Photosensitivity--genetics and clinical significance.

Photosensitivity is defined by the occurrence of spikes or spikes and waves in response to intermittent light stimulation (= photoparoxysmal response [PPR]). The EEG pattern can show a wide range of expression from solely occipital spikes to generalized irregular spikes and waves. The different types of responses represent different levels of expression of the same genetically determined trait. The photoconvulsive response of Bickford et al (7) is only a subtype of the PPR characterized by close relation to epilepsy. The average incidence of PPR is 7.6% in healthy children aged 1 to 16 years. Girls are more often affected than boys. The available data provide evidence for autosomal dominant inheritance of photosensitivity. Genetic heterogeneity must be considered. In studies starting from a neurobiological genetic point of view including the whole spectrum of PPR, the phenomenon could be shown to be a widespread condition being only loosely associated with epilepsy, but more often with symptoms of psychovegetative instability. Only about 3% of children with PPR of the given definition will manifest epilepsy up to age 20 years. According to family studies in epileptic and non-epileptic photosensitive probands, the phenotypical expression of the PPR and carriers' risk to manifest epileptic phenomena is decisively determined by other pathogenetic factors related to epilepsy. Possible interactions of photosensitivity within the multifactorial pathogenetic background of the epilepsies are reviewed.