More specific than any of the precipitants so far discussed are the stimuli which result in
so-called reflex epilepsy. Some young people have seizures induced by flashing lights, as in a discotheque, and this can be studied on an EEC In most of us, an obvious wave can be recorded from the back of the head (the occipital region) if a light is flashed in the eyes. With repeated flashes, these waves follow the flash frequency. At a critical frequency in a young person with photosensitive epilepsy, a totally different response of multiple spikes and waves—the photoconvulsive response—occurs, and a seizure may be induced. This of course is a laboratory situation, but seizures may result, in photosensitive children, from flickering light reflected from water, or by the interruption of steady light filtered through trees observed from a moving car.
The most common type of photosensitivity now encountered is television epilepsy. Experiments have shown that it is the normal sweep of the spots that make up the picture from side to side and down the face of the tube that is responsible, and not any malfunction of vertical picture or horizontal line hold. Susceptible children are most at risk when the screen occupies a considerable proportion of the visual field, as will occur if the size of the screen is large, and the child sits close to it, or approaches it to change the programme. The chances of seizures occurring are lessened by sitting far away from the screen. It may also help to reduce contrast between the screen and surroundings by placing the set near a lamp.
It has also been shown that the photoconvulsive response cannot be elicited if only one eye is exposed to the flashing light. It makes sense, therefore, for susceptible children to cover one eye if they approach the set. Remote programme selection by infra-red control is useful for such children. Both colour and monochrome television sets induce seizures, which are invariably generalized, though they may sometimes be of very short duration—just a few myoclonic jerks of arms and trunk muscles. Video games may also precipitate seizures. However, althought text on computer screens is occasionally associated with seizures, the problem is far less, and only occasional seizures have been reported.
Another type of visual reflex epilepsy occurs on looking at patterns such as squares of linoleum tiling. This may be regarded as typical of the highly specific reflex epilepsies occurring in a very few patients in which seizures may be induced by, for example, reading, hearing music (sometimes by only one particular phrase), or by performing mental arithmetic. The perception of such external stimuli must result in a particular pattern of nerve cell activity—this is presumably in part how we recognize tunes and words. One can only imagine that this particular set of activity in susceptible people acts as a specific template which, like a key in a lock, unlooses a seizure.
Non-specific stimuli—such as a loud noise, or a startle, however caused, may induce myoclonic jerks, and occasionally a generalized tonic-clonic seizure. This type of epilepsy is seen as an inherited feature in some strains of mice, and provides a model for the investigation of the physiology of such seizures, and a model for trying out the potential effectiveness of new anti-epileptic drugs.
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