Photic sneeze reflex (also whimsically called ACHOO, a backronym for Autosomal dominant Compelling Helio-Ophthalmic Outburst) is a medical condition by which people exposed to bright light involuntarily sneeze. It has been suggested that the photic sneeze reflex occurs only after someone has been adapted to the dark for at least five minutes, although this is not certain, and is not uniform amongst people with the photic sneeze reflex. The condition occurs in one-sixth to one-quarter of humans, with more common occurrence in Caucasians than other human races. The trait is passed along genetically, with a 50 percent chance of inheritance.
The probable cause is a congenital malfunction in nerve signals in the trigeminal nerve nucleus. The fifth cranial nerve, called the trigeminal nerve, is apparently responsible for sneezes. Research suggests that some people have an association between this nerve and the nerve that transmits visual impulses to the brain. Overstimulation of the optic nerve triggers the trigeminal nerve, and this causes the photic sneeze reflex. Another theory suggests that tears leaking into the nose through the nasolacrimal duct are a cause of the photic sneeze reflex. The speed of the reflex seems to favour the first theory, as it happens much too quickly for tears to be generated and drain into the nose.
The photic sneeze reflex is considered a risk factor to combat pilots: people suffering from photic sneeze reflex may not fly combat aircraft.
References
Semes LP, Amos JF, Waterbor JW. "The Photic Sneeze Response: Descriptive Report of a Clinic Population." Journal of American Optometry Association June 1995, 66(6): pp. 372–377 [On-line], Available: [1]
Deshmukh, N. "Sneezing photic response to bright light. Is it a cause of accidents?" The Guthrie Journal, 1995, 64(3): 104–105 [On-line], Available: [2]
Breitenbach, R.A, "The Photic Sneeze Reflex as a Risk Factor to Combat Pilots." Military Medicine, 158, December 1993: 806–809, Abstract: [3]
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