Equine head shaking can be an extremely challenging condition to treat and manage, and in many cases causes significant distress to both the horse and owner. It can make horses dangerous and unrideable, and may be the reason a horse is put to sleep.
As we better understand the cause of head shaking and the triggers involved for an individual, our management strategy can become more effective. Even so, the condition remains frustrating to treat or manage.
Below we will share some of the take-home points from a lecture on headshaking presented by Dr Kirstie Pickles.
What is head shaking?
Equine headshaking is a condition causing trigeminal nerve hypersensitivity, leading to assumed neuropathic pain that may cause burning, itching, tingling or electric shock-like pain along the route of the trigeminal nerve.
This presents in horses clinically, most commonly as a drop and sharp flick or jerk of the head in the vertical plane that can be quite violent. It can also include flicking or jerking in the horizontal plane, rubbing of the muzzle, licking or twitching of the lips and tongue, snorting, and is often coupled with an anxious expression.
Around 60% of head shakers become symptomatic in the spring and summer, and many owners report a progressive worsening of clinical signs.
Some of the triggers of head shaking can be bright sunlight, wind, and eating – especially dry hay. Less commonly, horses might experience rain, loud noises, flies and midges, pollen and snow as triggers.
Headshaking may also be triggered by musculoskeletal imbalances, ill-fitting tack, and fascial restrictions – causes that Vetrehabbers are well-equipped to deal with, and which I will not expand on in this blog.
Trigeminal nerve anatomy
The trigeminal nerve has two components, a central and peripheral component. The peripheral trigeminal nerve has 3 branches, the ophthalmic branch, maxillary branch and mandibular branch.
The central component includes the trigeminal nuclei and the spinal trigeminal tract. The component of nerves travelling through the spinal trigeminal tract travel up into the thalamus, where the conscious perception of pain rests. The spinal tract of the trigeminal nerve may extend to the C2 spinal cord segment.
Trigeminal nerve involvement
There has been confirmation of the involvement of the trigeminal nerve through sensory nerve conduction tests evoking action potentials of the trigeminal nerve. In two studies (Aleman et al., 2013, 2014), 6 control horses were compared to 6 head shakers through sensory nerve conduction tests.
They found that the headshaking horses had a lower stimulus threshold than the control group, with bilateral nerves being equally affected. There were no other differences in neurophysiological parameters, such as conduction verlocity.
One head-shaker in the study was seasonally affected and was in remission at the time of testing. For this horse, the stimulus threshold was identical to normal horses, suggesting that the nerve stimulus threshold is malleable and can be returned to normal ranges.
In further studies evaluating the structure of the trigeminal nerves (Aleman et al., 2014), the nerves were found to have normal structures, suggesting that there is no pathological process, but rather that there is an abnormal function of a normal structure.
Causes of Trigeminal nerve sensitivity
Chronic Trigeminal hypersensitivity or nerve pain can occur in humans after a reactivation of the varicella zoster herpesvirus, better known as the chickenpox virus. In humans there is a resultant constant intermittent spontaneous pain with mechanical allodynia of the affected area. This leads us to question whether equine headshaking is caused by a reactivation of the equine herpesvirus (EHV-1), which has a tendency to lie dormant in the trigeminal ganglia.
In 2012, Aleman et al., sought to answer this question, and found that there was no connection between the EHV-1 virus and equine head shaking. To learn more about potential causes of trigeminal nerve sensitization, head shaking and photic headshaking, as well as the similarities and differences between human trigeminal neuralgia and equine head shaking, be sure to listen to the webinar by Dr Kirstie Pickles in the Onlinepethealth Equine members portal.
Managing the condition
There are many anecdotally reported treatments for head shaking, and very few scientific efficacy trials or pharmacokinetic trials. Evaluation of efficacy is affected by many variables, including the lack of objective assessment, the fact that 5% of cases will spontaneously improve, seasonal remission, as well as the high occurrence of the placebo effect in owners who desperately want to see an improvement in the symptoms.
There are a number of management or treatment options reported, here are some of the most commonly used:
Physical protection: nose nets and UV protective masks
Nose nets have been shown to be one of the most useful management tools, but there isn’t one recommended nose net that works for all or most cases. When including this into the management, different kinds and styles of nose nets need to be tried before concluding that they don’t help.
