Visual stimuli photosensitivity and photosensitive epilepsy

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The most common reflex epilepsies are those induced by visual stimuli. Flashing lights, bright lights, moving visual patterns (e.g. escalators), eye closure, moving from dark into bright light, and viewing specific objects or colours have all been reported to induce seizures.

Photosensitive epilepsy is a form of simple reflex epilepsy. The term should be confined to those individuals who show unequivocal EEG evidence of photosensitivity, and differentiated from other, usually more complex, cases in which seizures can apparently be precipitated by visual stimuli but in whom EEG evidence of photosensitivity cannot be demonstrated. Photosensitivity (strictly defined) is present in the general population with a frequency of about 1.1 per 100,000 persons, and 5.7 per 100,000 in the 7-19 age range, and is very strongly associated with epilepsy. About 3% of persons with epilepsy are photosensitive and have seizures induced by photic stimuli (usually viewing flickering or intermittent lights or cathode ray monitors, bright lights or repeating patterns). The flicker frequency precipitating photosensitivity varies from patient to patient, but is most commonly in the 15-20 Hz range. The peak age of presentation of photosensitive epilepsy is 12 years, the male : female ratio is 2 : 3, and the propensity to photosensitivity declines with age. Most patients with photosensitivity have the syndrome of idiopathic generalized epilepsy, although photosensitivity also occurs in patients with focal epilepsy arising in the occipital region. In idiopathic generalized epilepsy, myoclonus, absence and tonic-clonic seizures can be precipitated by photic stimuli, and factors such as sleep deprivation or alcohol intake have additive effects—partying can involve all factors, and seizures are common the morning after the night before. Alternating patterns (such as in some video games, or when looking down large escalators) can precipitate seizures in photosensitive patients, as can disco lights or poorly tuned TV screens (which flicker at the mains alternating current frequency of 50 Hz in the UK and Europe, but not in the USA). Other common stimuli include bright light shimmering off moving water, or the flickering of light through trees from a moving vehicle, and the transition from relative darkness into bright light. Most photosensitive patients have non-photically induced seizures also, but photic seizures can be prevented or reduced by wearing glasses with tinted or polarized lenses, and by avoiding situations known to induce photosensitive responses. Photosensitivity also occurs in some patients with occipital lobe epilepsy and in some of the benign occipital focal epilepsy syndromes.

Television-induced seizures (and, far less common, seizures induced by video games or computer screens) can be reduced by taking the precautions listed in Table 1.33. In photosensitive persons with occipital lobe epilepsy, seizure discharges may also be caused by fixation-on or fixation-off stimuli.

Treatment with valproate, benzodiazepine drugs or levet-iracetam usually completely abolishes photosensitivity, even at doses that do not provide complete seizure control.

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