Headshaking — H&H busts the myths

Headshaking, either vertical head-tossing or repeatedly striking or rubbing the nose on a foreleg, is a frustrating and painful condition for horses, humans, and vets, and unfortunately, the causes of it are not fully understood by veterinary science.

Back in the 1980s, it was assumed to be bad behaviour until it was proven to be a medical condition, attributed to everything from seasonal allergies to over-tight nosebands (it isn’t these things). It has a significant impact on welfare.

It’s 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’s 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 can’t be ridden due to this condition, or it’s clearly in pain when not ridden, sometimes the only option is retirement or euthanasia. However, since the condition was identified in the ‘80s, veterinarians have been studying it and there are a range of treatment options that work in many — but far from all — affected horses.

In order to discover what the most up-to-date research has found, Horse & Hound spoke with Veronica Roberts, senior clinical fellow in equine medicine, from the University of Bristol, and Andrew Fiske-Jackson, a senior lecturer and interim head of RVC Equine, from the Royal Veterinary College, two of the UK’s leading authorities on headshaking.

Why horses headshake

There are many reasons horses shake their heads, but the type Roberts and Fiske-Jackson are studying is called trigeminal-mediated headshaking, which, they explain, is the sensitisation of the trigeminal nerve, the nerve in the head that provides sensory information around the eyes, teeth, nostrils, etcetera.

Roberts explained: “When you tap your hand, you can feel it. The nerve tells the brain what it should respond to. But neuropathic pain occurs when the nerve tells the brain stuff it shouldn’t.” Fiske-Jackson added that they assume the horses feel it as an electric shock sensation, based on the experiences of people who have an analogous condition called trigeminal neuralgia.

Seasons

About 60% of headshakers are seasonal — they get worse in the summer — which is why it’s sometimes assumed to be related to allergies. Roberts and Fiske-Jackson said that it’s related to the environment in some way, the horse responding to some environmental stimulus, but so far, they can’t say why.

Steroids and other drugs commonly used to treat allergies don’t have any significant effect on trigeminal-mediated headshakers. However, Fiske-Jackson said he has seen cases where moving to a different yard has made the horse better, or where the horse doesn’t headshake when it arrives at his clinic. This remains one of the most befuddling aspects of this disease. Roberts suggested to me that it could even be two different diseases, seasonal and not seasonal, but so far, no one has identified any reasons or aetiology for this.

While both vets loosely analogised it to trigeminal neuralgia in humans, it actually isn’t the same disease at all. In humans, it’s associated with delamination of the myelin sheath, the membrane that wraps around nerves. Roberts, working with a neuropathologist who was an expert on trigeminal neuralgia in humans, has looked at the myelin sheaths of affected horses and they found no damage.

“The nerve was functionally abnormal but looked normal,” she said. While this left her no more enlightened about the causes or mechanics of headshaking, she still felt it was a positive result because myelin sheath deterioration is generally not fixable. Instead, she suggested: “A switch got flicked, and we don’t know what switch got flicked, or how to flick it back.”

Bute

If a horse is a suspected headshaker, vets first of all recommend the owner do a bute trial, but they quickly emphasised that neuropathic pain isn’t affected by NSAIDs. Afterall, it’s the nerves responding as if they encountered a painful stimulus, but it’s a nerve misfire and not a ‘real’ stimulus (although the pain is real). If a horse responds positively to a bute trial, it has head pain somewhere, but it isn’t 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. But it still provides vets with information.

Nose nets

They also recommend that owners try a nose net — a mesh fitting over the noseband of a bridle or halter and covering the horses’ nostrils. Roberts and Fiske-Jackson said 25% of horses improve by about 75%. So, if you have a mild headshaker, this could be a useful piece of equipment to try. It was once thought that nose nets work because they affect the particles or the moisture and airflow within the nostrils. Now researchers don’t believe this is the case.

Fiske-Jackson explained: “You replace a painful sensation with a different sensation. The nose net puts pressure on a tingling sensation we think they are feeling.”

He likened it to rubbing part of your body after you bang it on something. It’s called the ‘gate theory.’ When you hurt yourself, you activate a pain neuron, and the rubbing stimulus then activates an inhibitory interneuron, which can inactivate, or block, the pain neuron, thereby reducing pain.

Nose nets are cheap and easy for horse owners, but if they don’t work, the horse needs a clinical diagnosis.

Trigeminal-mediated headshaking

Trigeminal-mediated headshaking is a different disease than other potential causes of headshaking-like behaviour. It isn’t the same pathology as headshaking caused by allergies, sinusitis, dental pain, and a number of other conditions. When horses come into the clinics at Bristol or RVC, the diagnostic procedures they undergo mainly rule everything else out. And it’s challenging to diagnose as horses can vary from day-to-day, or season to season, and the severity of symptoms and their interpretation can be heavily influenced by the placebo effect, on both owners and vets.

