Stupid Question ™
Dec. 24, 1998
By John Ruch
Q: Can sunlight make you sneeze? It makes me sneeze, but my friends say I’m crazy.
A: My Christmas present to you, Cale, is the knowledge that you’re not crazy. “Photic sneezing,” or “ACHOO” (Autosomal Dominant Compelling Helioophthalmic Outburst syndrome), is a real and apparently physiological phenomenon.
About 20 percent of the population are photic sneezers of varying sensitivity. They will sneeze when they see any bright light, typically when moving from a dark area into a brightly lit one. (Leaving a movie theater on a summer day is a classic set-up.)
The number of sneezes varies person to person, but for any given person remains about the same. The sneezing can be repeated ad infinitum by re-exposure to bright light. Some photic sneezers have learned they can help along a regular, dust-induced sneeze by looking at an electric lamp.
In tests, some photic sneezers could consciously restrain the sneezing, especially when they knew bright light was coming. No particular wavelength of light was more inductive, and sunglasses eliminated the effect.
Photic sneezing has been documented in three successive generations of a single family and may be genetically determined. (Geneticists are the ones who call it ACHOO.) It appears to occur more frequently in Caucasians.
Some photic sneezers say sneezing while driving (say, from out of a tunnel, or from shadow into sunlight) is a hazard.
Experiments to determine how the photic sneeze operates are difficult to perform and haven’t been done. Theories go back at least to Francis Bacon’s 1635 musings on moisture leaking from the brain, but modern consensus is that the mechanism is neurological.
The eyeball, tear glands, sinuses and facial skin all are sensorally linked by the ophthalmic nerve. It runs up the sinuses and into the ophthalmic ganglion, a wad of nerve cells in the back of the eye very near the optic nerve.
When the optic nerve—the one we see with—is stimulated by bright light, we feel discomfort or even pain. In photic sneezers, some of this stimulation somehow “jumps” to the ophthalmic ganglion and down the ophthalmic nerve, which experiences the stimulation as phantom irritation of the nose. It responds as it always does to nasal irritation—by setting off the sneeze reflex.
So it’s pain, not really light, that seems to be the key factor. An Ohio State University Medical Center ophthalmologist told me that after he removed a patient’s eye, she could touch a spot in the socket and induce sneezing at will.
The same phenomenon may affect horses, which are sometimes afflicted with headshaking, a syndrome of snorting, sneezing, bucking and shaking that can render them unrideable.