The Photic Sneeze Reflex

Have you ever sneezed as soon as you walk outside on a bright sunny day? I did today, and I have for just about as long as I can remember. This is a described human trait, occurring in about 1 out of 5 people. It’s called the photic sneeze reflex. It happens to me quite often in the summer, within seconds of walking outside from a building when I am suddenly exposed to sunlight. I then immediately sneeze 1-3 times.

The photic sneeze reflex was apparently described in the late 1970’s. It has also been called the Autosomal dominant Compelling Helio-Ophthalmic Outburst (also known as ACHOO). Scientists are unsure why such an optical stimulus as the sun would provoke a sneezing response, which usually occurs as a protective measure to some irritative nasal airway stimulus. I’m not quite sure if other people experience this, but to me it actually feels like something is in my nose, yet my nose may be completely clear. Apparently a lot of research has been done concerning this phenomenon since it was first discovered, and it is now known to have an autosomal dominant pattern of inheritance (meaning 50% of an affected person’s offspring will inherit the phenomenon). I guess I was one of the lucky ones.

Many hypotheses exist as to what exactly provocates the sneezing response to sunlight, but the leading theory says that it is a congenital "mix-up" of nerve fibers which result in a connection between the 1st cranial nerve (the optic nerve) with the 5th cranial nerve (the trigeminal nerve). The trigeminal nerve is involved with sneezing. Thus visual stimuli, which are transmitted by the optic nerve, trigger the trigeminal nerve, resulting in a sneeze reflex. Perhaps one day I may test this hypothesis by having affected subjects walk outside in the summertime after being indoors (for a specific amount of time), wearing a black blindfold over their eyes. If a large number of them still sneeze, the photic sneeze reflex is most likely not due to a mechanism involving optical stimulation. Although I am unsure of other people’s experiences with this phenomenon, it does not appear to occur with sudden exposure to other bright lights, but only with exposure to the sun. As well, I do not have to directly look at the sun in order for the reflex to occur. Thus there may be some other component of sunlight which initiates this reflex.
As you can see, the photic sneeze reflex is more of a nuisance, but nevertheless interesting. Perhaps it is just an anomaly? It would appear not to endow humans with some sort of adaptive ability or survival advantage if they had inherited it. In fact, due to it’s potential dangers, would-be pilots who have the photic sneeze reflex are not allowed to fly combat jets, for fear they would sneeze when exposed to the sunlight through their cockpits. As well, some people have reported car accidents due to this reflex! Nevertheless, perhaps in the future this unusual phenomenon will shed light on how neural circuitry develops from the code contained within our inherited genetic material.