Bright light, bright light! The ACHOO Syndrome

Bright light

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*. However, in 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...

* 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...

Trigeminal nerve
First, let's take a closer look at the trigeminal nerve or 5th cranial nerve (CN V). It consists of three major branches:

Trigeminal nerve with its three major branches. Three pairs of cranial nerves.

Normal sneezing
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:
1. Optic-trigeminal summation: stimulation of the ophthalmic nerve leads to enhanced irritability of the maxillary nerve, making it more likely to sneeze.
2. 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.
3. 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.

Calvin and Hobbes

Why bother?
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.

© MS

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. Peer reviewers said:
"Excellent and thorough presentation! Before now, I had never heard of the ACHOO syndrome, yet you managed to explain it so adequately that I feel as if I was already familiar with it."
"Fantastic! It is written in a very clear way. One can absolutely see that you understand the things Prof. Mason taught and that you know what you are talking about. I liked reading your paper very much."

Before the start of this MOOC, I didn't know anything about the brain. As the weeks went by I began to feel more brave in the complex world of nerves. I learned about:
- the tree-like structure of neurons, communication pathways (axons, synapses and the synaptic cleft) that trigger the release of neurotransmitters and how myelin makes communication go faster ('jumping' action potentials);
- the sympathetic system that screams for action (fight or flight) versus the parasympathetic system that tones everything down (rest and digest);
- how you can lose one side of the visual world while your eyes are still working perfectly;
- how to screen for conductive and sensorineural hearing loss using Rinne and Weber tests and how the ear echoes incoming sound back out (otoacoustic emissions);
- that it is possible to focus on a steady index finger while shaking your head from side to side (fast vestibulo-ocular reflex), but impossible to focus on a fast moving finger while keeping your head steady (slow visual system);
- the existence of the motor hierarchy and how it's possible that a person still has reflexes and stereotyped movements (e.g. walking and chewing, which involve central pattern generators, CPG's), but at the same time may be unable to make intentional movements (e.g. lifting an arm), and that smiling on command actually involves different pathways than smiling at a funny joke;
- that I can stand for hours and hours thanks to oxygen-dependent slow muscles (endurance), but that I can only jump for a few minutes because of glycogen-dependent fast muscles (speed and power)
- that the basal ganglia provide the Go / No Go for an intended movement and that the cerebellum is like the conductor of an enormous orchestra, coordinating all inputs and outputs to produce a smooth movement;
- that semantic memories (facts) can be accessed directly from 'storage' in neocortex, but episodic memories (autobiographical events) return to hippocampus for remembering and then are send back to neocortex again. The question is: are they still the same memories..?

References:
- Aristotle, The Book of Problems
- Abramson, D.C. Sudden unexpected sneezing during the insertion of peribulbar block under propofol sedation. Canadian Journal of Anesthesia, 1995.
- Breitenbach R.A., Swisher P.K., Kim M.K., Patel B.S. The photic sneeze reflex as a risk factor to combat pilots. Military medicine, 1993.
- Condliffe, J. How Staring At The Sun Can Make You Sneeze. www.gizmodo.com.au, 2014.
- Dean, Laura. ACHOO Syndrome, Medical Genetics Summaries, NBK109193, 2012.
- Langer, N., Beeli, G. and Jäncke, L. When the Sun Prickles Your Nose: An EEG Study Identifying Neural Bases of Photic Sneezing. PLoS One, 2010.
- Schrock, Karen. Looking at the sun can trigger a sneeze, article in Scientific American, 2008.