Photic sneeze reflex
Encyclopedia
The photic sneeze reflex (colloquially known as "sun sneezing") is a condition of uncontrollable sneezing
Sneeze
A sneeze is a semi-autonomous, convulsive expulsion of air from the lungs through the nose and mouth, usually caused by foreign particles irritating the nasal mucosa...

 in response to numerous stimuli, such as looking at bright lights or periocular injection. The condition affects 18–35% of the population, and its exact mechanism of action is not well understood.

Overview

Photic sneeze reflex, also sometimes referred to using the backronym
Backronym
A backronym or bacronym is a phrase constructed purposely, such that an acronym can be formed to a specific desired word. Backronyms may be invented with serious or humorous intent, or may be a type of false or folk etymology....

 Autosomal Dominant Compelling Helio-Ophthalmic Outburst Syndrome (ACHOO Syndrome), and the "Peroutka Sneeze Gene", is a hereditary trait
Heredity
Heredity is the passing of traits to offspring . This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve...

 which causes sneezing
Sneeze
A sneeze is a semi-autonomous, convulsive expulsion of air from the lungs through the nose and mouth, usually caused by foreign particles irritating the nasal mucosa...

, 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
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 (Problems
Problems (Aristotle)
The Problems is an Aristotelian or possibly pseudo-Aristotelian collection of problems written in a question and answer format as its authenticity has been under questioning. The collection, gradually assembled by the peripatetic school, reached its final form anywhere between the third century BC...

, 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.

Mechanism

The probable cause is a congenital malfunction in nerve
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...

 signals in the trigeminal nerve nuclei
Trigeminal nerve nuclei
The sensory trigeminal nerve nuclei are the largest of the cranial nerve nuclei, and extend through the whole of the midbrain, pons and medulla.The nucleus is divided into three parts, from rostral to caudal :...

. The fifth cranial nerve, called the trigeminal nerve, is apparently responsible for sneezes. Research suggests that some people have an association between this nerve and the nerve that transmits visual impulses to the brain. Overstimulation of the optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...

 triggers the trigeminal nerve
Trigeminal nerve
The trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...

, and this causes the photic sneeze reflex.

On the other hand, some people with the trait feel that it is advantageous. In the event that nasal discomfort occurs, but to an extent that is insufficient to induce a sneeze, intentionally seeking and finding a light source, or creating a mental image of a bright light source such as the sun, facilitates the sneezing process and is in turn a mode of relief.

This sneeze reflex can also be brought on by sudden inhalation of cold air or exposure to strong flavors. This implies an overstimulation of any nerve close to the trigeminal nerve can cause the sneeze reflex.

Management

While this phenomenon is poorly understood, recent research has shown that antihistamine
Antihistamine
An H1 antagonist is a histamine antagonist of the H1 receptor that serves to reduce or eliminate effects mediated by histamine, an endogenous chemical mediator released during allergic reactions...

s being used to treat rhinitis
Rhinitis
Rhinitis , commonly known as a stuffy nose, is the medical term describing irritation and inflammation of some internal areas of the nose. The primary symptom of rhinitis is nasal dripping. It is caused by chronic or acute inflammation of the mucous membrane of the nose due to viruses, bacteria or...

 due to seasonal allergies may also reduce the occurrence of solar sneezes in people affected by both conditions.

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
University of Alabama at Birmingham
The University of Alabama at Birmingham is a public university in Birmingham in the U.S. state of Alabama. Developing from an extension center established in 1936, the institution became an autonomous institution in 1969 and is today one of three institutions in the University of Alabama System...

, found that women represent sixty-seven percent of photic sneezers. Ninety-four percent 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
Retrobulbar block
A retrobulbar block is a regional anesthetic nerve block into the retrobulbar space, the area located behind the globe of the eye. Injection of local anesthetic into this space constitutes the retrobulbar block. This injection provides akinesia of the extraocular muscles by blocking cranial nerves...