UV protective masks can help manage photic head shakers, but they will only do so if the mask blocks 90% of UV rays. Many horses will feel a little disorientated in these masks to start with as their vision will be significantly reduced, but they soon adjust if it does improve their symptoms.
Magnesium supplementation can be an inexpensive and simple treatment measure, and shows relative success. Magnesium is a calming electrolyte that lowers the membrane permeability and reduces the hyperexcitability of the nerves. It interacts with GABA receptors in an inhibitory role, and blocks NMDA receptors to prevent glutamate release. Providing boron alongside magnesium can improve the availability of magnesium.
There are various kinds of magnesium available, to find out which are the most absorbable, as well as the daily dosage, please listen to the webinar by Dr Kirstie Pickles in the Onlinepethealth Equine members portal.
Electrotherapy can be used to alter nerve conduction and reduce pain, through transcutaneous or implanted electrical devices. The most commonly used methods include TENS, PENS, or electroacupuncture.
We are familiar with transcutaneous electrical nerve stimulation (TENS), but Percutaneous electrical nerve stimulation (PENS) may be a new term to you.
With the application of PENS, a conductive needle is inserted into the tissue to directly stimulate the nerve. It is recognized as a treatment for human patients with nerve pain, and provides relief to 70% of post-herpatic or traumatic trigeminal nerve pain sufferers (Johnson and Burchiel 2004).
In equines, PENS has shown a 50% success rate (Roberts et al., 2011, 2018) with reduced symptoms (or complete remission) for an average of 15 weeks, at which point the treatment can be repeated.
Devereaux 2017 has reported some success with the use of electroacupuncture, which may be more readily available than PENS.
For more information on the above treatment modalities, as well as a few more, be sure to watch Dr Kirstie Pickles lecture in the Onlinepethealth Equine Members portal, where she shares her recommendations.
While we now know that the trigeminal nerve is involved in head shaking, we still do not know why the abnormal firing of the nerve occurs, or how we can return the nerve stimulus threshold to normal levels. The condition remains challenging to manage, but electrophysiological therapies do appear to be helpful.
Please find below the resource list for Dr Kirstie Pickles lecture on Equine Head Shaking in the Onlinepethealth Equine Members portal. All the information in this blog was sourced from that lecture:
Aleman, M., Williams, D., Brosnan, R., Nieto, J., Pickles, K., et al. (2013) Sensory nerve conduction and somatosensory evoked potentials of the trigeminal nerve in horses with idiopathic headshaking. J Vet IntMed 27, 1571-1580
Aleman, M.R., Pickles, K.J., Simonek, G. and Madigan, J.E. (2012) Equine herpesvirus-1 in trigeminal ganglia of horses with idiopathic headshaking. J Vet IntMed 26 92–194
Devereux S. (2017) Electroacupunctureas an additional treatment for headshaking in six horses. Equine Vet Educ
Madigan JE, Bell SA. Owner survey of headshaking in horses. J Am Vet Med Assoc(2001) 219:334–7
Pickles, K.J, Madigan, J.E. and Aleman, M.R. (2014) Idiopathic headshaking: is it still idiopathic? (Review article) The Veterinary Journal 20121-30
Pickles, K.J., Madigan, J.E. and Aleman, M.R. (2014) Owner reported response to treatment in 130 headshaking horses. Proceedings of the American Association of Equine Practitioners 2014
Pickles, K.J., Berger, J., Davies, R., Roser, J. and Madigan, J.E. (2011) Use of Gonadotrophin Releasing Hormone immunisationin equine headshaking. Vet Record 168 (1) 19-22
Roberts, VL, Fews, D, McNamara, JM & Love, S, 2017,‘Trigeminal Nerve Root Demyelination Not Seen in Six Horses Diagnosed with Trigeminal-Mediated Headshaking’.Frontiers in Veterinary Science, vol4., pp. 72
Roberts, VLH, Patel, NK & Tremaine, WH, 2014,‘Neuromodulation using percutaneous electrical nerve stimulation for the management of trigeminal-mediated headshaking: A safe procedure resulting in medium-term remission in five of seven horses’.Equine Veterinary Journal, vol48., pp. 201-204