Electrophysiological studies of myoclonic jerks provoked by photic stimulation

A patient with a type of myoclonus produced by somatosensory and photic stimuli was studied electrophysiologically. Jerk-locked averaging showed cortical spikes in association with the spontaneous myoclonus. High amplitude somatosensory evoked potentials and high amplitude visual evoked potentials were observed. Somatosensory and photic stimuli produced 2 or 3 successive electromyographic (EMG) discharges. The time interval from the onset of the preceding EMG discharge to that of the following EMG discharge produced by photic stimulation corresponded to the latency of a long-loop reflex. It also corresponded to the time interval from the onset of the preceding EMG discharge to that of the following EMG discharge produced by somatosensory stimulation. Not only somatosensory but also photic stimuli might excite the loop of a long-loop reflex and cause myoclonic jerks.