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 sneezing reflex. Another theory suggests a passage connecting the tear ducts to the nose called the nasolacrimal duct. It enables tears to drain out of the eye. Tears leaking into the nose through the nasolocrimal duct are a possible cause of photic sneeze reflex.
The photic sneeze reflex is considered a risk factor to combat pilots: people suffering from photic sneeze reflex may not fly combat aircraft.
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Deshmukh, N. "Sneezing response to bright light. Is it a cause of accidents?" The Guthrie Journal, 1995, 64(3): 104-105 [On-line], Available http://www.cathouse.org/medical/photic_sneeze.html
Breitenbach, R.A, "The Photic Sneeze Reflex as a Risk Factor to Combat Pilots." Military Medicine, 158, December 1993: 806-809 [On-line], Available http://www.cathouse.org/medical/photic_sneeze.html References