A study of photosensitive epilepsy with particular reference to occipitial spikes induced by intermittent photic stimulation

Abstract

This thesis is the result of an attempt to find means of diagnostic value and understand the underlying mechanisms of photosensitive epilepsy which is a variety of epilepsy where seizures are provoked by photic stimuli. A detailed review of the relative literature has been endeavoured. The aspects dealt with were mainly concerned with electroencephalographic responses to intermittent photic stimulation. It has been found that intermittent photic stimulation is more effective on "eye-closure" than in any other eye state and more abnormalities are induced when the eyes are open than when they are closed. The significance of light in the provocation of "eye-closure" induced discharges has been emphasized. The characteristics of the occipital spikes induced by intermittent photic stimulation have been studied. The majority of photosensitive epileptic patients and those with epilepsy who are not clinically photosensitive but in whom E.G. abnormalities are provoked by intermittent photic stimulation, show occipital spikes alone or preceding photoconvulsive responses, during photic stimulation. Tbe occipital spikes were compared with the visual evoked responses of the same patients and of normal subjects. This showed that there was no simple relation between occipital spikes and components of visual evoked responses. The characteristics of the negative occipital spike show striking similarities to those of the recruiting response evoked by electrical stimulation of the non-specific thalamic nuclei as described by other authors. This may indicate that the non-specific thalamocortical system is responsible for the genesis of the "epileptogenic" occip:ital spikes and therefore implicated in the pathogenesis of photosensitive epilepsy. The intermittent photic stimulation showed an increased effectiveness when combined with patterns and this may be due to an increased susceptibility of the occipital cortex. It is suggested that seizures in photosensitive epilepsy are the result of discharges arising from abnormally activated,by photic stimuli, non-specific thalamic system and impinging upon a hypersensitive occipital cortex.