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Photosensitive epilepsy is a form of epilepsy in which seizures are triggered by visual stimuli that form patterns in time or space, such as flashing lights, bold, regular patterns, or regular moving patterns.
Of all persons who have been diagnosed as epileptic, between three and five percent are known to be of the photosensitive type (approximately two people per 10,000 of the general population). Often persons with PSE have no history of seizures outside of those triggered by visual stimuli. Females are more commonly affected than males, and there is distinct genetic correlation. Symptoms usually first appear during childhood or adolescence, with a peak at the beginning of puberty, and few people present with PSE after the age of 20.
Persons with PSE experience epileptiform seizures upon exposure to certain visual stimuli. The exact nature of the stimulus or stimuli that triggers the seizures varies from one patient to another, as does the nature and severity of the resulting seizures (ranging from brief absence seizures to full tonic-clonic seizures). Many PSE patients experience an “aura” or feel odd sensations before the seizure occurs, and this can serve as a warning to a patient to move away from the trigger stimulus.
The visual trigger for a seizure is generally cyclic, forming a regular pattern in time or space. Flashing lights or rapidly changing or alternating images (as in clubs, around emergency vehicles, in action movies or television programs, etc.) are an example of patterns in time that can trigger seizures, and these are the most common triggers. Static spatial patterns such as stripes and squares may trigger seizures as well, even if they do not move. In some cases, the trigger must be both spatially and temporally cyclic, such as a certain moving pattern of bars.
Several characteristics are common in the trigger stimuli of many PSE patients. The patterns are usually high in luminance contrast (bright flashes of light alternating with darkness, or white bars against a black background). Contrasts in color alone (without changes in luminance) are rarely triggers for PSE. Some patients are more affected by patterns of certain colors than by patterns of other colors. The exact spacing of a pattern in time or space is important and varies from one individual to another: a patient may readily experience seizures when exposed to lights that flash seven times per second, but may be unaffected by lights that flash twice per second or twenty times per second. Stimuli that fill the entire visual field are more likely to cause seizures than those that appear in only a portion of the visual field. Stimuli perceived with both eyes are usually much more likely to cause seizures than stimuli seen with one eye only (which is why covering one eye may allow patients to avoid seizures when presented with visual challenges). Some patients are more sensitive with their eyes closed; others are more sensitive with their eyes open.
Sensitivity is increased by alcohol consumption, sleep deprivation, illness, and other forms of stress.
Television has traditionally been the most common source of seizures in PSE. For patients with PSE, it is especially hazardous to view television in a dark room, at close range, or when the television is out of adjustment and is showing a rapidly flickering image (as when the horizontal hold is incorrectly adjusted). Modern digital television sets that cannot be maladjusted in this way and refresh the image on the screen at very high speed present less of a risk than older television sets.
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