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POLL: Are You a Photic Sneezer?

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Gattamalata writes:

Scientific American article Looking at the Sun Can Trigger a Sneeze: For some people, bright lights mean big sneezes by Karen Schrock link

Have you ever emerged from a matinee movie, squinted into the sudden burst of sunlight and sneezed uncontrollably? Up to a third of the population will answer this question with an emphatic "Yes!" (whereas nearly everyone else scratches their head in confusion). Sneezing as the result of being exposed to a bright light—known as the photic sneeze reflex—is a genetic quirk that is still unexplained by science, even though it has intrigued some of history's greatest minds.

Aristotle mused about why one sneezes more after looking at the sun in The Book of Problems: "Why does the heat of the sun provoke sneezing?" He surmised that the heat of the sun on the nose was probably responsible.

Some 2 ,000 years later, in the early 17th century, English philosopher Francis Bacon neatly refuted that idea by stepping into the sun with his eyes closed—the heat was still there, but the sneeze was not (a compact demonstration of the fledgling scientific method). Bacon's best guess was that the sun's light made the eyes water, and then that moisture ("braine humour," literally) seeped into and irritated the nose.

Humours aside, Bacon's moisture hypothesis seemed quite reasonable until our modern understanding of physiology made it clear that the sneeze happens too quickly after light exposure to be the result of the comparatively sluggish tear ducts. So neurology steps in: Most experts now agree that crossed wires in the brain are probably responsible for the photic sneeze reflex.

A sneeze is usually triggered by an irritation in the nose, which is sensed by the trigeminal nerve, a cranial nerve responsible for facial sensation and motor control. This nerve is in close proximity to the optic nerve, which senses, for example, a sudden flood of light entering the retina. As the optic nerve fires to signal the brain to constrict the pupils, the theory goes, some of the electrical signal is sensed by the trigeminal nerve and mistaken by the brain as an irritant in the nose. Hence, a sneeze.

But because this harmless (albeit potentially embarrassing) phenomenon doesn't seem to be linked with any other medical condition, scientific study of the subject has been scarce. Research has done little more than document its existence and attempt to gauge its prevalence. No rigorous studies exist, but informal surveys peg 10 to 35 percent of the population as photic sneezers. A study in the 1960s showed that the trait is autosomal-dominant—the gene is neither on the X nor Y chromosome and only one copy of the gene has to be present for the trait to be expressed—so if one parent sneezes when they look at a bright light, about half of his or her children will, too.

The genetic culprit remains unidentified, but scientists are starting to take an interest in trying to find out. "I think it's worth doing," says Louis Ptácek, a neurologist at the University of California, San Francisco, and an investigator at the Howard Hughes Medical Institute. Ptácek studies episodic disorders such as epilepsy and migraine headaches, and he believes that investigating the photic sneeze reflex could shed light on their related neurology.

Epileptic seizures are sometimes triggered by flashing lights and migraine headaches are often accompanied by photophobia. "If we could find a gene that causes photic sneezing, we could study that gene and we might learn something about the visual pathway and some of these other reflex phenomena," Ptácek says.

But until he and his colleagues find the right families for their study, the photic sneeze reflex will remain something of a genetic novelty act, like the ability to roll your tongue. Although a 1993 paper in the journal Military Medicine raised concerns that light-induced sneezing might endanger fighter pilots, for whom a split second of lost vision could be lethal in certain situations, such fear was largely put to rest when a small study found that wearing sunglasses eliminated the effect.

Beyond that blip of gravitas, papers published about photic sneezing have largely leaned toward the whimsical end of the spectrum. Consider one 1978 publication that took advantage of the then-raging acronym fad and suggested an alternate name for the photic sneeze reflex: Autosomal-dominant Compelling Helio-Ophthalmic Outburst syndrome, or, of course, ACHOO.

Author's followup in the comments section:

I mistakenly oversimplified the genetics involved when I wrote, "the trait is autosomal-dominant�the gene is neither on the X nor Y chromosome and only one copy of the gene has to be present for the trait to be expressed�so if one parent sneezes when they look at a bright light, about half of his or her children will, too."

If the parent who is a photic sneezer has only one copy of the gene, this is true. If a parent who sneezes has two copies of the gene, all of his or her children should be sneezers. And if both parents are sneezers and they both have one copy of the gene, each kid has a 75% chance of having the photic sneeze reflex.

Now, on to the fun stuff… Jables, your question about good eyesight is interesting. As far as I can tell, nobody has studied that. However, a somewhat related paper link from 1990 describes patients with neuropathic cystinosis, a metabolic disorder that causes the nonprotein amino acid cystine to accumulate in cells, who experienced an unusual number of light-induced sneezes during an eye examination. The researchers offer several hypotheses about why the crystal deposits on their corneas might have caused the photic sneeze reaction.

water_moon: there is a lot of anecdotal evidence that babies are far more likely to be photic sneezers than adults are, and that many babies sneeze in response to light even though neither parent does. There isn't any research on the topic, but I have an idea about why a baby might be a photic sneezer even if the parents are not. Babies are born with many more synaptic connections in their brains than adults have, and as they grow up these synapses are pruned so that only the essential ones remain and are strengthened. It could be that the gene for photic sneezing actually prevents some pre-existing connections from being destroyed, so that the light-reacting and sneeze-producing parts of the brain remain wired together (whereas in most people, these connections are severed during childhood). This idea is very similar to the current theory of the origin of synesthesia link , a sensory mix-up that causes some people to "taste sounds" or "hear shapes."

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Gattamalata suggests...

The first glint of sunlight in the morning, and I'm sneezing. Midday sun passes by the window, and the glare results in a sneeze or two, and I don't even have to look at the sun. Sunglasses does alleviate things, but who wears shades indoors, other than the cool kids? grin