In the U.S., epilepsy is the third most common neurological disorder after Alzheimer’s and stroke, affecting almost 3 million Americans. Epilepsy is a medical condition that causes seizures that affect a person’s mental and physical functions. A person is considered to have epilepsy when they have had two or more unprovoked seizures.
Seizures occur when nerve cells in the brain give abnormal signals that may briefly change a person’s consciousness, actions or movements. Seizures can be triggered by a variety of stimuli or mental processes such as hearing music or reading. Seizures that are habitually triggered by specific external stimulus and are not spontaneous are referred to as reflex epilepsy.
The most common type of reflex epilepsy is photosensitive epilepsy in which all or almost all seizures are provoked by flickering or flashing lights, both natural and artificial. Some patterned effects such as checks and stripes or contrasting dark and light patterns may trigger seizures for some people with photosensitive epilepsy. Note however, flashing lights and patterned effects may make some people ill, disoriented or uncomfortable but that does not necessarily mean that they have photosensitive epilepsy.
Several types of seizures can be provoked by flickering or flashing light but the most prevalent is the tonic-clonic (grand mal) seizure which is sometimes preceded by mild clonic jerking. Myoclonic, absence, or less common simple or complex partial seizures are the other forms of seizures that may occur. Almost 90% of persons who develop photosensitive epilepsy were previously affected by juvenile absence or juvenile myoclonic seizures in childhood. Photosensitive epilepsy largely occurs between the ages of 7 and 20 with the incidence of seizure being highest around age 12 and 13 suggesting a correlation with early puberty.
More girls than boys are affected and occurrence diminishes as the person approaches adulthood. Less than 5% of people with epilepsy are photosensitive and there are photosensitive persons who are yet to experience a seizure as not all flashing lights or visual patterns induce them. The intensity, duration and rate of the flashing light are what matter. A light with the flashing frequency of between 15 and 20 flashes per second will most likely trigger a seizure while a rate of below 3 flashes per second may not.
The electroencephalogram (EEG) is used to diagnose epilepsy by recording brainwave patterns from the small electrical signals that continually come from the brain. If looking at flashing or strobe lights during the EEG triggers epileptic activity in the brain it may indicate that the person has photosensitive epilepsy.
The risk of a seizure for persons with photosensitive epilepsy can be significantly reduced by avoiding stress and tiredness as well as ensuring that your vision is not completely filled by the trigger. It has been shown that in over 90% of persons who have photosensitive seizures, simulation of only one eye is much less able to trigger a seizure than simulation in both eyes. Hence shielding one eye when exposed to a potential trigger can significantly diminish the chances of a seizure.