Photic Trigeminal Neuralgia or Photic Headshaking
Neuralgia = pain at the nerve endings, usually induced by a disturbed "food" supply of a nerve
What is called Photic Headshaking in our
horses is called Photic Sneeze Reflex in human. The ongoing
research in horses is based on the ongoing research in human.
The website "Photic Sneezing" authored by Mr. Martin Haegenheim, reports of 4 existing theories:
Several theories have been advanced to explain the phenomenon, but the generally accepted explanation is the " crossed wires " theory.
The sudden exposure to bright light sends a signal through the optic nerve fibers. These fibers run close to and into the same hub as the trigeminal nerve. The second division of the trigeminal nerve supplies sensory fibers to the nasal mucosa. The assumption is that nerve impulses in the optic nerve cause a sympathetic discharge in the trigeminal nerve fibers. which produces the sneezing.
A second theory involves the simultaneous activation of neighboring parasympathetic branches. In this case, the stimulus to the retina also causes tears to be formed, leading to congestion in the nose and a " trickling sensation ". A third scenario posits a more generalized parasympathetic system hypersensitivity, and could be linked to the subject's emotional state. The fourth theory links intraorbital trigeminal nerve stimulation and increased ocular sensitivity to light ; this would account for photic sneezing in cases of nephropathic cystinosis.
These theories are discussed at length in Everett (1964) and Whitman and Packer (1993) in the bibliography.
Re-published with friendly permission by above author.
At the website Wikepedia we can read follwing article:
Photic sneeze reflex, also whimsically called
ACHOO, a backronym for Autosomal dominant Compelling Helio-Ophthalmic
Outburst) is a medical condition by which people exposed to bright
light involuntarily sneeze. It has been suggested that the photic
sneeze reflex occurs only after someone has been adapted to the dark
for at least five minutes, although this is not certain, and is not
uniform amongst people with the photic sneeze reflex. The condition
occurs in one-sixth to one-quarter of humans, with more common
occurrence in Caucasians than other human races. The trait is passed
along genetically, with a 50 percent chance of inheritance. It is
more often seen in children than in adults.
Headshaking appeared to be light-stimulated in approximately 60% of the horses examined (Madigen 1998 Nov;(27):28-9.) http://www.ncbi.nlm.nih.gov/pubmed/10485000
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