 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. 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
Snatiation
Snatiation is a medical disorder characterized by uncontrollable bursts of sneezing brought on by fullness of the stomach, and typically observed in sufferers immediately after a large meal. It is thought to be passed along genetically as an autosomal dominant trait...

 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
Sneeze
A sneeze is a semi-autonomous, convulsive expulsion of air from the lungs through the nose and mouth, usually caused by foreign particles irritating the nasal mucosa...

 occurs in response to irritation in the nasal cavity, which results in an afferent
Afferent
Afferent is an anatomical term with the following meanings:*Conveying towards a center, for example the afferent arterioles conveying blood towards the Bowman's capsule in the Kidney. Opposite to Efferent.*Something that so conducts, see Afferent nerve fiber...

 signal propagating through the ophthalmic
Ophthalmic nerve
The ophthalmic nerve is one of the three branches of the trigeminal nerve, the fifth cranial nerve. The ophthalmic nerve carries only sensory fibers.-Branches:*Nasociliary nerve**sensory root of ciliary ganglion**posterior ethmoidal nerve...

 and maxillary branches of the trigeminal nerve
Trigeminal nerve
The trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...

 to the trigeminal nerve nuclei
Trigeminal nerve nuclei
The sensory trigeminal nerve nuclei are the largest of the cranial nerve nuclei, and extend through the whole of the midbrain, pons and medulla.The nucleus is divided into three parts, from rostral to caudal :...

 in the brainstem. The signal is interpreted in the trigeminal nerve nuclei
Trigeminal nerve nuclei
The sensory trigeminal nerve nuclei are the largest of the cranial nerve nuclei, and extend through the whole of the midbrain, pons and medulla.The nucleus is divided into three parts, from rostral to caudal :...

, and an efferent
Efferent
Efferent is an anatomical term with the following meanings:*Conveying away from a center, for example the efferent arterioles conveying blood away from the Bowman's capsule in the kidney. Opposite to afferent....

 signal goes to different parts of the body, such as mucous gland
Mucous gland
Mucous glands, found in several different parts of the body, typically stain lighter than serous glands during standard histological preparation...

s and the diaphragm
Thoracic diaphragm
In the anatomy of mammals, the thoracic diaphragm, or simply the diaphragm , is a sheet of internal skeletal muscle that extends across the bottom of the rib cage. The diaphragm separates the thoracic cavity from the abdominal cavity and performs an important function in respiration...

, thus producing a sneeze. The most obvious difference between a normal sneeze and an 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
Ophthalmic nerve
The ophthalmic nerve is one of the three branches of the trigeminal nerve, the fifth cranial nerve. The ophthalmic nerve carries only sensory fibers.-Branches:*Nasociliary nerve**sensory root of ciliary ganglion**posterior ethmoidal nerve...

 branch of the trigeminal nerve
Trigeminal nerve
The trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...

 may enhance the irritability of the maxillary branch, resulting in an increased probability of sneezing. This is similar to the mechanism by which photophobia
Photophobia
Photophobia is a symptom of abnormal intolerance to visual perception of light. As a medical symptom photophobia is not a morbid fear or phobia, but an experience of discomfort or pain to the eyes due to light exposure or by presence of actual physical photosensitivity of the eyes, though the term...

 develops by persistent light exposure relaying signals through the optic nerve and trigeminal nerve
Trigeminal nerve
The trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...

 to produce increased sensitivity in the ophthalmic
Ophthalmic nerve
The ophthalmic nerve is one of the three branches of the trigeminal nerve, the fifth cranial nerve. The ophthalmic nerve carries only sensory fibers.-Branches:*Nasociliary nerve**sensory root of ciliary ganglion**posterior ethmoidal nerve...

 branch. If this increased sensitivity occurred in the maxillary branch instead of the ophthalmic
Ophthalmic nerve
The ophthalmic nerve is one of the three branches of the trigeminal nerve, the fifth cranial nerve. The ophthalmic nerve carries only sensory fibers.-Branches:*Nasociliary nerve**sensory root of ciliary ganglion**posterior ethmoidal nerve...

 branch, a sneeze could result instead of photophobia.

