ACHOO? You’re kidding, right?!
No, actually I’m not. It is a real syndrome and I ‘suffer’ from it. ACHOO sounds like sneezing and that’s exactly what it is.
So, what is it?
ACHOO is short for Autosomal dominant Compelling Helio-Ophthalmic Outburst. It is a Photic Sneeze Reflex and symptoms are uncontrollable sneezing, even multiple times, when suddenly exposed to bright light. The ACHOO syndrome is a genetic dominant disorder. It affects both males and females and the chances of passing on the gene(s) are 50:50. It is very common: about 1 in 3-4 people are affected.
A short history of sneezing
The ACHOO Syndrome is as old as mankind. Aristotle mentioned in his Book of Problems: “Why does the heat of the sun provoke sneezing?” He concluded that the heat of the sun on the nose is the cause. Another funny detail: Aristotle also said that sneezing is medicine for the brain, it cleans the brain of evil in the same way that coughing cleans the lungs. He who cannot sneeze, will die soon… Howeverin the 17th century English philosopher Francis Bacon stepped into the sunlight with his eyes closed and didn’t sneeze. Therefore the ‘heat of the sun’ theory was replaced by the ‘sunlight makes the eyes water and this moisture irritates the nose’ theory. However, both Aristotle and Bacon were wrong.
First, let’s take a closer look at the trigeminal nerve or 5th cranial nerve (CN V). It consists of three major branches:
- The ophthalmic nerve carries sensory information from e.g. the skin of the forehead and scalp, upper eyelid and cornea and to part of the mucous membrane of the nasal cavity. It supplies parasympathetic innervation to the lacrimal gland (tear production).
- The maxillary nerve senses information from the lower eyelid, mucous membrane, nasal cavity, upper lip and teeth. It supplies parasympathetic innervation to to the lacrimal and nasal glands.
- The mandibular nerve picks up sensation from the lower teeth and lip, part of the tongue, chin and jaw. It innervated chewing muscles.
A normal, healthy person sneezes because the ophthalmic and maxillary nerve pick up some irritation in the nasal cavity. In response the trigeminal nerve nuclei send out signals to the mucous glands and the diaphragm. The result: ACHOO!
What causes ACHOO?
A person with ACHOO Syndrome not only sneezes in response to irritation in the nasal cavity, but also when exposed to bright light. The photic sneeze reflex does not depend on specific wavelengths, but rather on a sudden change in light intensity entering the retina, which is sensed by the optic nerve (cranial nerve II) . Upon exposure to bright light, the optic nerve fires a signal to the brain to constrict the pupils. And the trigeminal nerve happens to be close to the optic nerve..
There are 3 theories about ACHOO:
- Optic-trigeminal summation: stimulation of the ophthalmic nerve leads to enhanced irritability of the maxillary nerve, making it more likely to sneeze.
- Parasympathetic generalization: one stimulus activates multiple neighboring nerve fibers, leading to a trickling sensation in the nose, which the brain mistakes for nose irritation and we start to sneeze.
- Parasympathetic hypersensitivity, particularly within the nasal mucosa.
A recent experiment subjected photic and non-photic sneezers to bright light while making an EEG. This showed that photic sneezers have a higher excitability of the visual cortex to standard visual stimuli. The photic sneeze reflex is not a reflex that occurs only at the level of the brainstem, but also involves specific cortical areas. Although the results do not contradict theory 1, they support theory 2 and 3 more (which involve cortical circuits rather than just brainstem circuits).
People with ACHOO Syndrome might just be more sensitive for bright lights that evoke a trickling sensation. In addition, there is also a possibility that the organization of the visual cortex is different in photic sneezers. Furthermore, photic sneezers may anticipate the exposure to visual stimuli differently than normal people.
The ACHOO gene has not yet been found. But why bother, might you ask. Because the genetic causes for photic sneezing may shed some light on the visual pathway and other reflexes. It may teach us more about epileptic seizures, which can be triggered by flashing lights, or migraine headaches, which are often accompanied by extreme sensitivity to light.
This article was submitted as my final project for the MOOC Understanding the Brain: The Neurobiology of Everyday Life by Peggy Mason, University of Chicago, 2015.