Despite being involuntary and often unwanted, sneezing is an important reflex that the human nervous system employs to remove irritants from the nose. For example, when foreign objects enter through the nostrils, the brain can sense that these objects are inside the nose and proceed to initiate the sneezing reflex to expel them. These two phases of normal sneezing are called the sensory phase and motor phase. Sneezing is primarily controlled by the trigeminal nerve, the largest of the set of nerves inside the head called cranial nerves and the one responsible for facial sensation. In the sensory phase, nasal irritation is sensed by the trigeminal nerve, which carries the information back to a region in the brainstem. This region then initiates the motor phase of the reflex, resulting in a sneeze [1, 2].
Outside of normal sneezing exists an entirely separate group of sneezing-related phenomena. One particularly common condition that affects up to 35% of Americans is called the photic sneeze reflex . This reflex is conveniently abbreviated as ACHOO (Autosomal Dominant Compulsive Helio-Ophthalmic Outburst syndrome), so named because it involves sneezing in response to sudden increases in light intensity [1, 3]. Those afflicted with ACHOO experience uncontrollable bursts of sneezes upon exposure to bright light, such as the flash from a camera or moving from a dark room into the sunlight. The number of sneezes during each of these bursts is variable among people, but consistent for each person; it’s usually under 10, but there has been a reported case of 43 sneezes per burst ! The condition is associated with several genetic variants, and because it’s characterized by an autosomal dominant inheritance pattern, an affected person’s offspring have a 50% chance of inheriting the condition .
The mechanism behind ACHOO remains elusive. However, there are two prevailing theories; both involve unintentional “cross-talk” between neurons, but one focuses on the visual system while the other focuses on the parasympathetic nervous system, a network that mainly controls involuntary body functions, such as digestion . According to the first theory, there are physical variations in the brains of people with ACHOO, and these differences give rise to abnormal interactions between the visual system and the trigeminal nerve. Visual information is carried from the eyes to the brain through the optic nerve, which can potentially activate nearby neurons depending on its positioning and sensitivity, both of which could differ in photic sneezers. In fact, the sensitivity of the optic nerve could be so high that it abnormally excites entire regions of the brain in addition to the trigeminal nerve . This ability to activate other nerves and brain regions could be the reason that the optic nerve can trick the brain into initiating a burst of sneezes in response to bright light.
The second theory revolves around the parasympathetic nervous system (PNS). In the head region, there are several branches of the PNS that execute important functions. Since these branches are usually activated by a central signal from deeper in the brain rather than a localized signal at each small branch, sometimes there is unintentional activation of neighboring PNS branches when the brain is attempting to activate one specific branch. This unintentional activation is called “parasympathetic generalization” . In the head, there are two PNS branches that potentially interact to cause ACHOO: the first branch constricts the pupils in response to bright light, while the second branch is responsible for nasal secretions and congestion . When the brain attempts to activate the first branch in response to bright light, there could be a generalization of this PNS signal, thus inadvertently stimulating the second branch. The resulting fluid secretion in the nose would then be detected by the trigeminal nerve as nasal irritation, triggering the sneeze reflex.
Next time you walk out from a building or take a photo with flash, pay attention to whether anyone sneezes a few times. Once you’ve found an ACHOO victim, take some time to explain to them in detail how exactly their sneezes might be arising!
Staff Writer, Signal to Noise Magazine
MD/PhD Candidate, Molecular Biology Interdepartmental Doctoral Program, UCLA
 Songu, M. & Cingi, C. Sneeze reflex: facts and fiction. Therapeutic Advances in Respiratory Disease 3, 131–141 (2009).
 Nonaka, S., Unno, T., Ohta, Y. & Mori, S. Sneeze-evoking region within the brainstem. Brain Research 511, 265–270 (1990).
 Dean L. ACHOO Syndrome. 2012 Oct 15 [Updated 2015 Jul 27]. In: Pratt V, McLeod H, Rubinstein W, et al., editors. Medical Genetics Summaries [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2012-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK109193/
 Collie, W. R., Pagon, R. A., Hall, J. G. & Shokeir, M. H. ACHOO syndrome (helio-ophthalmic outburst syndrome). Birth Defects Orig. Art. Ser. XIV(6B), 361-363 (1978).
 Eriksson, N. et al. Web-Based, Participant-Driven Studies Yield Novel Genetic Associations for Common Traits. PLoS Genetics 6(6), e1000993 (2010).
 Langer, N., Beeli, G. & Jäncke, L. When the Sun Prickles Your Nose: An EEG Study Identifying Neural Bases of Photic Sneezing. PLoS ONE 5(2), e9208 (2010).