Seasonal Trigeminal Neuralgia or Seasonal Headshaking
Neuralgia = pain at the nerve endings, usually induced by a disturbed "food" supply of a nerve
Seasonal means symptoms appear during a certain period of time depending on the season of the year.
An every year repeating distinct seasonal pattern is to observe: the headshaking begins in the spring, for the Western hemisphere, mainly March/April or later in May/June, when sunlight becomes more intense, days and nights are warmer than during cold fall and winter months; symptoms getting worse over the course of the summer and starting to vanish in fall, when sunlight is less intense and temperatures drop during the day and night. Such horses are often completely symptom free over the winter, some others might show on occasional "nicking" on bright sunny days, and the temperatures rise. (Madigan et al, 1995).
Research has found that melatonin (and serotonin), a hormone produced in the brain, plays a key role in some HS horses during the spring and summer; melatonin regulates the system's day and night, and seasonal changes. An unusual decrease in melatonin production in these horses causes branches of the trigeminal nerves to be over-stimulated by bright sunlight. 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
Melatonin, or 5-methoxy-N-acetyltryptamine, is a
hormone produced by pinealocytes in the pineal gland, located in the
center of the brain. It is a derivative of the amino acid tryptophan.
Melatonin helps regulate sleep-wake or circadian rhythms. Normally,
production of melatonin by the pineal gland is stimulated by
darkness and inhibited by light.
A few cases have been observed, where the problem develops in fall and disappears all of a sudden in winter. It is suggested that a seasonal problem develops into a year-round problem (Mair and Lane, 1993).
In other cases allergies appear at the same time as the seasonal headshaking sets on again; there seems to be a very strong connection when referring to a lot of horse owner's and vet's reports of successfully treating allergy symptoms with the result that the headshaking stopped. Lane & Mair (1987,1993) reported that allergic rhinitis is an underlying cause for HS, with even stronger symptoms during bright sunny days, exactly the same characteristics as observed in human.
Melatonin produces a substance called Arginine Vasotocin. This secretion has an inhibiting effect upon cortisol, the main stress hormone produced by the adrenal glands. The melatonin level is rising again when the days become 'shorter' and colder.
Mair, T. and Lane, G. (1993). Equine Practice 2. Headshaking in Horses. Bailliere Tindall. 109-119.
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