Abstracts

Rationale: Photosensitive epilepsy where seizures are triggered by environmental flicker occurs in both idiopathic focal and idiopathic generalized epilepsies (IGE). Photosensitivity occurs in 20-30% IGE and is mandatory in the idiopathic focal epilepsy syndrome of idiopathic photosensitive occipital epilepsy (IPOE). We have previously shown that there is overlap between IPOE and Juvenile Myoclonic Epilepsy (JME).The aim of this study was to analyze phenotypic patterns in families with photosensitive epilepsy.Methods: 29 families where at least 2 relatives had idiopathic epilepsy and electroclinical photosensitivity were studied. Affected family members underwent detailed electro-clinical assessment. A 21-channel EEG recording including intermittent photic stimulation and hyperventilation was performed. An additional Oz electrode was used in some cases. Seizure types were classified according to the international classification and the phenotypic patterns in each family were analysed. Results: In the 29 families, there were 110 affected members who underwent detailed characterization; 84 were photosensitive (62 female, 22 male) and 26 had epilepsy without definite photosensitivity (13 female, 13 male). Three main categories of idiopathic photosensitive phenotypes were observed in individual subjects: pure IGE, mixed IPOE/ IGE, and IPOE. Within each category, subjects with seizures only with photosensitive stimuli were noted. Comparison of phenotypes between family members with photosensitive epilepsy and those with non-photosensitive epilepsy showed a significant female bias (p=0.02) and earlier age of seizure onset (p=0.03)for those with seizures associated with photosensitivity. Photosensitive epilepsy was associated with absence seizures, myoclonic seizures and simple visual hallucinations rather than generalized tonic-clonic seizures. For families, phenotypic heterogeneity was typical with a spectrum of phenotypes showing electroclinical overlap between IPOE and a range of IGE subsyndromes such as Childhood Absence Epilepsy.Conclusions: Analysis of photosensitive epilepsy phenotypes shows considerable overlap between the idiopathic focal and IGE. They manifest as a continuum from IGE to mixed IPOE /IGE and IPOE. Within each of these categories, subjects with purely photosensitive seizures may be seen. Photosensitivity is an endophenotype that is likely to be useful in dissecting the polygenic aetiology of the IGE.