaftspotential. This is a slow negative potential beginning around one second prior to movement and maximal over the vertex (3). It is illustrated by the thin electroencephalographic (EEG) traces in Figure 30.1. In contrast, a slow premovement potential does not precede organic spontaneous myoclonus, although a very much briefer cortical spike discharge or sharp wave may precede the EMG discharges in cortical myoclonus by an interval of 20 to 40 ms, depending on whether the muscle under investigation is in the upper or lower limb (2). The bold EEG traces in Figure 30.1 show the absence of premovement potential prior to EMG bursts in a patient with abdominal flexion jerks due to tics.

The presence of a long premovement potential beginning more than 800 ms prior to jerks is very good evidence in favor of psychogenic myoclonus during the recording session (4), although it will not distinguish those patients with entirely psychogenic jerks from those with an organic movement disorder who elaborate with a preponderance of psychogenic jerks.

The presence of an abbreviated premovement potential, beginning less than 800 ms prior to the jerks is less helpful. Libet (5) showed that healthy subjects can have such shortened premovement potentials if they do not deliberately think about the timing of the movement, so it is likely that some psychogenic myoclonus may also be accompanied by abbreviated premovement potentials. On the other hand, some organic forms of jerks may also be accompanied by what effectively appear to be abbreviated premovement potentials. This may, for example, be true of the minority of patients with tics. Karp et al. found very brief premovement potentials (similar to the very late NS’ component of the normal premovement potential) in two out of their five patients with tics (6). However, Obeso et al. reported a complete absence of premovement potentials preceding tics in their cases (7), and Tijssen et al. reported three patients with an unusual late-onset posttraumatic tic disorder that was also not preceded by any premovement potential (8). Shibasaki et al. reported premovement potentials in the chorea of choreoacanthocytosis (9), but this is difficult to interpret, given that the chorea in Huntington disease involves no such EEG potential. There has also been a report of abbreviated premovement potentials prior to the jerks of myoclonus-dystonia, although this has not been replicated (10).