​SINGAPORE – It may be April Fool’s Day, but Achoo syndrome is no joke.

Autosomal dominant compelling helio-ophthalmic outburst syndrome, ostensibly named for its backronym, refers to a mysterious sneezing reflex in some people that is triggered by bright light, usually after coming out from a dark environment.

In severe cases, some studies showed that exposure to bright light can cause more than 40 consecutive sneezes, said Dr Tan Zhibin, an associate consultant at the Department of Neurology at the National Neuroscience Institute.

Dr Tan said sudden uncontrollable sneezing can be dangerous for pilots and drivers who are exiting a dark tunnel, though there have not been documented cases. Other researchers have brought up the dangers of the reflex to baseball outfielders and high-wire acrobats.

The exact mechanism behind the reflex has not been confirmed. However, there have been some theories.

A 2010 Swiss study published in a scientific journal by the Public Library of Science found neural differences between photic – or light-stimulated – sneezers and non-photic sneezers by measuring their brain activity.

In all normal subjects, light stimulates the occipital lobe in the brain located in the visual cortex, which receives and processes visual stimuli, said Dr Tan.

“But in patients with the photic sneeze reflex, this area of the brain is particularly overactive or oversensitive to visual stimuli. So it is triggered more strongly in response to bright lights.”

But what happens afterwards and how it leads to a sneeze is unclear, he said.

Another theory suggests that the reflex is caused by a crossing in pathways taken by electrical signals, said Dr Tan.

When there is an irritant in the nose, a person’s trigeminal nerve senses the irritation and sends signals to his brain to trigger a sneeze. This pathway is known as the sneeze reflex arc.

And when someone is exposed to bright light, the optic nerve sends signals to the brain to constrict the pupils, which is known as the pupillary light reflex arc.

For photic sneezers, the two pathways may cross, be connected, or be closer to each other than in other individuals, causing signals fired by the optic nerve to activate the trigeminal nerve. This results in the brain processing a sensation of nasal irritation, resulting in a sneeze, Dr Tan explained.

According to a 1987 study, it is estimated that 17 per cent to 35 per cent of the world’s population have the reflex.

While no one has successfully identified the exact genes at play, the photic sneeze reflex has been found to be passed on in an autosomal dominant pattern – meaning there is a 50 per cent chance of a child inheriting the gene causing this trait from a parent.

There are no recent studies on the prevalence of photic sneezing, as the sneezing does not require urgent attention, said Dr Tan.

Individuals with Achoo syndrome who spoke to The Straits Times said it was nothing more than a quirk.

Communication studies undergraduate Kimberly Pang, 21, said she usually sneezes after exiting a car or stepping out of her house into intense sunlight. But rather than an inconvenience, it has become a fun fact she shares with people.

“You can control it as much as you can control holding a normal sneeze back,” she said, adding that her mother and brother have the reflex as well.

On the flip side, Mr Donggeun Ahn, a business undergraduate in Germany, found the reflex uncontrollable and annoying.

He said the number of sneezes can exceed five in a row, hurting his throat.

Even then, the 23-year-old said the most annoying aspect was not the sneezing but the discomfort in his eyes, nose and mouth, which get itchy and can sometimes have a burning sensation.

Dr Tan said although there is no immediate urgency to learning about the reflex, understanding the mechanisms of Achoo syndrome may be beneficial.

“Given that understanding normal anatomy and physiology is often the first step to understanding human diseases, further research will not be harmful to the practice of medicine, and can potentially open doors to treatment of other disorders.”