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BLOG: Why do we sneeze at the sun?
Roughly 25% of Americans have a tendency to sneeze when exposed to a bright light. Does this sound crazy to you? Or do you think that number is too low?
The photic sneeze can be a conversation starter at a dinner party or something your patient might mention during an exam.
It seems to have some variability, but is essentially this: When a person is dark-adapted and then is exposed to a bright light, a sneeze reflex is triggered. Depending on the person, it can trigger a single sneeze or multiple sneezes, but once it calms, there is a refractory period of several hours or longer.
Sometimes called sun sneezing, the condition has been demonstrated with several different light sources, so the more common term is photic sneezing. While there certainly are more debilitating conditions that eye doctors could be concerned about, photic sneezing is one that we should be aware of, if only to reassure a concerned patient or parent that this condition is normal. Of course, photic sneezing is not harmless. A driver emerging from a long tunnel only to sneeze three times while traffic merges around him is a scary situation in which some of us have found ourselves.
I am a photic sneezer, and a fairly bad one at that. For me it probably happens four or five times a week and is triggered by a full stomach, which is also reported in the literature. (Snatiation is a portmanteau of sneeze and satiation, but is an actual condition that some people experience as they sneeze when they experience fullness of the stomach.) Sometimes it’s bad enough that I have to go into a dark room and sit there until the sensation passes. Then I’ll be fine until the next day. I come from a family of five; my mother is a photic sneezer, but my father is not. Both of my siblings, Jessica and Adam, are photic sneezers. Our family is relatively typical of the epidemiology of photic sneezers: it has been shown to have an autosomal dominant mode of inheritance and is much more common in white people.
Henry Everett wrote the landmark study, “Sneezing in response to light,” in 1964 at Johns Hopkins in Baltimore, and found photic sneezers in: 2% of black males, 2% of black females, 28% of white males and 15% of white females. Prevalence studies in America have shown as low as 11% and as high as 36%, 24% in Sweden and 25% in England, to name a few (I couldn’t find any studies out of Asia or Africa, but perhaps this re-emphasizes that it’s rare there?).
To understand why a photic sneeze happens, it helps to review why any sneeze happens. The sneeze reflex happens in two phases. The first is a sensory/afferent phase, and the second is a motor/efferent phase. The sensory part of a sneeze is mainly related to the maxillary (or second) branch of the trigeminal nerve. Small branches of that nerve innervate the nasal mucosal epithelium, and when they are triggered by any number of different stimuli, they transmit an impulse back to the trigeminal ganglion and then on to the sneezing center in the lateral medulla. Once a threshold is reached in the medulla, the efferent phase begins. This consists of “eye closing, deep inspiration and then a forced expiration with initial closing of the glottis, and increasing intrapulmonary pressure” (Songu et al.). Interestingly, during my research I found a case of a patient with a tumor in the dorsal medulla impinging on the nucleus and the trigeminal tract. This patient had an inability to sneeze, presumably due to the interruption of the sneeze reflex, and the condition was termed “asneezia,” which I think is just about the best name for any condition ever.
Essentially, the sneeze is our body’s reflexive way to wash out mucosal irritants and debris. So what’s the physiology of photic sneezing? There are three theories. The first is thought of as crossed stimulation. If you recall the anatomy of the cranial nerves, you know that the maxillary branch of the trigeminal nerve runs very close to the optic nerve as it makes its way to the medulla. In some people’s anatomy, it’s possible that the nerves are close enough that a large depolarization of the optic nerve (ie, a bright light after being dark adapted) is enough to trigger a depolarization along the maxillary nerve – a referred innervation, if you will.
The second theory is another crossed stimulation, but this time happening in the ciliary ganglion. Recall that in the pupillary light reflex, retinal fibers innervate the pretectal nuclei and then the Edinger-Westphal (EW) nuclei. The parasympathetic fibers from the EW nuclei and the trigeminal afferent fibers both pass through the ciliary ganglion, where they may participate in referred innervation.
Finally, the third theory is known as parasympathetic generalization and it’s a tricky one, but essentially thought of as stimuli that activate one branch of parasympathetic neurons tend to activate other branches at the same time. This happens often elsewhere in the body, but in the photic sneeze example is essentially this: When the second cranial nerve is exposed to bright light, the brain sends a message down the third cranial nerve to constrict the pupils but also down the seventh cranial nerve to cause lacrimation. This triggering of seventh cranial nerve can also cause nasal congestion and secretion, which can then cause a “trickling” sensation that then triggers the fifth cranial nerve and initiates a sneeze (Everett). This etiology might not account for why some people have the condition while others don’t, except to say that photic sneezers may have a greater sensitivity to parasympathetic stimuli in the nasal mucosa.
Sneezing was mentioned twice in the Odyssey, as a positive omen for both Odysseus and Penelope. It was generally thought of as favorable in Western society until the great plague of the 14th century, when sneezing was considered foreboding of death, and it became common to say a short prayer for loved ones apres-sneeze. In some Eastern cultures, when a person sneezes it signifies that someone else is talking about them; one sneeze means speaking good things about you and two sneezes mean bad things. If you’re a photic sneezer like me, then I wish you all single sneezes.
References:
Collie WR, et al. Birth Defects: Original Article Series. 1978;14(6b):361-363.
Everett H. Neurology. 1964;14;483.
Forrester JM. Hum Hered. 1985;doi:10.1159/000153527.
Langer N, et al. PLoS ONE. 2010;doi:org/10.1371/journal.pone.0009208.
Songu M, et al. Ther Adv Respir Dis. 2009;doi:org/10.1177/1753465809340571.
Whitman B, et al. Neurology. 1993;43:868-871.