Abstracts

Some patients with reflex seizures have apparently generalized or bilateral ictal manifestations triggered by stimuli classically associated with specific cortical sensory territories, including flickering light or geometric pattern. Activation of less restricted brain areas by stimuli such as calculation or reading can also trigger generalized seizures. Many such patients appear clinically to have an idiopathic generalized epilepsy, especially JME. We reviewed the clinical and experimental data regarding these different types of reflex seizures. Photosensitive patients present with seizures when exposed to environmental stimuli producing flickering light or arranged in geometric patterns with specific characteristics of luminance, colour and contrast. Seizures induced by thinking are triggered by non-verbal higher mental activity (e.g. calculation). Praxis-induced seizures are triggered by similar mental activities accompanying the use of the hands (e.g.writing). Language[ndash]induced seizures are usually triggered by verbal higher mental activity (e.g. reading). Functional imaging and other advanced methods have recently contributed to a further understanding of the cortical regions involved during these tasks and how seizures may be triggered. It appears that patients with generalized reflex seizures present regions of cortical hyperexcitability overlapping the areas physiologically activated during specific sensory stimulations (flash and pattern), and cognitive (thinking, praxis and reading) or motor (praxis and reading) activities. In visual sensitivity the relevant system is occipital. For verbal and non-verbal cognitive tasks the system involves complex networks extending over multiple cortical areas in both hemispheres. When these areas receive appropriate afferent volleys and a critical mass of cortex is activated, an epileptic activity is produced that ultimately involves the cortico-reticular or cortico-cortical pathways, with the final result of a generalized or bilateral epileptic event. We believe that three major patterns of cortical activation of generalized or bilateral reflex seizures can be distinguished:
1) Sensitivity to visual stimuli, originating in the occipital cortex.
2) Sensitivity to non-verbal cognitive stimuli. This includes thinking-induced seizures involving predominantly the non-dominant or both parietal lobes, and praxis-induced epilepsy in which, in addition, the sensorimotor cortex is involved.
3) Sensitivity to verbal cognitive stimuli, involving both hemispheres with a major role played by the dominant lateral frontal cortex.
Although the underlying mechanisms are similar, as are the resulting seizure types, the triggering stimuli are different. All depend on cortical hyperexcitability which is diffuse but not necessarily uniform. This gives additional evidence that the cortical machanisms of IGE are complex and non-uniform.