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Editor,—Photosensitive epilepsy was first reported by Gowers in 1885 who described a girl who had attacks when going into bright sunshine, and a man who had a visual aura of bright blue lights and subsequent fitting evoked by looking at a bright light.1 The first report of epileptic seizures caused by watching television was made by Livingstone in 1952, and since then numerous cases have been documented.1
Almost 10% of newly diagnosed cases of epilepsy in 7–19 year olds demonstrate photosensitivity on electroencephalogram (EEG) and have seizures triggered by watching television or by computer games.2 Photosensitive epilepsy can usually be controlled by sodium valproate3; however, an important part of management of this condition is the avoidance of stimuli that provoke the fit.
A 9 year old boy was referred to a paediatric neurologist because of episodes of absence seizures on watching television. This at times progressed to generalised tonic-clonic seizures. He was clinically normal otherwise apart from slight clumsiness, slow speech, and poor handwriting.
His mother and paternal grandmother suffered with grand mal epilepsy, but neither were photosensitive.
An EEG showed interseizure epileptiform activity that increased on photic stimulation.
He was started on sodium valproate that was gradually increased to a dose of 1000 mg twice a day (39.9 mg/kg/day). This initially reduced his seizure frequency but in his teenage years, he suffered more frequent attacks of tonic-clonic seizures. Compliance, both with medication and with avoidance of environmental stimuli, had by this age become an issue. Conventional polarised glasses over his own myopic correction improved his condition but he continued to have attacks four or five times a month with everyday stimuli such as flickering light and television.
At age 16 years he was prescribed spectacles polarised at 90 degrees for the right eye and 180 degrees for the left eye. He was delighted with these glasses as these allowed him to visit amusement arcades and watch television without having an attack. This effect was substantiated on EEG tracings. The response to the photic stimulus when wearing cross polarised glasses was abolished (Fig 1). We explored the effect of polarisation of light at different axes using spectacles with variable polarisation (Fig 2). We found that cross polarisation at right angles to the horizontal and vertical was most effective and polarisation of the two eyes at the same axis least effective.
There was a considerable increase in EEG activity on photic stimulation with no glasses on.
A blood sample taken randomly and at the time of EEG tracing showed no trace of antiepileptic drugs. This suggests that our patient was using the cross polarised spectacles instead of his medication to avoid photosensitive epilepsy.
Closing one eye is known to be an effective way of avoiding photosensitive epilepsy.4 Flickering environmental stimuli from reflected sources are an important cause of photosensitive epilepsy.1 Since reflection from a plane surface is associated with polarisation, we reasoned that polaroid filters that obstructed horizontally polarised light from one eye and vertically polarised light from the other might be helpful in photosensitive epilepsy. This is because the patient would be rendered effectively monocular for reflected stimuli.
We were initially surprised by the effectiveness of cross polarised spectacles in eliminating photosensitivity in our patient on direct stimulation from a flickering light source and not just from reflected surfaces. However, it would appear that with a poorly defined flickering image, identical polarisation of the stimulus for each eye may be necessary for the brain to summate the two flashes perceived by each eye into a single enhanced photic stimulus.
Our patient’s polarising lenses reduced the visual acuity from 6/5 to 6/6. Like most readily available polarising lenses intended for sunglasses, these lenses incorporated some additional tint; it should be possible to provide lenses that are lighter and more cosmetically acceptable. The greater effectiveness in our patient of lenses polarised at right angles to each other, rather than in the same plane, confirmed that the benefit of the glasses was not simply due to a darkening effect.
Further investigation of the effects of other methods of disturbing binocular perception may throw light on the mechanism by which occlusion of one eye reduces photosensitivity. The effectiveness of cross polarised spectacles in other patients with photosensitive epilepsy needs to be investigated.
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