The horse repeatedly tossing the head vertically or rubbing its nose on a foreleg are typical signs of the condition known as headshaking in horses. This frustrating and painful syndrome can have a significant impact on the horse’s welfare.
Back in the 1980s, headshaking was assumed to be bad behaviour but is now known to be a distressing and debilitating medical condition.
Headshaking is found in 4.5% of the UK horse population, and one in five diagnosed horses do it at rest, while the remainder only headshake when ridden or lunged. It is reported to be more common in geldings than mares and stallions, and although it can appear at any age, it often first occurs between the ages of six and 12.
If a horse cannot be ridden due to this condition, or it’s clearly in pain when not ridden, then retirement or euthanasia may have to be considered. Since the 1980s a range of treatment options have been developed that can help many horses, but they do not work in all affected equines.
Why do horses headshake?
There is a range of potential reasons for a horse to shake its head. Some cases are caused by allergies, sinusitis, dental pain, and a number of other conditions. These all need to be ruled out as part of the clinical investigations into the cause of the condition.
The form of headshaking called trigeminal-mediated headshaking is a different disease than other potential causes of headshaking-like behaviour. It is caused by hypersensitivity of the trigeminal nerve – the nerve in the head that provides sensory information around the eyes, teeth and nostrils among other areas.In the case of trigeminal-mediated headshaking, the nerve responds as if it has encountered a painful stimulus, but it’s a nerve ‘misfire’ rather than a ‘real’ stimulus, although the pain felt is real.
Experts believe that when triggered, the nerve sends an electric shock-like sensation to the horse’s brain, leading to the extreme response. This is based on the experiences of people who have a condition called trigeminal neuralgia, which is similar in certain respects.
While vets have compared headshaking to trigeminal neuralgia in humans, it is not the same disease. In humans, it’s associated with delamination of the myelin sheath, the membrane that wraps around nerves. Researchers have looked at the myelin sheaths of affected horses and found no damage, thus the nerve functions abnormally while appearing normal. This has lead to speculation that there is another underlying condition.
About 60% of headshakers are worse in the summer, which leads some to believe the condition is related to an allergy of some kind. Experts believe that while it is related to the environment in some way, as these horses are responding to an environmental stimulus, they can’t yet say what or why in every case. Known triggers include bright sunlight, wind and pollen.
Ways to manage headshaking
Steroids and other drugs commonly used to treat allergies do not have any significant effect on trigeminal-mediated headshakers. However, there have been cases where a horse has improved when moved to a different yard, or where the horse doesn’t headshake when it arrives at a veterinary clinic for further investigations. This remains one of the most confusing aspects of this disease.
If a horse is a suspected headshaker, vets sometimes recommend the owner does a bute trial, while emphasising that neuropathic pain is unlikely to be altered by this. If a horse responds positively to a bute trial, it has head pain somewhere, but it is not a trigeminal-mediated headshaker. Often with bute trials, it’s assumed that if the bute doesn’t work, the problem is behavioural, but because of the nature of neuropathic pain, that’s not the case. However, it can provide vets with information.
Nose nets are usually the first line of treatment recommended — a mesh fitting over the noseband of a bridle or halter that covers the horses’ nostrils. This leads to an improvement in about 25% of horses by around 75%. So, if you have a mild headshaker, this could be a useful piece of equipment to try. It was originally thought that nose nets work because they affect the particles or the moisture and airflow within the nostrils. But now experts believe they work by constantly stimulating sensory nerve endings in the nose, which can suppress the overactive nerves that are sending sharp pain signals to the brain. This reduces the painful sensation rather like rubbing part of your body after you bang it on something.
Diagnosing trigeminal-mediated headshaking
When horses come into a specialist equine hospital, the diagnostic procedures they undergo mainly rule everything else out. The condition can be challenging to diagnose as horses can vary from day-to-day, or season-to-season, so video footage supplied by the horse’s owner can be very helpful.
There is only one test that definitively diagnoses trigeminal-mediated headshaking, and it involves stimulating nerves on the gum of a horse under general anaesthesia and measuring the response, but because of the risks involved, this is not performed in the UK. The vet will do an examination of the eyes and mouth and possibly a CT scan of the head, which will show any structural abnormalities including dental disease, sinusitis, or any other ‘gross pathology’ — meaning something you can see.
