23. Visuo-motor connectivity in photosensitive epilepsies

Abstract

Objective Visual and motor cortex are involved in the origin/propagation of the photoparoxysmal response (PPR), a stronghold for the diagnosis of generalized photosensitive epilepsies. The study of the functional connection between these two distant areas with transcranial magnetic stimulation (TMS), could help clarify the pathophysiology of photosensitivity. Methods Eleven patients with idiopathic generalized epilepsy and photosensitivity were compared to 11 healthy subjects. We measured the resting motor threshold of the left hand motor area and the phosphene threshold. The paradigm consisted in a conditioning stimulus (CS) delivered over the phosphene hotspot of the visual cortex and followed at random interstimulus intervals (15–40 ms) by a test stimulus over the left motor cortex to evoke a motor evoked potential (MEP) of ∼1 mV from right FDI. The conditioned-MEP was expressed as a percentage of the unconditioned-MEP amplitude. Results In healthy subjects the CS over the visual cortex produced significant inhibitory changes on the corticospinal excitability with a time course between 18 and 40 ms. Patients with IGE and photosensitivity showed a failure of the physiological inhibition. Conclusions In photosensitive patients the alteration of the normal visuo-motor functional connectivity could be responsible for the origin/propagation of the PPR from the hyper-excitable visual cortex to the motor cortex.