Reflex, photic sneeze: A disorder characterized by nearly uncontrollable paroxysms of sneezing provoked in a reflex fashion by the sudden exposure of a dark-adapted subject to intensely bright light, usually to brilliant sunlight. The number of successive sneezes is usually two or three but can be up to about 40. The Achoo syndrome is also called the photic sneeze reflex or the helio-ophthalmic outburst syndrome.
The syndrome is a frequent genetic trait. It is inherited in an autosomal dominant manner, affecting males and females alike, who have a 50:50 chance of passing the Achoo gene to each of their children who will have the disorder.