They would also do an endoscopy of the upper airway and look for any pathology such as lesions or swelling inside the nasal passages or throat that might cause the behaviour.
Obviously if the vets see anything, they treat it, but if the CT scan and endoscopy do not show any abnormalities, that moves the horse towards a presumptive diagnosis of trigeminal-mediated headshaking. Effectively, it’s a diagnosis of exclusion.
On occasions the vets will do a nerve block of the maxilliary nerve. If this eliminates the behaviour, it means the problem is in that part of the nerve, but a negative response does not rule neuropathic pain out. The source of the pain could be behind the nerve block further up the same nerve closer to the brain. But as with other diagnostics, it may give the vet more information. The procedure is tricky to perform and can exacerbate the headshaking, hence it is not always undertaken.
Treatment options: can drugs help?
While there are drugs that treat neuropathic pain in humans, such as Gabapentin, they do not work well in horses. Some combinations of anti-histamines and anti-epileptics have been tried with mixed results, but they are not licenced for use in horses and can cause sedation as a side effect. Dexamethasone, a steroid, has been tried, with no clear evidence of improvement. Also, plenty of horse owners have tried supplements and homeopathy, but none of these have withstood rigorous scientific assessment, although anecdotally some may help in some cases.
Most owners are aware of neurectomy as a treatment for lameness — cutting a nerve so the horse no longer feels pain. So, might this work for trigeminal-mediated headshaking? It was tried on 18 horses in the 1980s. Three got better, but it left the horses unable to feel their face, which caused serious welfare issues. While it helped to advance veterinary medicine’s understanding of the disease, it was not humane.
The only accepted surgery for headshaking involves placing platinum coils in the back of the intraorbital canal. The coils expand gradually and cause pressure on the nerve. It’s done under general anaesthesia and has significant risks. After surgery, it can irritate the nostrils and nasal cavity and be quite distressing for the horse and its handlers. The success rate is around 50%. However, out of 58 horses in a study that had the surgery, four were put down due to the serious side effects. It’s generally only chosen in cases where the headshaking is severe and has not responded to any less invasive treatment, leaving euthanasia as the only other option.
One treatment option that is currently in use is called EquiPENS. It’s a form of neuromodulation — altering the nerve activity by firing either an electrical or chemical stimulus into it — that was first used in humans (called PENS therapy). A probe is passed through the skin and placed over the nerve in the face and it sends an electrical impulse while the horse is under standing sedation. In a study of 168 horses, half responded positively but only one-quarter of those stayed in remission long-term. However, with top-up treatments, some of those who reverted back to headshaking stayed in remission longer.
The good news is that PENS treatments have few side effects or complications, but it is costly at around £700 per treatment. Horses initially get three treatments, but may need further ones which means the cost easily gets prohibitive to the average owner.
A similar treatment called electroacupuncture, which also stimulates the nerve with electric impulses, is currently being studied at the Royal Veterinary College. A separate study using six horses showed encouraging remission times in three of the horses.
Electroacupuncture is far cheaper than PENS, and if it withstands a rigorous study, it’s potentially a viable option for more owners. The treatment can be performed at the stable yard, avoiding the need for the horse to travel.
Researchers at Bristol University are conducting a study of amino acids that stabilise nerve membranes in people, rats, and cats. Amino acids have less side effects than conventional pharmaceuticals and are competition-legal.
A recently published study from the University of Californina, Davis, California, has show that supplementation of the diet with magnesium and boron can reduce headshaking behaviour in some affected horses
There is much still to be learnt about this condition as it is very variable and the majority of cases only headshake when tacked up and lunged and ridden, and far fewer horses do it just in the field or stable.
Currently the future for horses diagnosed with trigeminal-mediated headshaking is questionable at best. Some cases can be managed by simply wearing a nosenet during ridden exercise and appear unbothered by the condition while not working, while others clearly suffer from considerable ongoing pain, leaving euthanasia as the only option on welfare grounds. However, it is hoped that the continued research into new treatments for the condition will mean a brighter outcome for horses in the future.