Photic sneeze reflex

The photic sneeze reflex (also known as photoptarmosis, Autosomal Dominant Compelling Helio-Ophthalmic Outburst Syndrome (ACHOO)^[1] and colloquially sun sneezing) is a condition that causes variable difficulty to control sneezing in response to numerous stimuli, such as looking at bright lights or periocular (surrounding the eyeball) injection. The condition affects 18–35% of the population in the United States,^[2] but its exact mechanism of action is not well understood.^[3]

Overview

The photic sneeze effect is a genetic tendency to begin sneezing, sometimes many times consecutively (due to naso-ocular reflex^[4]), when suddenly exposed to bright light. This condition tends to occur more severely after one has emerged into the light after spending time in a dark environment.^[5] Although the syndrome is thought to affect about 18% to 35% of the human population, it is relatively harmless and not widely studied.^[3]

The photic sneeze effect has been documented for many centuries. The Greek philosopher Aristotle was one of the first to contemplate this strange phenomenon in 350 BCE, exploring why looking at the sun causes a person to sneeze in The Book of Problems: "Why does the heat of the sun provoke sneezing?"^[6][7] He hypothesized that the sun’s heat caused sweating inside the nose, which triggered a sneeze in order to remove the moisture.^[8] In the 17th century, English philosopher Francis Bacon disproved Aristotle’s theory by facing the sun with his eyes closed, which did not elicit the ordinary sneeze response. Bacon therefore guessed that the eyes played a vital part in triggering photic sneezing. He assumed that looking at the sun's light made the eyes water, and then that moisture proceeded to seep into the nose and irritate it, causing a sneeze.^[7] Although plausible, scientists later determined this theory to also be incorrect because sun-induced sneezing occurs too quickly after sunlight exposure; watering of the eyes is a slower process, so it could not play a vital part in triggering the reflex.^[7]

Today, scientific attention has mainly focused on a hypothesis proposed in 1964 by Henry Everett, who was the first to call light-induced sneezing “The Photic Sneeze Effect.” Since the nervous system transmits signals at an extremely fast pace, Dr. Everett hypothesized that the syndrome was linked to the human nervous system, and was perhaps caused by the confusion of nerve signals.^[9] The genetic basis of photic sneezing still remains unclear, and single genes for this condition have not been found and studied. However, the condition often occurs within families, and it has been suggested that light-induced sneezing is a heritable, autosomal-dominant trait.^[10] A 2010 study demonstrated a correlation between photic sneezing and a single-nucleotide polymorphism on chromosome 2.^[11]

Management

While this phenomenon is poorly understood, recent research has shown that antihistamines being used to treat rhinitis due to seasonal allergies may also reduce the occurrence of photic sneezes in people affected by both conditions.^[12]

Those affected by photic sneezing may find relief by shielding their eyes and/or faces with hats, scarves, and sunglasses.^[1]

Symptoms

The photic sneeze reflex manifests itself in the form of uncontrollable sneezing in response to a stimulus which would not produce a sneeze in people without the trait. The sneezes generally occur in bursts of 1 to 10 sneezes, followed by a refractory period that can be as long as 24 hours.

Photic sneezing

A photic sneeze is the most common manifestation of the photic sneeze reflex. A photic sneeze results from exposure to a bright light. The photic sneeze reflex seems to be caused by a change in light intensity rather than due to a specific wavelength of light.^[3] Photic sneezes are harmless annoyances for the most part, but can pose physical risk to people who are operating vehicles or machinery that involve precise movement and reactions.

A study conducted by the School of Optometry at the University of Alabama at Birmingham, found that females represent 67% of photic sneezers, and Caucasians represent 94%. The study also found statistically significant correlations between photic sneezing and the presence of a deviated nasal septum. The study also showed that photic sneezing is more likely to be acquired than inherited.^[13]

Response to periocular injection

During surgeries in and around the eye, such as corneal transplant surgery, the patient often requires injection of a local anesthetic into the eye. In patients who show the photic sneeze reflex, an injection into the eye, such as that undergone in a retrobulbar or peribulbar block, can often elicit a sneeze from the patient. During these procedures, the patient may be sedated prior to the periocular injection. The patient begins to sneeze just as the needle is inserted into the eye, often resulting in the anesthesiologist having to remove the needle before injecting the local anesthetic in order to avoid damaging the patient's eye.^[14]

Sneezing after eating

A condition called gustatory rhinitis can cause some individuals to sneeze after eating, particularly after the consumption of spicy foods.^[15] Stomach fullness is another example of a stimulus that can cause uncontrollable sneezing. Those who exhibit this symptom or disorder, called snatiation, undergo uncontrollable fits of 3–15 sneezes immediately after eating large meals that completely fill the stomach, regardless of the type of food eaten. Snatiation is not believed to be an allergic reaction of any kind.^[16] Even less well understood than photic sneezing and sneezing in response to periocular injection, the trait appears to be inherited in an autosomal dominant fashion.^[17]

Sonar sneezing

Another related condition is sonar sneezing, in which exposure to loud noises or loud environments triggers a sneeze response.

Pathophysiology

There is much debate about the true cause and mechanism of the sneezing fits brought about by the photic sneeze reflex. Sneezing occurs in response to irritation in the nasal cavity, which results in an afferent signal propagating through the ophthalmic and maxillary branches of the trigeminal nerve to the trigeminal nerve nuclei in the brainstem. The signal is interpreted in the trigeminal nerve nuclei, and an efferent signal goes to different parts of the body, such as mucous glands and the diaphragm, thus producing a sneeze.^[18] The most obvious difference between a normal sneeze and one caused by the photic sneeze reflex is the stimulus – normal sneezes occur due to irritation in the nasal cavity, while the latter can result from a wide variety of stimuli. Some of the theories regarding the mechanism of a sneeze resulting from an unusual stimulus are below. There is also a genetic factor that increases the probability of photic sneeze reflex [19]. The C allele on the rs10427255 SNP is particularly implicated in this [19], although the mechanism by which this gene increases the probability of this response is unknown.

