Ever get caught in a sneeze when the dentist shines bright light into your eyes? You are not alone ̵
There is actually a name for this: the photic sneeze reflex. Or, if you want to have fun with it, call it Autosomal Compelling Helio-Ophthalmic Outburst (ACHOO) Syndrome. This reflex affects somewhere between a fifth and a third of the world's population – but what causes it? Let's see.
What is the photic sneeze reflex?
If this reflex persisted all the time you were in bright light, you could suffer. Millions of people would sneeze every time they spent a day in the sun. Fortunately, the photic sneeze reflex only hits when you first encounter a bright light. Then it disappears.
Normally sneezing helps us to remove irritants from the nose. That's why you sneeze more when you are sick or allergic. But the photic sneeze reaction clearly serves this purpose – and scientists don't actually know exactly why it exists.
What we do know is that it is a genetic trait. The gene for this reflex appears near a gene that plays a role in light-induced seizures, so there may be a link between the two. However, because the photic sneeze reflex is not particularly common or harmful, it has not been widely studied.
What triggers this reaction?
Light of any kind appears to cause the photic sneezing reaction – or more specifically, the transition from a dark area to a brightly lit area.
Everything from stepping in the sun to turning on a lamp to seeing a camera flash can cause a sneeze. Some people even use the reflex on purpose to cause a sneeze they feel build up. It doesn't matter what the light source is, as long as it follows a period of relative darkness: both natural and artificial light can cause it.
Many people can live with this reflex without realizing they have it because occasional sneezing is quite normal. You can easily test this response on yourself and your friends by using a flashlight or other convenient light source.
Something about the way light stimulates the eyes causes people with this genetic trait to sneeze. Researchers do not yet know exactly what that is.
Some believe that the link can be found in the trigeminal nerve, which connects the eyes and nose. It is possible for light to stimulate this nerve in a way that causes irritation in the nose. The nose misinterprets the nerve signal and makes you sneeze.
Or it may be that the light causes tears in the eyes, which flow into the nose and can cause sneezing. One study also linked the photic sneeze reflex to an aberrant septum, suggesting the causes go beyond just being genetic.
Although the photic sneeze reaction is usually harmless, it can occasionally pose a hazard. For example, a driver who starts to sneeze when leaving a dark tunnel is more likely to have an accident. Fortunately, if you want to combat this reaction, all you have to do is put on sunglasses.