The different patterns of the photoparoxysmal response--a genetic study.

In order to investigate the significance of different patterns of the photoparoxysmal response (PPR), an electroencephalographic family study on 135 probands and 371 relatives was carried out. The PPR was subclassified in 4 phenotypically different types. Considering all types of PPR, the incidence was equal in siblings of probands with epilepsy and in siblings of probands without epilepsy. Type 4 (generalized spikes and waves) occurred more often both in probands with epilepsy and their siblings than the respective controls. Thus, the coincidence of photosensitivity will appear as higher if only a PPR with generalized spikes and waves is considered to be indicative of photosensitivity. A striking age dependency of the phenotypical expression of the PPR was found. These findings suggest that the phenotypical expression of the PPR is age-related and modified by other factors predisposing to generalized epilepsy, the varied patterns of the PPR only representing different levels of expression of the same genetically determined trait. The importance of an adequate technique of intermittent light stimulation is discussed.