91av

Are you a photic sneezer? Take the questionnaire and find out

I WAS rounding the corner to the bus stop when it hit me – a bright shaft of sunlight smack between the eyes. My reaction was immediate: an unpleasant prickling in my nose, a quickening of my breath, an uncontrollable watering of my eyes. Then, almost as quickly as the sensation came, relief, blessed relief. Aaaaa-tisshoo! A sneeze.

It wasn’t the first time. In fact, the same thing happens every time I go into the sun. For a long time, I thought it was a quirk all of my own. Then a friend mentioned she was similarly afflicted. Next my mother came out of the closet. With a bit of digging around I came to a startling realisation: not only am I not alone, but the “photic sneeze reflex” is actually common. Quite how common, no one knows exactly – but anything between 1 in 10 and 1 in 3 of us might be affected.

The more I looked, the more mysterious things became. Sunlight is the most widespread, but by no means the only, odd stimulus that sets off sneezing. Thinking about sex, eyebrow tweezing, eating chocolate or a mint, or drinking a glass of wine – all these activities can leave us groping around for handkerchiefs. Members of one Kuwaiti family were even – a phenomenon dubbed . Then there was the case of the medical student who sneezed with almost clockwork precision around 8.20 every morning.

All these oddities are faithfully recorded in the medical literature, almost always accompanied by an earnest call for further investigation. But a teensy problem has always stood in the way. Despite the fact that everyone does it, we still don’t fully understand the way the nervous system coordinates a normal sneeze, let alone a photic sneeze.

A sneeze, one of the most violent actions your body will ever perform, should be triggered by an irritant in the mucus-producing membranes of our nostrils. This stimulates the endings of the trigeminal or fifth cranial nerve, which mediates sensory inputs from all over our face. The result is a cascade of reactions throughout our body: mucus production in the nose, a momentary closing of the eyes, and a wave of motor impulses down into our torsos to generate an explosive expulsion of air at up to 150 kilometres an hour – an extremely localised personal hurricane if you like.

The forces are considerable. Instances of whiplash through sneezing are not unknown, and a sneezing fit in 2004 threw Chicago Cubs baseball legend Sammy Sosa’s back into spasm, knocking him out of the game for two weeks. I once gave myself a hernia during a pollen-induced sneezing fit in the bath.

“Instances of whiplash through sneezing are not unknown”

Coordinating such a ferocious manoeuvre – together with the mucus production and blink reflex – is no mean feat. Sneezes are orchestrated by the parasympathetic nervous system, that part of our subconscious control hardware that regulates reflex housekeeping activities from producing tears and saliva to moving the waste products of digestion down the gut to our bowels.

The nerves within the parasympathetic nervous system that coordinate sneezing feed into a part of our brainstem known as the medulla oblongata. A series of experiments conducted by researchers from Asahikawa Medical College, Japan, in 1990 showed this was the case for cats (Brain Research, ), and it seems to be true for humans too since some people with damaged medullas lose the ability to sneeze (Neurology, ).

Unfortunately, current brain imaging techniques are not sensitive enough to pin down exactly which neurons within the medulla control the sneeze response. It is within this elusive “sneeze centre” of the brain that the mystery of photic sneezing lies.

This mystery has a long history. In the 4th century BC, Aristotle asked why the heat of the sun prompts us to sneeze, whereas the heat of the fire does not. A partial answer came two millennia later, when the English natural philosopher Francis Bacon showed that his photic sneeze had nothing to do with heat at all: if he closed his eyes when going into the sun, he didn’t sneeze even though the heat was still there. While Bacon’s application of the scientific method was beyond reproach, his conclusions are distinctly iffy to a modern nose. “The cause is not the heating of the nostrils,” he asserted, “but the drawing down of the moisture of the brain.”

It took a long time, however, before modern science could better that explanation. Henry Everett, a consultant psychiatrist at the Johns Hopkins University Hospital in Baltimore, Maryland, was the first to make a systematic attempt to understand the condition. Noting in 1964 that six of the 18 junior psychiatric doctors at Johns Hopkins were photic sneezers, he questioned 75 of his patients and 169 of his students in detail about their sneezing habits (Neurology, ).

In the group of patients, 18 per cent reported a photic sneeze reflex; among the students, it was 24 per cent. Small sample sizes in this study and subsequent research have made it difficult to work out with any certainty how common the condition is. About 35 per cent of my colleagues at 91av turned out to be photic sneezers, which is significantly higher than Everett’s findings but seems to fit the upper bound set by studies since.

Everett also thought to ask his volunteers about photic sneezing among their nearest and dearest. Whereas 80 per cent of sneezers reported other sneezers among their close relatives, only 20 per cent of non-sneezers did. As Everett perceptively observed, the results were likely to be skewed. People with the condition tend to be attuned to other people’s sneezing (apart from me, that is), whereas those without it often don’t notice when other members of their family are sneezers.

Nevertheless, the correlation was too significant to ignore, suggesting photic sneezing is an inherited rather than acquired response to environmental conditions, as had previously been assumed. Subsequent studies have borne out that hunch, with patterns of inheritance suggesting that it is carried on a dominant gene (Birth Defects, vol 14, p 361), so anyone with just one copy would be afflicted. This is known as autosomal dominant transmission, giving scientists the unmissable opportunity to rename the condition “autosomal-dominant compelling helio-ophthalmic outburst” – ACHOO for short.

