The interval between the positive peak of premyoclonus spike and the onset of myoclonus is shorter than the cortical latency in cortical myoclonus.

In 6 patients with cortical myoclonus, the intervals between the positive peak of the premyoclonus cortical spike and the onset of the myoclonic jerk [interval (Sp-Mcl)] were precisely measured by the computer-assisted jerk-locked averaging method. The intervals (Sp-Mcl) were 13.0-15.8 ms for the abductor pollicis brevis, 8.0-10.4 ms for the extensor digitorum communis and 5.5 ms for the biceps brachii, respectively. These were apparently shorter than the normal latencies of the responses elicited by transcranial cortical stimulation. The difference between the interval (Sp-Mcl) and the spinal latency was -0.8 to 3.5 ms, which was not long enough for the time required for the descending volleys going down from the cortex to the spinal cord. We have concluded that the motor cortex is activated before the positive peak of the premyoclonus spike in cortical myoclonus.