Central Motor Pathway Response to Flashes: a Magnetic Stimulation Study in Normal and Photosensitive Subjects
Roberto Cantello, Maria Gianelli, Carlo Civardi, Alessandra Cavalli, Roberto Mutani. University Department of Neurology.
Ospedale Maggiore, 28100 Novara, Italy.
In photosensitive (PS) patients, sudden flashes not only may produce paroxysmal EEG abnormalities, but also a variety of motor events. This implies malfunction of visuomotor connections. We studied visuomotor electrophysiology of normal and PS subjects by transcranial magnetic stimulation of central motor pathways.
We examined ten healthy volunteers (mean age 28 SD 6 ys) and three consenting female de novo patients (mean age 20 SD 5 ys), whose diagnosis was pure PS epilepsy (two cases) and idiopathic generalized epilepsy with photosensitivity (one case). We used a Dantec Mag2 electromagnetic stimulator, with a round coil centered at the vertex. Stimulus intensity was just suprathreshold for evoking motor evoked potentials (MEPs) in the relaxed right first dorsal interosseus muscle (FDI). Subjects were instructed not to react to the flash perception. In a random order, we obtained eight control MEPs and eight MEPs conditioned by a prior flash (conMEPs), for interstimulus intervals (ISIs) of 20-140 msec. We measured the average amplitude of conMEPs and expressed it as % of control. We also studied the flash-induced changes of the H-wave in the right soleus muscle, to get separate information on the spinal motor nucleus behaviour.
In the normal group, the significant
We describe the timing and type of the motor neuron electrical response to sudden flashes in normal humans. It consists of area 4 inhibition with an 80-msec latency, and of a widespread facilitation of the central motor pathway with a 100-msec latency. We document it was abnormal in PS patients. This may be a key pathophysiological aspect of photosensitivity.