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Individual differences |
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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, and other forms of stress.
Television has traditionally been the most common source of seizures in PSE. An example of this is the infamous episode of Pokémon, Dennō Senshi Porygon. 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.
Some PSE patients, especially children, may exhibit an uncontrollable fascination with television images that trigger seizures, to such an extent that it may be necessary to physically keep them away from television sets. Some patients (particularly those with cognitive impairments, although most PSE patients have no such impairments) self-induce seizures by waving their fingers in front of their eyes in front of bright light or by other means.
While computer displays in general present very little risk of producing seizures in PSE patients (much less risk than that presented by television sets), video games with rapidly changing images or highly regular patterns can produce seizures, and video games have increased in importance as triggers as they have become more common. Some people with no prior history of PSE may first experience a seizure while playing a video game. Often the sensitivity is very specific, e.g., it may be a specific scene in a specific game that causes seizures, and not any other scenes.
As with video games, rapidly changing images or highly regular patterns can trigger seizures in people with photosensitive epilepsy. Two sets of guidelines exist to help web designers produce content that is safe for people with photosensitive epilepsy:
- The World Wide Web Consortium have produced Web Content Accessibility Guidelines. Version 1, produced in 1999, suggest designers allow users to control flickering or avoid causing the screen to flicker at all .
- In the USA, websites provided by federal agencies are governed by section 508 of the Rehabilitation Act. This says that pages shall be designed to avoid causing the screen to flicker with a frequency greater than 2 Hz and less than 55 Hz. 
Diagnosis may be made by noting the correlation between exposure to specific visual stimuli and seizure activity. More precise investigation can be carried out by combining an EEG with a device producing Intermittent Photic Stimulation (IPS). The IPS device produces specific types of stimuli that can be controlled and adjusted with precision. The testing physician adjusts the IPS device and looks for characteristic anomalies in the EEG, such as photoparoxysmal response (PPR), that are consistent with PSE and/or may herald the onset of seizure activity. The testing is halted before a seizure actually occurs.
Sometimes diagnostic indicators consistent with PSE can be found through provocative testing with IPS, and yet no seizures may ever occur in real-life situations. Many people will show PSE-like abnormalities in brain activity with sufficiently aggressive stimulation, but they never experience seizures and are not considered to have PSE.
Treatment and prognosisEdit
No cure is available for PSE, although the sensitivity of some patients may diminish over time. Medical treatment is available to reduce sensitivity, with sodium valproate being commonly prescribed. Patients can also learn to avoid situations in which they might be exposed to stimuli that trigger seizures and/or take steps to diminish their sensitivity (as by covering one eye) if they are unavoidably exposed. These actions together can reduce the risk of seizures to almost nil for many PSE patients.
Some PSE patients have trigger stimuli that are so specific that they are never likely to encounter them in real life. Their PSE may only be discovered by accident in an unusual situation or during examination for other complaints.
- Harding, Graham et al. (1994). Photosensitive Epilepsy. Mac Keith Press, London. (A definitive reference, and the source for most of the information in this article.)
- Henri Gastaut, neurologist
- Strobe light
- Bucha effect
- Psychogenic non-epileptic seizures
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