We thought this unusual and performed a literature search of the topic. In the 19th century, a link between the nose and the genitals was recognized. Both Watson in 1875¹ and Mackenzie in 1884² had noted a link between sexual excitement and sneezing in some individuals, but there was no credible reason given for the phenomenon. Then Fliess, a young German otolaryngologist and close friend of Freud, developed his theory of ‘nasal reflex neurosis’^3,4 linking the nasal mucous membranes with the genital areas, perhaps due to the finding of erectile tissue in both areas. How the erectile tissue of these distant anatomical sites is linked in its response was never explained, but Fliess went on to expand the theory, suggesting that dysmennorhoea was due to nasal disorder. He expanded even further, suggesting that the nose was linked to menstruation and that menstruation was an underlying cyclical timing of all organs in the body. He consequently suggested that a huge number of neurological, psychological or sexual complaints could be treated with medication or surgery of the nose. His ideas became so fanciful that he was ridiculed and fell out of favour with both his medical contemporaries and with Freud.

The only subsequent report of this phenomenon in the medical literature dates from a letter to the Journal of the American Medical Association of 1972⁵ where a 69-year-old man is described who complains of severe sneezing immediately following orgasm, again with no psychiatric morbidity. There were two replies to this report, one of which propounded the theories of Fliess and suggested that ‘undoubtedly’ this was due to the similarities between the erectile tissue of the nose and that of the penis, and suggesting topical nasal anaesthesia. The other reply was from Everett, who had investigated sneezing in response to sunlight⁶ and stated that a few patients had also mentioned sneezing in response to sexual ideation during his investigations. However, he too eventually also suggested treatment with topical nasal anaesthesia for this symptom.

From the sporadic reports in the medical literature, we surmised that the phenomenon of sneezing in response to sexual ideation or in response to orgasm is more common than recognized. As this is a problem that does not lead to morbidity and may be seen as embarrassing to discuss it may be under-reported. We performed a search of Internet ‘chat rooms’ where this problem may be discussed in an anonymous forum, using the Google search engine and with terms ‘sex, sneeze OR sneezing’. We searched in June 2007, and again six months later in December 2007. We found many discussions of the phenomena, with 17 people of both sexes reporting sneezing immediately upon sexual ideation,^7–18 and three people after orgasm.^19–21 Non-expert responses to the queries proposed all manner of explanation, but none that are credible. In four of the threads, respondents reported that they too suffered the same problem. In the reports it appears the sneezing occurs immediately upon sexual ideation, or very soon after orgasm. It also seems that the two phenomena do not occur in the same person, i.e. that these are two distinct entities: sneezing with sexual ideation or sneezing with orgasm. No people mentioned that they suffered neurological or rhinological disease.

Although Internet reports do not give us an accurate incidence of these phenomena, our findings do suggest that it is much more common than recognized.

Sneezing most commonly occurs in response to nasal irritation. Animal studies show that afferent sensory nerve fibres from the nasal mucosa carried in the first two trigeminal nerves relay to the ventromedial spinal trigeminal tract and the adjacentpontine-medullary reticular formation ( Figure 1).²² In humans too, the lateral medulla (close to the spinal trigeminal tract) is thought to be the likely location of the sneezing centre; evidenced by case reports of inability to sneeze in patients with lateral medullary syndrome or medullary neoplasm,^23–25 and a case report of paroxysmal sneezing in a patient with an infarct in the lateral medulla.²⁶ The efferent arc of the reflex involves an initial inspiratory phase then an expiratory phase where air is expelled at speeds of around 150 km/h, with the soft palate and uvula depressed to direct air through the nasal passages. This arc therefore requires the integrated activity of the respiratory, pharyngeal and laryngeal musculature. Of note, the response to nasal stimuli, including the sneeze response, also includes a parasympathetic nervous system mediated increase in nasal and lacrimal secretions.²⁷

It is recognized that stimulation of non-nasal branches of the trigeminal nerve may provoke sneezing, for example with eyebrow plucking. However, several unusual triggers of sneezing have been described that do not seem to involve a trigeminal pathway. A well-recognized entity is the photic sneeze reflex, sometimes also referred to as the autosomal dominant compelling helio-ophthalmic outburst (ACHOO) syndrome,^6,28 thought to affect as much as 24% of the population.²⁹ The syndrome is defined by bouts of sneezing when looking at sunlight. It appears to be inherited as an autosomal dominant trait, but the genetic locus has not been identified. In separate reports, two families have been described who appear to sneeze after meals, seemingly provoked by fullness of the stomach.^30,31 Again this appears to be inherited in an autosomal dominant manner but with the genetic locus unknown.

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 (Table 1).

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 ( Figure 2).

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.