The photic sneeze reflex (also known as ACHOO syndrome, a contrived acronym for Autosomal-dominant Compelling Helio-Ophthalmic Outburst[1]) is an inherited and congenital autosomal dominant reflex condition that causes sneezing in response to numerous stimuli, such as looking at bright lights or periocular (surrounding the eyeball) injection.

[3] A survey conducted by the School of Optometry at the University of Alabama at Birmingham found that 67% of respondents who identified as photic sneezers were female and 94% were Caucasian.

Those who exhibit this symptom or disorder, called snatiation, undergo uncontrollable fits of 3–15 sneezes immediately after eating large meals that completely fill the stomach, regardless of the type of food eaten.

In particular, photic sneezing poses a considerable risk to pilots, due to the frequent presence of bright sunlight and the precise reactions needed to successfully control the aircraft.

The reflection of the sun from surrounding water has a high probability of producing at least one photic sneeze for pilots who have the reflex.

Propofol has been shown to temporarily suppress inhibitory neurons in the brainstem, which is also where the trigeminal nucleus – the "sneeze center" of the brain – lies.

[11] A small-scale study comprising a Caucasian family from Spain done by Sevillano et al. in July 2016 revealed that prominent corneal nerves might be associated with the photic sneeze reflex.

[15] While this phenomenon is poorly understood, recent research has shown that antihistamines being used to treat rhinitis due to seasonal allergies may also reduce the occurrence of photic sneezes in people affected by both conditions.

[16] Those affected by photic sneezing may find relief by shielding their eyes and/or faces with hats, scarves, and sunglasses,[1] or by applying transverse pressure on the philtrum with their finger.

"[20][21][22] He hypothesized that the sun's heat caused sweating inside the nose, which triggered a sneeze in order to remove the moisture.

[23] In the 17th century, English philosopher Francis Bacon disproved Aristotle's hypothesis by facing the sun with his eyes closed, which did not elicit the ordinary sneeze response.

He assumed that looking at the sun's light made the eyes water, and then that moisture proceeded to seep into the nose and irritate it, causing a sneeze.

[22] Although plausible, scientists later determined this hypothesis to also be incorrect because sun-induced sneezing occurs too quickly after sunlight exposure; watering of the eyes is a slower process, so it could not play a vital part in triggering the reflex.

[25] A common myth or old wives tale is that the photic sneeze reflex held an evolutionary advantage for caveman for when they were sleeping or residing in caves.