Redefining Headshaking
 

There are several articles on the internet that describe the basics of headshaking syndrome. I would like to refer the reader to those articles to get a general idea about the disorder. The topic to be covered here is the important task to redefine headshaking syndrome. Headshaking is described on the website for the University of Lincoln (my favorite place for a thorough explanation of headshaking) as a "presenting sign" and not a diagnosis. This is very important because headshaking in itself it is not the same as headshaking syndrome. A syndrome is described as a number of essential characteristics, which when concurrent lead to the diagnosis of the condition (see criteria page). Diagnosis matters because it will define the treatment. Failures in treatment contribute to depression and loss of hope, for the owner, and some treatments may even cause an increase in pain for the horse.

 

Many articles give numerous (about 60) potential causes for headshaking which include insect irritation, ear mites, dental problems, tack and rider contributions, allergies, sunlight sensitivity, chiropractic issues, vaccinations and so on. These may be triggers for headshaking but they are not necessarily the cause of headshaking syndrome. Dr.Madigan reports in his study, Evaluation and Treatment of Headshaking Syndrome (1997) that although these and many more causes are suggested, "Few, if any, responded to treatment appropriate for these diagnosis". It should be also pointed out that responses to treatments need to be carefully scrutinized for the occurrences of relapses and what would be normal periods of remission.
Developing Criteria
 

All horses shake their head at one time or another horizontally, whereas horses with headshaking syndrome exhibit repetitive, involuntary headshaking that is more vertical. These horses may also have horizontal headshaking but the hallmark sign is the vertical movement. The motion is a sudden, intense downward flick of the nose and in severe cases the whole head and neck may be involved. This is a differentiating sign that is very useful in defining headshaking syndrome.

 

Another mandatory criterion is that headshakers rub their nose and muzzle on objects. It is not a brief gentle rub but the rubbing can become vigorous, incessant and even frantic. They may even get sores on their face or bang their heads on stall walls to relieve the unbearable pain running along the nostrils and face. This is due to the tingling and pricking pain of the syndrome. It is important to understand that the site of pain does not necessarily coincide with the source of pain.

What and Why?
 

Many veterinarians have connected this to the irritation or inflammation of the trigeminal (facial) nerve that runs from the back of the head, around the ears, along both sides of the face and terminates in the nose and muzzle area. Symptoms often reveal the area most affected. The ear will twitch when that part is most affected, the eye area shows up as photophobia and the muzzle area will twitch when nerves in that area are irritated. Most experts agree at this time that the condition is very similar to trigeminal neuralgia in people. Dr. Madigan's conclusion at the end of the aforementioned study is that headshaking horses "demonstrate evidence of neuropathic pain as a basis for the behavior". This statement is very powerful in defining headshaking syndrome and differentiating it from other disorders. It also shows the need for the understanding that horses that are thought to be displaying"bad" behavior associated with headshaking syndrome are most likely displaying pain behavior.
The "seasonal" features of sneezing, tearing, rhinorrhoea and eye swelling associated with headshaking may be due to cranial autonomic and specifically parasympathetic activation. Also described as "excessive cranial parasympathetic autonomic reflex activation to nociceptive input in the ophthalmic division of the trigeminal nerve" (Goadsby, PJ, 2005). These suggestions help explain the pathophysiology behind cluster headaches in humans and optic-trigeminal summation which is similar to headshaking syndrome in horses.

 

The seasonality continues to be perplexing. There are many theories including one that relates to information on day length that is sent thru the eye, along the retinohypothalamic tract, to the suprachiasmatic nucleus (the biological clock of the body) in the hypothalamus, along the sympathetic nervous system to the pineal gland where melatonin is produced. The malfunction may be in the hypothalamus or somewhere else in the pathway to the production of melatonin. I hope to have more on the role this plays in headshaking and welcome all others to give input on this important topic. There is also ongoing research investigating the role of hormones to explain seasonal onset. This looks very promising. There is, in addition, quite a bit of anecdotal evidence that suggests an allergic component in relation to the occurrence of headshaking in the spring and summer and this topic should remain an important consideration and area of study.
There are many more areas to consider such as the role of nitric oxide, calcium channels, gender, the effect of differing wavelengths of light and so on. I hope this website will spark interactive, enlightening and scientific conversations that will lead to a cure for headshaking syndrome.

 
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