Epilepsy affects about 50 million people worldwide according to the World Health Organization, and about 3% of people with epilepsy experience seizures triggered by flashing lights or visual patterns. This chronic condition is called photosensitive epilepsy and it should be a major consideration for web designers. Problematic visual elements could be dangerous, and improving the content’s accessibility will yield other benefits (including fewer barriers for other people).
However, photosensitive epilepsy is widely misunderstood. Here are some of the most common misconceptions surrounding photosensitivity and seizures — along with some tips for creating websites that reduce the hazard.
Myth 1: All visually triggered seizures involve convulsions
A seizure that leads to muscular stiffening and uncontrolled, jerking bodily movements is classified as tonic-clonic, but that’s just one type of seizure. In fact, there are more than 40 types of seizures, with symptoms ranging from waking loss of consciousness to visual hallucinations to convulsions. Common types of photo-induced seizures include:
- Eyelid myoclonia, characterized by fluttering eyelids and eyes rolling back.
- Generalized myoclonic jerks, in which muscles or groups of muscles contract and relax quickly.
- Absence, or a sudden, brief loss of awareness.
Regardless of their characteristics, seizures can be disorienting and uncomfortable. Avoiding large flashing lights and high-contrast geometric patterns on your site can help your users navigate safely.
Myth 2: Only children experience light-induced seizures
It’s true that photosensitive epilepsy is most common among children and adolescents, and that the risk of developing this condition diminishes with age. However, photosensitivity can persist well beyond the mid-20s, even if those cases become less common in older populations.
One epilepsy study found that "there was no specific age limit of freedom from photosensitivity in patients, especially in those with family history of photosensitive epilepsy." Even if your audience consists primarily of older adults, it’s important to avoid design elements that increase the risk of seizure.
Myth 3: Flashing lights are the only triggers of photo-induced seizures
A strobe effect is indeed a trigger for people with photosensitive epilepsy, but it isn’t the only trigger. As mentioned above, geometric patterns with high contrast are another common visual trigger, especially if the pattern moves quickly or changes directions. Developers should be careful with geometric contrasts of light and dark, including repeated patterns of stripes, bars, grids, or checkerboard designs.
Myth 4: All flashing lights are equally likely to cause seizures
Some flashing frequencies create a greater risk than others. Lights that blink between three and 30 times per second are most likely to trigger seizures. That said, some people experience seizures when exposed to flashing frequencies outside this range, so the best practice is to avoid large flashing areas on your site wherever possible.
Myth 5: Only people with epilepsy are affected by flashing lights and high-contrast patterns
Many site visitors may feel disoriented, uneasy, or nauseated by exposure to flashing lights or geometric patterns. People with eye conditions or sight loss may be more affected by these design choices. That doesn’t mean these users have photosensitive epilepsy, or are at risk for seizure, but it’s not a great user experience. As we mention frequently, accessible web design frequently enhances usability for all visitors.
Designing your website to consider people with photosensitivity
The simplest way to keep your site accessible for people with photosensitive epilepsy and other light-sensitivity disorders is to avoid flashing lights and high-contrast geometric patterns entirely. For a more nuanced approach, follow guidance in from the Web Content Accessibility Guidelines (WCAG).
To conform with WCAG success criteria, developers should follow these recommendations:
- If content must flash, keep it below three flashes per second.
- If content absolutely must flash above that threshold, keep it small — below 25% of the user’s 10-degree visual field (find more on that technical detail here).
- Use a tool to ensure content doesn’t violate the three-flash threshold or a related limit on saturated red flashes, as that color can increase the risk of seizures.