Photic sneeze reflex

The photic sneeze reflex (also known as photoptarmosis, colloquially known as "sun sneezing") is a condition of uncontrollable 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, but its exact mechanism of action is not well understood.

Overview
Photic sneeze reflex, also sometimes referred to using the backronym Autosomal dominant Compelling Helio-Ophthalmic Outburst (ACHOO) syndrome, and the "Peroutka Sneeze Gene", is a hereditary trait which causes sneezing, possibly many times consecutively (due to naso-ocular reflex ) when suddenly exposed to bright light. The condition affects 18-35% of the human population.

The first mention of the phenomenon is probably in the later work attributed to Aristotle (Problems, book XXXIII). (between the third century BCE to the 6th century CE).

There does not appear to be evidence that this sneeze reflex is actually inherited in an autosomal dominant fashion. Some SNPs have been associated with photic sneeze reflex, yet much of the heredity remains to be understood.

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 solar sneezes in people affected by both conditions.

Some people may find relief by shielding their eyes and/or faces with hats, scarves, and sunglasses.

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 disorder. 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 is a sneeze that 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. Photic sneezes are harmless annoyances for the most part, but can pose physical risk to people who are operating vehicles or machinery, which involves 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. 94% of those were Caucasian. The study also found statistically significant correlations between photic sneezing and the presence of a deviated septum. The study also showed that photic sneezing is more likely to be acquired than inherited.

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.

Response to fullness after eating
Stomach fullness is another example of a stimulus that causes uncontrollable sneezing that may be linked to the photic sneeze reflex. Some people who exhibit this symptom undergo uncontrollable fits of 3-15 sneezes immediately after eating large meals which completely fill the stomach. The so-called snatiation reflex occurs independently of the choice of food eaten, and is not believed to be an allergic reaction of any kind. Although 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.

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. 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.

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.

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.

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.

Propofol-induced inhibitory suppression
Uncontrollable sneezing during a periocular injection while sedated by propofol is likely caused by the propofol. 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.

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 driving or operating machinery.

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.

Vehicle operation
A fit of sneezing while operating a vehicle can cause the operator to lose control of that vehicle, resulting in damage to the person, to the vehicle and/or to the 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 incredibly 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.

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.

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.

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 is at risk for experiencing a photic sneeze during a flight, he can wear polarized goggles to block out 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 his 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.