Achoo syndrome: 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 2 or 3, 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 much more common than has been generally recognized. In one study it was found in 23% of medical students. The syndrome is one of the most frequent of all known genetic traits. 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. For those affected, being in tunnels and shadows created by tall buildings and forested terrain can induce sneezing upon re-entering the bright sunlight.

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