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Photic Sneeze Reflex

January 9, 2014

Have you ever looked into a bright light to help you sneeze? Ever walked out of a dark room into the sun and suddenly sneezed without reason? If these sound like ridiculous questions, it’s likely that you are not among the 20-30% of people with a photic sneeze reflex. Sometimes this condition is referred to as Autosomal-dominant compelling helio-ophthalmic outburst syndrome, or “ACHOO” syndrome. Oh scientists, you slay me.

The photic sneeze reflex has intrigued scientists for ages, though. Even famous Greek dudes like Aristotle would write about it. He mused that the heat of the sun upon his nose stimulated sneezing, a theory that was later disproved by simply walking into the sun with eyes closed.


Other scientists hypothesized that bright light made the eyes water and subsequently drain into the nasal cavity, stimulating the sneeze reflex. This was a good guess, but the time between stimulus and sneeze is much too short for it to be plausible.

So if it’s not heat, and it’s not physical stimulation of the nasal passages… where does that leave us? Modern day scientists point toward a neurological basis for the photic sneeze reflex. You’ve all heard of the optic nerve, that big bundle of neurons behind your eyeball that sends information from your retina to your visual cortex. Well, there is another large nerve in your face called the trigeminal nerve. It has three branches that innervate your whole face (including your schnoz), and its the one that causes sneezing.


Because the optic nerve is in close proximity to the trigenimal nerve, scientists believe the photic sneeze reflex is the result of some minor “wire crossing.” A very large stimulus to the optic nerve can accidentally activate the trigeminal nerve, triggering a sneeze even without nasal irritation.

Clever acronym aside, we can actually gather a good amount of information about this condition just from the name. The term “autosomal-dominant” means that it’s inherited from your parents (we think), and you only need to get one copy of the gene to have the syndrome. The term “compelling” indicates that the sneeze happens involuntarily, which probably isn’t all that weird considering most sneezes happen that way. “Helio” is Greek for sun, the originally identified stimulus, although any bright light can trigger the reflex. “Opthamalic” means that it has to do with stimulation of the eye, and “outburst” just means sneeze. So there you have it – ACHOO!

Humans are such beautiful creatures

So if the condition itself is harmless, why should anyone care? Apart from discussing your weird genetic problems at parties (because everyone does that, right?), there’s a growing interest in studying how optic signals can activate inappropriate responses. For example, some forms of epilepsy are triggered by strong light stimuli, particularly when those lights are flashing. We don’t fully understand why this specific stimulusĀ triggers a seizure, although we have some knowledge about the neural defects that allow it to propagate. Our current understanding of neural circuitry can be summarized as follows:


So THAT is why a funky sneeze reflex is interesting to scientists! The more we can understand about the way our bodies are put together, the easier it will be to find causes/cures when things go wrong.


From → Biology

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