If you find yourself sneezing when you come from the dark into the light, you’re not alone. Jason G Goldman investigates why this sudden syndrome strikes.

In 1991, a University of Manchester pathologist named Emyr Benbow wrote a letter to the editor of the British Journal of Ophthalmology. "Even trivial symptoms are more easily tolerated if you can put a name to them," he wrote, "even if that produces only an illusory understanding of their significance". The name he was referring to was "photic sneezing". Benbow suffered from a curious phenomenon where moving from darkness into very bright light, caused him to reflexively sneeze. He found it of some comfort that "it occurs in normal people".

The first formal investigation of the reflex was probably made in the early 1950s by a French researcher named Sedan. He discovered that some patients sneezed when he shined his ophthalmoscope, used to examine the retina, into their eyes. His continuing inquiry into six such photic sneezers established that they would also sneeze when exposed to bright sunlight, flash photography, and in one case, an ultraviolet light. In describing the phenomenon, he noted that the sneeze only occurs just as the patient becomes exposed to light; they don't continue to sneeze even if continually awash in the bright glow of the Sun (or an ophthalmoscope).

Since Sedan wasn't able to find any discussion of light-related sneezing in the medical literature, he concluded it must be quite rare. But by the time that physician H C Everett, who seems to have coined the phrase "photic sneeze reflex", wrote about it in the journal Neurology in 1964, it seems as if quite a bit more was known about the condition.

For one thing, it was known to be somewhat more common than Sedan had assumed. Researchers would go on to estimate that it afflicts some 17% to 35% of the world's population: some 23% of medical students in Everett's study, and 24% of blood donors in another.

The reflex was known to some perhaps as early as the third century BC

The tendency of some people to sneeze in response to bright light wasn't only just noticed in the last century; the Greek philosopher and scientist Aristotle observed the phenomenon as well. In his Book of Problems, he (or possibly his students) asked, "why does the heat of the Sun provoke sneezing, and not the heat of the fire?" He concluded that the Sun's heat aerosolises the fluids within the nose, which triggers a sneeze. The heat of a fire, on the other hand, not only vaporises those fluids, but also consumes them, thus drying out the nose, which actually inhibits a sneeze.

Never mind that he wasn't exactly spot on either in the cause for the sunny sneeze – it's light, not heat – nor in the explanation, but it means that the reflex was known to some perhaps as early as the third century BC.

Bright lights, big sneeze

We now know quite a bit more about the biology that underlies the photic sneeze reflex. For example, the reflex is now also known by the hilariously apt acronym Achoo, which stands for Autosomal Dominant Compelling Helio-opthalmic Outburst. "Autosomal" because the affiliated gene is located on one of the non-sex-linked chromosomes, and "dominant" because you only need to inherit it from one of your parents to express the trait.

In 2010, a group of geneticists led by Nicholas Eriksson of the genetic testing company 23andMe identified two single nucleotide polymorphisms, or SNPs, that were associated with the sunny sneeze by assessing the genotypes of nearly 10,000 23andMe customers. These SNPs are alterations to single letters within a person's genetic library. One is called rs10427255 and the other, about which there is somewhat less statistical certainty, is called rs11856995. One of them is located nearby a gene known to be involved in light-induced epileptic seizures, which raises the possibility that there might be some kind of biological link between the two syndromes.

There are precious few laboratory studies of the condition, but a Cleveland doctor named Harold H Morris III provided one case report. A 55-year-old woman referred to him had a history of suffering from seizures, both spontaneous and in response to light. She described herself as an "easy sneezer", but never really noticed if her sneezes could be a response to light as well. To find out, Morris shined some bright lights into her eyes in a variety of different ways.

He reported in 1989 in the Cleveland Clinic Journal of Medicine that he could make her sneeze by flashing lights into her eyes at a rate of 15 Hz. On average, the sneeze followed 9.9 seconds after the first flash. The flashes typically made her sneeze twice in a row, but at least once they induced three sneezes in rapid succession. "The intersneeze interval was two to four seconds" Morris carefully noted. As for the particular frequency at which his patient managed to sneeze, he suspects that it was nothing more than coincidental; there is no biological reason to suspect that there's anything unique or special about 15 Hz.

It can actually prove quite dangerous in certain situations

Despite the information that researchers have managed to amass on the subject, nobody quite knows exactly how optical stimulation of the eyes leads to a sneeze, but one possibility is that the eyes and the nose are connected via the fifth cranial, or trigeminal, nerve. Or it could be the result of a process called "parasympathetic generalisation". When a stimulus excites one part of the body's parasympathetic nervous system, other parts of the system tend to become activated as well. So when bright light causes the eye's pupils to constrict, that may indirectly cause secretion and congestion in the nasal mucus membranes, which then leads to a sneeze.

It sounds like it could be a fairly trivial issue but as Benbow pointed out in his 1991 letter, it can actually prove quite dangerous in certain situations. "I found that sudden exposure to sunlight when emerging from a road tunnel of sufficient length is sure to induce a sneeze, with accompanying momentary blindness," he wrote. Sunshine sneezing could also be detrimental for baseball outfielders and high-wire acrobats. Perhaps the need to avoid a mile-high sneeze also gave birth to that great fashion accessory: aviator shades.

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