Current hypothesis of the pathway leading to
headshaking syndrome in horses
Author: Pam Neff, RN, BSN, BS
Headshaking syndrome (HS) in horses is a perplexing disorder that appears to be international in distribution, seasonal, gender-biased towards geldings, often involves light sensitivity and is seen mostly in ages over 10. It is characterized predominantly by vertical head tossing or flicking and muzzle irritation observed as rubbing on objects and sneezing. This is thought by experts on the subject to be a result of neuropathic pain along the trigeminal nerves in the face. Although there are many triggers known to elicit these symptoms, the cause is still unknown. Without knowing the cause of HS the treatments are trial and error and at this time there is no treatment that has been proven to eliminate HS completely or permanently.
The purpose of this paper is to examine the cause and source of the irritation of the trigeminal nerves. It is based on a year of literature review, consultations with veterinary experts in the field and my own theories and background in medicine.
Comparisons of Headshaking Syndrome in Horses and Cluster-tic Syndrome in Humans
A syndrome is defined as a number of essential characteristics which, when concurrent, lead to the diagnosis of a condition. It is imperative to be able to diagnose the condition in order to provide effective treatments. Many similarities exist between headshaking syndrome and cluster-tic syndrome. Since there is a great deal of research done with people that have cluster headaches with trigeminal neuralgia, and there are some very valid, proven treatments, it is wise to use this information to help horses with HS.
Cluster-tic syndrome is characterized by two types of pain that coexist. One is periocular, with evident autonomic features of eye tearing, rhinorrhea, nasal congestion and eyelid edema, and daily attacks for weeks or months with periods of remission. These autonomic features have been shown to be a result of parasympathetic hyper-stimulation. There have, in addition, been reports of light sensitivity, jaw and neck pain and restlessness. The second associated pain is characterized by electric shocks along the facial nerves. The first pain is the cluster headache and the second is trigeminal neuralgia (tic).
Cluster headaches are classified as vascular headaches. The intense pain is caused by the dilation of blood vessels which creates pressure on the trigeminal nerve . Cluster headache research has shown that the highest incidence is in older males and it is more prevalent during the solstices of June 21 and December 21. The hormonal aspect appears to be related to low levels of testosterone that have been found in males with cluster headaches . The seasonal aspect has been found to be related to melatonin, which is responsible for the entrainment of the biological clock, which has levels that also appear to be low. In humans, the biological clock is situated in the hypothalamus. The hormonal imbalances of melatonin and testosterone appear to be due to a malfunction of the hypothalamus.
Trigeminal neuralgia is characterized by sudden bursts (paroxysms) of face pain. The current opinion is now in favor of a neurovascular conflict: an artery that has an offending contact with the trigeminal nerve root.  Trigeminal neuralgia is diagnosed as having pain with distribution along one or more of the trigeminal nerves, the pain is sudden, intense, precipitated by triggers , has periods of remission, no neurological deficits and all other causes have been excluded.
Both of the disorders above have been described as “the most painful affliction to man”. This emphasizes the importance of continued research in this area to provide permanent and adequate relief from such pain.
Headshaking syndrome in horses has many characteristics shared with the above two conditions. Defining characteristics include, intense facial pain associated with nasal discharge, eye tearing, sneezing, triggers, seasonality with periods of remissions. It can therefore be deduced that headshaking syndrome in horses is essentially the same as cluster-tic syndrome in humans.
My current proposed theory is that headshaking syndrome in horses is essentially the same as cluster-tic syndrome in humans and both share the same pathways. The predominant pathway appears to be vasodilation which is responsible for the neurovascular conflict in trigeminal neuralgia and the vascular effects in the cluster headache. Nitric oxide is a potent vasodilator and may be the origin of this pathway. The other pathway is based on an abnormality in the hypothalamus that may cause increased sensitivities to triggers originating from imbalances in sex steroids, corticosteroids and other neurohormones.
 Wikipedia, Cluster Headache
 Medscape, The Treatment of Cluster Headache and Facial Pain
 Joffroy, A., Trigeminal neuralgia, pathophysiology and treatment, Acta neurol.belg.,2001, 101, 20-25.
 Madigan, J., Bell, S., Evaluation and Treatment of Headshaking Syndrome, 1997