Photic reflex myoclonus: a neurophysiological study in progressive myoclonus epilepsies

To investigate the neurophysiological features of photic reflex myoclonus (PRM) in patients with progressive myoclonus epilepsies (PMEs) of different types (Unverricht-Lundborg disease, Lafora's disease, cryptogenic).

All patients underwent computerized video-polygraphic recordings, collecting electromyographic (EMG) activity from several cranial and limb muscles. PRM was elicited by intermittent photic stimulation (IPS).

IPS could evoke PRM with a 1:1 relation at frequencies up to 12 Hz. Back-average of the EEG, triggered from the onset of PRM at the upper limbs, showed a contralateral positive-negative transient in central region, preceded by approximately 10 ms by a similar, ipsilateral occipital wave. When IPS induced bilateral jerking, a time lag of approximately 10 ms between the homologous muscles of the two sides was observed, paralleled by a similar delay between the associated contralateral EEG transients in the two central regions, suggesting spread of cortical myoclonic activity from one hemisphere to the other via transcallosal fibers. PRM propagated in different cranial and limb muscles according to a rostrocaudal pattern, with latencies compatible with a transmission along fast-conducting corticospinal motor pathways.

In our PME patients, PRM presented uniform neurophysiological features, indicating the participation of both occipital and motor cortices, with bilateral spread presumably mediated by transcallosal connections and propagation down the spinal cord via fast-conducting corticospinal pathways.