Reflex epilepsy · Grey Matter

Reflex epilepsy is epilepsy in which seizures are reliably set off by a specific stimulus or activity, and it is the rare case where a seizure can be provoked on demand, which makes it both a clinical problem and a window on how a network tips over.

Reflex epilepsy. Reflex epilepsy is epilepsy in which seizures are reliably set off by a specific stimulus or activity, and it is the rare case where a seizure can be provoked on demand, which makes it both a clinical problem and a window on how a network tips over.

Flashing light is by far the most common trigger, accounting for most reflex seizures. In a photosensitive person, light flashing at about 15 to 25 times a second is the most provocative, red flashes and high-contrast stripes or checkerboards are worse than other colours and patterns, and the risk grows with how much of the visual field the stimulus fills. The EEG test is the same stimulus under control: a strobe swept through frequencies while the recording watches for a photoparoxysmal response.

Other triggers engage other networks. Reading can provoke jaw jerks or focal seizures in reading epilepsy; calculation, decision-making or playing games can trigger seizures in some generalised epilepsies; music, eating, hot water and startle are rarer triggers.

Photosensitivity is common and photosensitive epilepsy is rare. Abnormal EEG responses to flashes are found in a small percentage of the population, while epilepsy with light-triggered seizures affects about one person in 4,000, mostly young, and it is common in juvenile myoclonic epilepsy.

Broadcast and web rules come from it. Guidelines limit content to no more than three flashes in any second and restrict saturated red flashes, and broadcasters screen programmes with automated testers.

Treatment combines avoidance and drugs. Covering one eye reduces the area stimulated, and valproate and similar drugs lower the sensitivity.

A reflex seizure shows that the threshold is local.

The stimulus pushes one network, the one it uses, and a network sitting close to its edge is the one that tips.

Questions: Why can light flashing at 15 to 25 times a second trigger seizures in photosensitive people? Each flash drives large numbers of neurons in the visual cortex at the same instant, so a rhythmic flash imposes synchrony from outside, and in a cortex prone to synchronise this can grow into a self-sustaining discharge. Flash rates of about 15 to 25 per second are the most provocative, red and high-contrast patterns are worse, and the risk grows with how much of the visual field the stimulus fills. Why some brains respond this way is only partly known, with a genetic contribution, and photosensitivity is most common in children and adolescents.