Photosensitivity in Epilepsy Syndromes. Age Differences?

Epileptic visual sensitivity is defined by the presence of paroxysmal abnormalities of brain electrical activity provoked by intermittent photic stimulation (the Photoparoxysmal Response or PPR) or by spatially structured visual stimuli (Pattern Sensitivity). Photosensitivity can be observed in patients in whom seizures are only seen if precipitated by visual stimuli (reflex seizures), in the context of specific genetic generalized or focal epilepsy syndromes; less frequently the PPR is observed in the context of brain disorders, as is the case for progressive myoclonic epilepsies. In some individuals, the phenomenon is confined to a pure electroencephalographic pattern and patients may never present overt seizures. Evidence from family studies has suggested that in these patients the PPR is the expression of a genetic trait. Photosensitivity is an age-dependent feature, usually first seen in childhood and adolescence, while photosensitive epilepsies accounts for 10% of all childhood-onset epilepsies. Visual sensitivity can be recorded in the EEG of patients at virtually any age, but it is most frequent between the ages of 8 and 25. In a longitudinal study on 454 patients, the mean age of onset was 13.7 years, confirming earlier reports of an age of epilepsy onset of 14.4 years in a group of 103 photosensitive patients, with the incidence of PPR declining after 20 years of age. Abnormalities tend to persist over time in a large (60%) proportion of patients with PPR only, as suggested in a long-term follow-up study in children with no history of epileptic seizures. About two-thirds of patients with photic-induced seizures remain photosensitive in adult age, even though 85% of them can become seizure free suggesting a separate developmental trajectory for the electrographic and clinical aspects of the phenotype. Adult-onset visually induced seizures have been less frequently reported in the context of both focal and generalized electroclinical syndromes. Photic-induced occipital seizures are the most prevalent phenotype (63%); both focal and generalized phenotypes share positive family history of epilepsy and facilitation by external factors such as tiredness and sleep deprivation. It is not yet known whether these age-related differences reflect the peak age of occurrence of the associated epilepsy syndromes or are the expression of true maturational differences in cortical excitability and brain connectivity patterns. EEG features associated with adult-onset photosensitivity are morphologically different from those seen in pediatric age, being often of lower voltage, limited to the posterior region of the scalp, failing to persist after the stimulation is extinguished and are usually not associated with overt ictal clinical manifestations.