An uncontrollable sneeze when stepping into bright sunlight is a response known as the photic sneeze reflex, or ACHOO syndrome. It is an inherited genetic trait that causes some individuals to sneeze when their eyes are suddenly exposed to a bright light. The reflex is not an illness and is mild, though its intensity can vary from person to person.
The Science Behind the Sneeze
The name ACHOO is an acronym for Autosomal Dominant Compelling Helio-Ophthalmic Outburst. “Autosomal dominant” describes its pattern of inheritance; it is a genetic trait not linked to sex chromosomes and can be passed down from a parent to a child with a 50% probability if just one parent carries the gene. While the specific genes are not fully identified, this inheritance pattern explains why the tendency to sneeze in bright light often runs in families.
The exact biological mechanism of the photic sneeze reflex is not completely understood, but a leading theory involves crossed signals in the nervous system. This hypothesis suggests a close relationship between the optic nerve, which transmits light signals to the brain, and the trigeminal nerve, which controls sneezing. When the optic nerve is suddenly stimulated by an intense change in light, it is thought to inadvertently activate the nearby trigeminal nerve.
This cross-activation tricks the brain into thinking the nose is irritated, initiating the sneeze reflex. The stimulus is not the light itself, but rather the sudden change in light intensity, such as when exiting a dark tunnel into the sun. This results in one or more sneezes that are unrelated to allergens or irritants in the air.
Potential Risks and Management
For most individuals, the photic sneeze reflex is a harmless curiosity. However, an unexpected fit of sneezing can pose a risk in certain situations. This is true for drivers leaving a garage or tunnel and for pilots who are frequently exposed to bright sunlight.
The reflex can also affect athletes, such as a baseball outfielder who might look up into the sun to catch a fly ball. While there is no cure for ACHOO syndrome, management focuses on preventative strategies.
Practical management techniques are straightforward. Wearing sunglasses, particularly those with polarized lenses, can dampen the intensity of light reaching the optic nerve. Putting down a car’s sun visor before emerging from a dark area can prevent the sudden light change. Shielding one’s eyes with a hand or hat can also prevent an episode.
Prevalence and Diagnosis
The photic sneeze reflex is quite common, with estimates suggesting it affects between 18% and 35% of the global population. Some research has indicated that certain demographic populations may show a higher incidence of the trait, though it is found in people worldwide.
There is no formal medical test or procedure to identify ACHOO syndrome. Diagnosis is made based on a person’s own consistent report of the phenomenon. If an individual regularly experiences a prickling sensation in their nose followed by sneezes after exposure to bright light, they have the reflex. The number of sneezes can range from a couple to more than a dozen in a single burst.