Photosensitivity in epilepsy. Electrophysiological and clinical correlates.

Photosensitivity is a rare phenomenon found, often more or less accidentally, in approximately 5% of epileptic patients. Its pathophysiology still remains largely unsolved and the clinical significance of photosensitivity is controversial. The literature on the subject is impressive, yet predominantly anecdotal. In this thesis we describe the results of an extensive and standardized study of 100 consecutive photosensitive patients with special emphasis on the clinical history, the seizure history and the electrophysiological findings. These are then compared to identical data of an age and sex matched control group, obtained from the same population of epileptic patients, referred to a special (tertiary care) epilepsy clinic. In chapter I, the literature is reviewed and photosensitivity as a special form of "reflex epilepsy" is discussed. A distinction is made between normal and abnormal reactions on intermittent photic stimulation (IPS) during electroencephalographic (EEG) registrations and the criteria of true photosensitivity are formulated. The relation between the presence of photosensitivity during EEG examination and the occurrence of visually-induced epileptic seizures in daily life is discussed. As self-induction of seizures has been associated with photosensitivity, an extensive review, including a special reference list (see appendix A), is given concerning such self-inducing behaviour in photosensitive patients. A review of photosensitivity as a genetic marker and model of epilepsy concludes this chapter. In chapter II the aims of this study are outlined. Some general conceptions about photosensitive epilepsy have become widely accepted in clinical practice without much scientific support, i.e., the idea that the finding of photosensitivity is synonym with the diagnosis of primary generalized epilepsy. Furthermore, photosensitivity is generally believed to be a genetically determined, benign type of epilepsy in childhood and adolescence but when associated with self-inducing behaviour is interpreted as a sign of mental subnormality. Whether or not these conceptions are valid and whether photosensitivity is or is not a special subtype of epilepsy remains unsolved. In this study we thus set out to answer the following questions: A. Are photosensitive epileptic patients different from non-photosensitive patients with epilepsy, with respect to clinical history and, more specifically, to seizure history and family history for seizures? B. Is the degree of photosensitivity, established as photosensitivity range, predictive for the liability to visually-induced seizures in daily life? Are detailed laboratory findings concerning sensitivity to television and black-and-white striped patterns of clinically predictive value, e.g. can patients, liable to TV epilepsy or pattern-induced seizures, be identified by EEG investigations?