So it seems I can blame my genes for my photic sneeze. I don’t have to look far for the culprit: my mother, caught red-handkerchiefed.

But that was only half the answer. Now I wanted to know what exactly this aberrant gene was doing. How come it made both my mother and me sneeze when our eyes, not our noses, were stimulated?

Curious conditions

Clues might lie, I thought, in some of the other curious conditions. So I phoned Mahmood Bhutta of Wexham Park Hospital in Slough, UK. Together with his colleague Harold Maxwell, he caused a stir in December last year with his description of patients who sneezed at orgasm or even in response to having sexual thoughts (Journal of the Royal Society of Medicine, ).

The connection between sex and sneezing is not a new observation. As far back as 1875, scientists had an inkling that the root cause was the erectile tissues common to both the nether regions and the glands of the nasal cavity. The theory was that nitric oxide released during arousal to dilate blood vessels in the genitals could diffuse through the body and excite the erectile tissue in the nasal cavity, triggering a sneeze.

Bhutta doesn’t have much time for this explanation – the diffusion would take far too long to explain the rapidity of symptoms, he says. “The speed and involuntary nature of the response means the cause is likely to be neurological.” That chimed with something I had read in Everett’s paper, where he proposed a series of possible neurological explanations for photic sneezing.

It might be, for example, the result of confused signalling from a hyperactive trigeminal nerve as it gathers tactile information from across the face. Somewhere along the nerve the impulses from different nerve endings around the eye and in the nose may become scrambled, confusing the brain about the origin of the different signals. That seemed plausible enough, and since the nerve endings in the eyebrows also belong to the trigeminal, something similar might also account for sneezing when tweezing.

Alternatively, Everett suggested, the photic sneeze could be explained by a special connection between trigeminal and optic nerves. That was attractive, since it might also lie behind the mysterious phenomenon of .

There was still something vaguely unsatisfactory about these explanations, though, since none can explain all those different types of sneezing at one fell swoop. What about orgasmic sneezing and snatiation, for example?

The answer to that, Bhutta suggests, might lie in another one of Everett’s hypotheses: that the confusion arises in the way the medulla regulates our reflex actions. Everett originally proposed this idea to explain just photic sneezing, but Bhutta thinks it could explain all the strange sneezing conditions, since all of the triggers involve stimulation of a parasympathetic nerve response controlled by the medulla. When bright sunlight hits our eyes, our pupils contract involuntarily – a parasympathetic response. When our stomachs are full, the parasympathetic system kicks in to start our gastric juices flowing. When we think of sex, parasympathetic action stimulates blood flow to our genitals.

All these nerve responses flow to and from regions of the medulla close to where the sneeze centre is located. This suggests that far from being a neat system of discrete responses to individual stimuli, our reflex systems at their base in the medulla are often a tangled web of cross-talking nerve wires. Sometimes when bright sunlight hits our eyes, the parasympathetic system responds appropriately and our pupils constrict. But for certain people whose medullas are wired differently, sunlight triggers a different reflex response, such as a sneeze.

Nervous overkill is no deal breaker in the survival stakes as long as the right reflexes are also stimulated at the right time, so aberrant genes that cause confused reflexes in some individuals would have been conserved by evolution. “It’s a mess,” says Bhutta, “because it’s never had to be anything else.”

All this is just a hypothesis, not established fact, Bhutta emphasises, and is likely to remain so until we fashion better tools for studying the activity of individual nerve pathways in living humans. This is a sentiment echoed by , a neurogeneticist at the University of California, San Francisco. “People speak as if they know what the hell’s going on,” he says. “In reality, we don’t.”

That doesn’t make Ptácek any less interested in the photic sneeze. On the contrary, he hopes work on the disorder could shed light on far more serious conditions, such as migraines and epilepsy, which are also caused by crossed wires in the nervous system. These disorders are diverse and often bizarre. In a condition called paroxysmal dyskinesia, for example, a startle or sudden voluntary movement can induce dance-like involuntary movements lasting for minutes on end; and there are instances of epilepsy induced by listening to music or reading.

Certain parallels between these conditions and photic sneezing are obvious. “We know, for example, that some people with epilepsy have seizures if you flash strobe lights at them,” says Ptácek. “But measure the brain waves of an epileptic with no strobe response when a strobe is switched on, and you still see sudden spikes in brain activity.” That looks rather like just a more generalised version of photic sneezers’ over-the-top response to a light stimulus.

The fact that photic sneezing is so common and is almost certainly also an inherited trait could provide an unparalleled opportunity to get to grips with these neurological confusions. Ptácek is hoping someone will study the condition in detail and pin down its pathology once and for all (see “Are you a sneezer?”). “It would be just the thing for some ambitious young doc to make his or her mark with,” he says.

The photic sneeze has long been overlooked because its effects are generally less than serious. The US military once studied it as a , true, but otherwise reports have concentrated on more esoteric concerns such as its potential .

Ptácek thinks the connection to the kind of disorders he studies means that a change of perception is long overdue. “Sometimes to discover things that make you go ‘wow’ in science,” he says, “you have to follow your nose.”

From a darkened room I’ll say “bless you” to that.

Are you a photic sneezer? Take the questionnaire and find out