Optic-trigeminal summation

Stimulation of the ophthalmic branch of the trigeminal nerve may enhance the irritability of the maxillary branch, resulting in an increased probability of sneezing. This is similar to the mechanism by which photophobia develops by persistent light exposure relaying signals through the optic nerve and trigeminal nerve to produce increased sensitivity in the ophthalmic branch. If this increased sensitivity occurred in the maxillary branch instead of the ophthalmic branch, a sneeze could result instead of photophobia.^[18]

Parasympathetic generalization

The parasympathetic nervous system has many neighboring fibers that respond to different stimuli. When one stimulus activates multiple nerve fibers of the parasympathetic nervous system, parasympathetic generalization is occurring. There is a possibility that sensory input from the eyes could travel to the neurons in the cortex that interpret such signals, but neighboring neurons which are involved in sneezing are also activated, due to the generalization. This could lead to a sneeze in response to a stimulus other than nasal irritation.^[18]

Increased light sensitivity

When the trigeminal nerve is directly stimulated, there is the possibility that increased light sensitivity in the ocular nerve could result. An example of directly stimulating would be plucking an eyebrow or pulling hair. In many people who show the photic sneeze reflex, even this direct stimulation can lead to a photic sneeze.^[18]

Propofol-induced inhibitory suppression

Uncontrollable sneezing during a periocular injection while sedated by propofol is likely caused by the drug. Propofol has been shown to temporarily suppress inhibitory neurons in the brainstem, which is also where the trigeminal nucleus – the "sneeze center" of the brain – lies. This chain of events leads to increased sensitivity to stimulation and reduced threshold for involuntary responses. In this hypersensitive state, the periocular injection stimulates the ophthalmic and/or maxillary branch of the trigeminal nerve, which results in summation in the trigeminal nuclei. This summation can lead to a sneeze in the unconscious patient.^[19]

Risks

Sneezing generally does not present any particular risks to the individual, and is usually more of an annoyance than a risk of injury. The fits of sneezing brought about by the photic sneeze reflex can, however, have dangerous implications during certain scenarios and activities, such as operating a vehicle, or while undergoing operations (dental, optical) and having bright lights directed towards the patient's face.

Disease transmission

Perhaps the most universal risk of sneezing is the spread of disease. Bacterial infections can spread to susceptible uninfected people via the spread of microscopic organisms suspended in the droplets expelled by a sneeze. Bacteria which commonly spread by sneezing include bacterial meningitis, strep throat, and tuberculosis. Viral infections can also be spread by sneezing. When a virus is expelled by a sneeze, its mucous membrane evaporates, and the virus becomes a droplet nucleus which can be inhaled by another person, thus spreading the virulent infection. Examples of virulent infections that spread by sneezing include measles, mumps, rubella, and influenza.^[20]

Vehicle operation

A fit of sneezing while operating a vehicle can cause the operator to lose control of that vehicle, resulting in injury to the person, and damage to the vehicle and/or surroundings. In particular, photic sneezing poses a considerable risk to pilots, due to the frequent presence of bright sunlight and the precise reactions needed to successfully control the aircraft. For the pilot of a fighter aircraft, if an uncontrollable fit of sneezing were to occur during aerial combat, the pilot could be incapacitated when his or her situational awareness needs to be greatest. A plane landing on an aircraft carrier or shoreline also requires precise movements and quick reflexes. The reflection of the sun from surrounding water has a high probability of producing at least one photic sneeze for pilots who have the reflex. Any amount of sneezing while attempting to land could cause the pilot to lose control, potentially resulting in disaster.^[3]

Medical procedures

Uncontrollable fits of sneezing are common in patients under propofol sedation who undergo periocular or retrobulbar injection. A sneeze by a sedated patient often occurs upon insertion of a needle into or around their eye. The violent and uncontrollable movement of the head during a reflexive sneeze has potential to cause damage within the patient's eye if the needle is not removed before the sneeze occurs.^[19]

Preventive measures

There is currently no definitive way to cure the sneezing fits brought on by the photic sneeze reflex. Photic sneezing can be combated by shielding one's eyes with hats or sunglasses. There are many remedial fixes for sneezing, such as placing a finger horizontally below the nose or holding the nose closed when the beginnings of a sneeze are felt.

Another remedy is to deliberately cause the onset of sneezing in a safe environment before moving into an environment where the condition could be a danger. The person will then be protected as long as the refractory period lasts.

The most helpful way to avoid the risks stated above is to be aware of any inclination to sneeze in response to strange stimuli. If a pilot knows he or she is at risk for experiencing a photic sneeze during a flight, he or she can wear polarized goggles to block the sun, or at the very least be prepared for a sneeze and have measures planned to minimize the risk from such a sneeze. Any patient with a history of uncontrollable sneezing who requires periocular surgery should tell the doctor or anesthesiologist, so that they can take appropriate measures to minimize the risk of injury in case of a sneeze during the surgical procedure. People who know they have a tendency to experience sneezing fits after consuming a large meal can make an effort to reduce the size of their meals, since snatiation seems to occur only as a result of an extremely full stomach.

See also

• Snatiation
• Sexually induced sneezing

References

External links

• Healthline
• Online Mendelian Inheritance in Man entry