So why may sexual ideation or orgasm trigger sneezing? And does this serve any function? Without experimental investigation we can only surmise, but analysis of existing literature may provide theoretical support.
One possibility is a psychiatric response, with the sneeze representing a forceful emission of sexual tension, but sneezing is reflex and not volitional (although psychosomatic sneezing has been described, typically in teenage girls32
). Another possibility (in keeping with the theories of Fliess) is a humoral mechanism, whereby nitric oxide released to cause genital tumescence enters the bloodstream and causes engorgement and irritation of the nasal mucosa too. However, this would take a long time to be affected, and studies have shown that in penile erection, nitric oxide is released locally but does not enter the systemic circulation to an appreciable extent.32
It seems that a neurological rather than a humoral cause for this phenomenon is more probable because of the immediacy of the response. Here, the other recognized unusual causes of sneezing may provide a clue as to the cause. The most frequent and the most investigated of the ‘unusual’ triggers of sneezing is light (i.e. the photic sneeze reflex), but the cause of this is still unknown. Everett who was the first to investigate the phenomenon suggested several theories.6,28,34
The first is that the eye responds to bright light with photophobia via output to the first trigeminal nerve (ophthalmic) and this may cause an inadvertent response in the second division of the trigeminal nerve (maxillary). This maxillary nerve stimulation suggests there to be a source of nasal irritation, which triggers the sneeze reflex. However, although triggering of other branches of the trigeminal nerve is known to induce sneezing, there is no evidence that this is more frequent in the population affected by the photic sneeze reflex. The second theory is that light leads to lacrimation, and the tears produced drain via the nasolacrimal duct to cause irritation of the nasal mucosa in sensitive individuals. However, this would take a long time to be affected and seems unlikely to cause the almost immediate sneezing response that is observed. A third theory proposed by Everett is of ‘parasympathetic summation’, whereby parasympathetic neuronal output to one organ is indiscrete and accompanied by parasympathetic output to other organs. Here, parasympathetic nervous system pupillary miosis in response to light also leads to parasympathetic mediated secretion from the nasal mucosa, triggering sneezing. It seems to us that sneezing in response to sexual ideation or in response to orgasm may also be an effect of parasympathetic summation and support the latter of these theories.
It is known that the neurones that will subsequently form the cranial outflow of the parasympathetic nervous system originate in the vagal region of the embryonic neural tube, in a region not too distant from the putative future sneeze centre. Although there is a separate sacral region of the neural tube for formation of sacral parasympathetic outflow, recent ablative studies of vagal neural crest cells in the chick embryo suggest that the vagal centre plays a central role in all parasympathetic innervation.35
Furthermore, the parasympathetic system is phylogenetically old and is not arranged somitically. There is evidence of persisting links between different components of the parasympathetic nervous system, whereby stimulation of one parasympathetic response will lead to other parasympathetic responses. For example during sleep, bradycardia and hypotension may be accompanied by penile erection. These interconnecting pathways have not been researched in depth, but there is evidence of descending projections from the parasympathetic nucleus of the eye (the Edinger-Westphal nucleus) to the brainstem36
which would support the notion of aberrant stimulation of extra-ocular parasympathetic pathways in response to light.
We note that all of the reported triggers of sneezing that arise independent of a nasal trigger have parasympathetic outflow as a common variable. For the photic sneeze response there is parasympathetic activity to cause pupillary constriction in response to light. When the stomach is full there is parasympathetic activity to stimulate gastric peristalsis and acid secretion. When there is sexual ideation, there is parasympathetic outflow to cause venous dilation leading to penile or clitoral tumescence. At orgasm, although ejaculation may be sympathetically mediated, there is also parasympathetic activity to effect secretion from glands producing components of male or female ejaculate ().
Hypothesized parasympathetic efferents as a trigger for sneezing
We surmise that the key to understanding all unusual triggers of sneezing is that parasympathetic nervous system outflow is interconnected and indiscrete, in what Everett termed ‘parasympathetic summation’. Thus, any trigger that stimulates a parasympathetic response will mean that efferents from the relevant parasympathetic nucleus will also project back to their embryonic origin at the vagal parasympathetic nuclei, which may in turn stimulate other parasympathetic responses. This may include a response to the parasympathetic greater superficial petrosal nerves, which relay via the pterygopalatine ganglion to effect nasal secretion and consequent irritation. This nasal irritation will then result in a sneezing response ( ).
Hypothesized parasympathetic summation as a trigger for sneezing
The finding that both sneezing in response to light and in response to fullness of the stomach are inherited in an autosomal dominant pattern suggests the possibility of a genetically determined aberration in parasympathetic neuronal embryogenesis as an explanation. We have reported sneezing in response to sexual ideation or orgasm, and it is possible that this too could be inherited in an autosomal dominant manner. It may be difficult to know, because in general, parents and children are unlikely to discuss such phenomena with each other due to social inhibitions.
We report here that sneezing in response to either sexual ideation or after orgasm is an under-reported phenomenon, and may be much more common than expected. We surmise that a mechanism of parasympathetic summation is the cause of this unusual response, and that this mechanism may also account for other reported unusual causes of sneezing – exposure to light and fullness of stomach. Further investigation in this field may help us to understand the sneeze reflex in more depth, and also allow us to give explanation and reassurance to the possibly significant number of people affected by this curious phenomenon.