Parasympathetic generalization

The parasympathetic nervous system
Parasympathetic nervous system
The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system . The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously...

 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 neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

s 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
Trigeminal nerve
The trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...

 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
Propofol
Propofol is a short-acting, intravenously administered hypnotic agent. Its uses include the induction and maintenance of general anesthesia, sedation for mechanically ventilated adults, and procedural sedation. Propofol is also commonly used in veterinary medicine...

 is likely caused by the propofol
Propofol
Propofol is a short-acting, intravenously administered hypnotic agent. Its uses include the induction and maintenance of general anesthesia, sedation for mechanically ventilated adults, and procedural sedation. Propofol is also commonly used in veterinary medicine...

. Propofol
Propofol
Propofol is a short-acting, intravenously administered hypnotic agent. Its uses include the induction and maintenance of general anesthesia, sedation for mechanically ventilated adults, and procedural sedation. Propofol is also commonly used in veterinary medicine...

 has been shown to temporarily suppress inhibitory neurons in the brainstem, which is also where the trigeminal nucleus
Trigeminal nerve nuclei
The sensory trigeminal nerve nuclei are the largest of the cranial nerve nuclei, and extend through the whole of the midbrain, pons and medulla.The nucleus is divided into three parts, from rostral to caudal :...

 - 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
Ophthalmic nerve
The ophthalmic nerve is one of the three branches of the trigeminal nerve, the fifth cranial nerve. The ophthalmic nerve carries only sensory fibers.-Branches:*Nasociliary nerve**sensory root of ciliary ganglion**posterior ethmoidal nerve...

 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.

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
Meningitis
Meningitis is inflammation of the protective membranes covering the brain and spinal cord, known collectively as the meninges. The inflammation may be caused by infection with viruses, bacteria, or other microorganisms, and less commonly by certain drugs...

, strep throat, and tuberculosis
Tuberculosis
Tuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...

. 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
Measles
Measles, also known as rubeola or morbilli, is an infection of the respiratory system caused by a virus, specifically a paramyxovirus of the genus Morbillivirus. Morbilliviruses, like other paramyxoviruses, are enveloped, single-stranded, negative-sense RNA viruses...

, mumps
Mumps
Mumps is a viral disease of the human species, caused by the mumps virus. Before the development of vaccination and the introduction of a vaccine, it was a common childhood disease worldwide...

, rubella
Rubella
Rubella, commonly known as German measles, is a disease caused by the rubella virus. The name "rubella" is derived from the Latin, meaning little red. Rubella is also known as German measles because the disease was first described by German physicians in the mid-eighteenth century. This disease is...

, and influenza
Influenza
Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...

.

Vehicle operation

A fit of sneezing while operating a vehicle can cause the operator to lose control of the vehicle, resulting in damage to the person, vehicle, and surroundings. In particular, photic sneezing poses a considerable risk to pilots, due to the presence of the bright sun and the precise reactions needed to successfully control an aircraft. For the pilot of a fighter aircraft
Fighter aircraft
A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...

, 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 off of the surrounding water has a high probability of producing at least 1 photic sneeze for pilots who have the reflex. Any amount of sneezes during a critical moment during landing could cause the pilot to lose control, resulting in potential disaster.

Medical procedures

Uncontrollable fits of sneezing are common in patients under propofol
Propofol
Propofol is a short-acting, intravenously administered hypnotic agent. Its uses include the induction and maintenance of general anesthesia, sedation for mechanically ventilated adults, and procedural sedation. Propofol is also commonly used in veterinary medicine...

 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.

Preventative 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
Snatiation
Snatiation is a medical disorder characterized by uncontrollable bursts of sneezing brought on by fullness of the stomach, and typically observed in sufferers immediately after a large meal. It is thought to be passed along genetically as an autosomal dominant trait...

seems to occur only as a result of an extremely full stomach.
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