ACHOO syndrome: Why some people sneeze every time they see the sun

Adel Manuel doesn't have any mutant superpowers, but he has the next best thing: He can sneeze on command.

"I can’t fly, but I can sneeze," he says during a phone call.

Ever since he was a kid, Manuel, a 40-year-old resident of southeastern Michigan, knew that if he stared into a bright light — the sun or a bright lightbulb — he'd automatically sneeze.

"I can shoot off five, six of them in a row, just looking at bright light," he tells me. And the brighter the light, the better: "I have a bright LED light in my basement — if I look straight at it, I’m going to go."

I called Manuel because I wanted to know what it's like to live with the photic sneeze reflex, a genetic quirk that affects between 15 and 30 percent of people (it's more common than green eyes) but is very poorly understood.

Manuel agreed with my characterization that it's a bit of a superpower. It's also helpful if he feels a sneeze coming on and just can't get it out. A bright light "will push it through," he says.

I wish I had that ability.

How the photic sneeze reflex works

Jeroen / Flickr

Photic sneeze reflexes don't seem to make any sense at first. The purpose of sneezing is to clear unwanted irritants or germs from the nose. Bright light produces neither.

The phenomenon was first described in the 1950s, after a French doctor discovered several patients sneezing in response to the light of his ophthalmoscope, (the device doctors use to examine the retina). Further investigation revealed that it wasn't just any light that brought on the sneezing, but sudden flashes. These patients would reliably sneeze when quickly exposed sunlight, flash photography, and, in a few cases, even ultraviolet light.

In the 1970s, the condition was formalized under the acronym ACHOO. (Yes, really.) It stands for "autosomal dominant compelling helio-ophthalmic outburst syndrome."

But since it's not a pressing medical concern ("It’s not like you’re crippled to go outside," Manuel says), photic sneeze reflexes haven't been researched very well. There are just a couple dozen papers investigating the condition.

Over the years, researchers have floated a few theories for why the sneezes happen.

1) Photic sneezing may have its roots in our parasympathetic nervous system, which controls many of our involuntary actions. One possibility is that the sudden activation of one component of this system — i.e., the constriction of pupils and tearing up in the face of bright light — spontaneously activates another component, sneezing. But that wouldn't explain why some people have photic sneezing and others don't.

2) Another possibility: Perhaps the optic nerve, which responds to light, gets its wires crossed with the fifth cranial nerve, which controls some facial movements. "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 [fifth cranial] nerve and mistaken by the brain as an irritant in the nose," author Karen Schrock explained in a 2008 Scientific American article. That interaction results in a sneeze.

This might explain why only some people do it. One paper in the journal Eye speculates that genetic variation might bring the optic and cranial nerves in closer contact.

That's just a guess. "The precise [neurological] pathway is not known," Louis J. Ptáček, a neurologist at the University of California San Fransisco who has studied the reflex, says in an email.

3) It could also be something a bit more deeply rooted in the brain: A 2010 study out of Switzerland found that the visual cortices in the sneezers' brains are more easily excited than those in non-sneezers. This suggests that the reflex isn't a classic reflex generated in the brain stem — such as the reflex to move your hand away from a hot stove — but involves deeper brain processing.

4) It's possible the trait is a leftover of evolution: that at one point, it was advantageous for a baby's survival to be more prone to sneezing.

Ultimately, we don't really know what causes photic sneezing.

And it's a scientific mystery that has existed since antiquity. Even Aristotle was perplexed by it. He wrote in his Book of Problems, "Why does the heat of the sun provoke sneezing?" His best guess: "Because the heat of the sun doth dissolve." Whatever that means.

Can photic sneezing be dangerous? Occasionally.

In 1991, British pathologist E.W. Benbow wrote a letter to the editor of the British Journal of Ophthalmology in response to a paper about photic sneezing. Benbow charged that the medical journal had neglected to mention a major hazard for sneezers: driving.

Specifically: driving through a dark tunnel and then reemerging into the light. Benbow wrote this is enough to trigger a sneeze (he had the trait) and produce "momentary blindness." That's not ideal.

Similarly, a group of Swiss researchers reported that "photic sneezing could be dangerous for individuals in certain professions, such as baseball outfielders, high-wire acrobats, and airplane pilots." (If you're a high-wire acrobat who has struggled with photic sneezing, please, please contact me. The world needs to hear your story.)

In 1993, a team of scientists in the Air National Guard determined that the reflex posed a risk for combat pilots. They worried it "could trigger an unexpected sneezing episode during critical periods of flight." But there's an ingenious solution to this problem: sunglasses.

Photic sneezing is surprisingly common

Comprehensive numbers don't exist, but the best estimates suggest around 15 to 30 percent of humans have this trait, and it may be more common among some ethnic backgrounds than others.

One survey on American medical students in the 1960s (not exactly a representative sample) found that 28 percent of the white males and 16 percent of the white females had it. The largest survey — of around 500 blood donors in Sweden circa 1983 — came up with a figure of around 20 percent.

Photic sneezing is thought to be a lot less prevalent among African Americans (the best estimate is around 2 percent of African Americans have it).

But the studies that do exist suggest the trait is dominant. Which means if you're a photic sneezer, there's a 50 percent chance your offspring will have it too.

In 2010, the gene testing company 23andMe conducted a big opt-in study on its customers, looking to link certain behaviors with genes. The results, published in the journal PLOS, showed there was indeed a genetic marker for the reflex. This genetic marker was located near a gene that's known to be associated with seizures. That suggests further study of the gene may yield insights for epilepsy.

"There may be a link between photosensitive epileptic seizures and photic sneeze reflex (triggered by a sudden switch from being dark-adjusted to light) providing a possible link between this genetic region and photic sneeze reflex," Joanna Mountain, 23andMe's senior director of research, says in an email.

But, again, like so much of the research into this odd, but oddly common, phenomenon, this is just a hunch.

Are there any other weird stimuli that cause people to involuntarily sneeze?

Why yes! There's:

• Sexually induced sneezing. ("Sneezing in response to either sexual ideation or after orgasm ... may be much more common than expected," a paper in the journal of the Royal Society of Medicine reads.)
• Sneezing induced by feeling full. (One case study of a relatively normal 32-year-old man found that "fullness of the stomach immediately after meals invariably results in three or four uncontrollable sneezes."
• And the weirdest: For some, eating chocolate can induce some unwanted sneezes.

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