There is only one test that definitively diagnoses trigeminal-mediated headshaking, and it involves sensitising nerves on a horse under general anaesthesia, but because of the risks of GA and side-effects, no one in the UK does this. Rather, they do an oral exam of the eyes and mouth and a CT scan of the head, which will show any structural abnormalities (but not in soft tissue), including dental disease, sinusitis, or any other ‘gross pathology’ — meaning something you can see.

They also do an endoscopy of the upper airway and look for any pathology like 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 don’t show any abnormalities, that moves the horse towards a presumptive diagnosis of trigeminal-mediated headshaking. Effectively, it’s a diagnosis of exclusion.

The vets will then do a nerve block of the maxilliary nerve. If they get a positive response, it means the problem is in that part of the nerve, but a negative response doesn’t rule neuropathic pain out. The source of the pain could be behind the nerve block. But as with other diagnostics, it gives the vet more information.

Drugs

What about drugs, owners wonder. Afterall, there are drugs that treat neuropathic pain in humans. Bute, as stated above, won’t work. Fiske-Jackson said a drug called Gabapentin has been well-established as a treatment for humans.

“In theory, it should work. In horses, there are no studies, but I know of only one case that has responded. It doesn’t work that well.”

He also said combinations of anti-histamines and anti-epileptics have been tried, and while they have seen some mixed results, it sedates the horses and they eventually develop resistance — it’s not really a viable option. Dexamethasone, a steroid, has been tried, with no improvements. And plenty of horse owners have tried supplements and homeopathy, but Fiske-Jackson emphasised that none of these have withstood rigorous scientific assessment and are no better than a placebo.

Neurectomy

Most owners are aware of neurectomy as a treatment for lameness — cutting a nerve so the horse can’t feel pain anymore. So, they might ask if there is a similar surgery for trigeminal-mediated headshaking. Roberts told me that this was in fact done on 18 horses in the 1980s. Three got better, but, she added: “The side-effects were horrible. You don’t want them to not feel their face.” It did advance veterinary medicine’s understanding of the disease and helped dispel the myth that it was entirely behavioural, but it wasn’t humane.

Coil surgery

The only accepted surgery for headshaking involves placing four to six 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. If you can manage it, the success rate is 50%. However, out of 58 horses in a study that had the surgery, four were put down due to the side-effects. Fiske-Jackson says he only does it in cases where the headshaking is severe and hasn’t responded to any less invasive treatment, and euthanasia is the only other option.

“I’ve seen it work well. Equally, I’ve seen it where the horse is traumatised,” he said. Owners need to go into it with their eyes open. Roberts told me she doesn’t like doing the surgery at all and feels there are better options now.

EquiPENS

The one that’s exciting vets the most is called EquiPENS. It’s a form of neuromodulation — altering the nerve activity by firing either an electrical or chemical stimulus into it — was first used in humans (called PEN therapy). A probe is placed over the nerve in the face and it fires an electrical impulse. The horse is standing sedated. They find it a little alarming at first, but Roberts and Fiske-Jackson said they soon relax and seem to find it pleasant. Roberts first did it on seven horses, and five went back to ridden work. In a subsequent 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.

Roberts said: “It’s not the answer but it’s the best we have for now. I feel we’re barking up the right tree but in the wrong direction.”

The good news is that PENS treatments have few side-effects or complications, but the downside is that it is expensive, £700 per treatment, and horses initially get three treatments but may need further ones. It goes without saying that the cost easily gets prohibitive to the average owner.

Article continues below…


You might also be interested in:


New research

Fiske-Jackson told us about a similar treatment called electroacupuncture, which also stimulates the nerve with electric impulses, but it uses a different algorithm than PENS. While it’s had some success in a small group of cases, both vets stressed to me that there are no rigorous studies yet for this treatment. However, the vets at RVC are working on a study of electroacupuncture because it is far cheaper than PENS, and if it withstands a rigorous study, it’s potentially a viable option for more owners.

At Bristol, Roberts has funding to trial a completely different approach to treatment, an amino acid used in humans, cats, and rats that stabilises nerve membranes. As of this writing, she is still looking for subjects and told us: “I don’t know how much the horse will absorb and I don’t know if the nerve is unstable.” However, it’s safe, and unlike other drugs that have been tried, you can compete on it. For the study, she needs 50 horses, properly diagnosed with CT scans, etcetera, and they will get on the trial for free. It’s a double-blind study, so neither the owners nor the researchers will know whether or not any given horse has had the drug or the placebo (although they can unblind any individual if the horse has a reaction). The horses will get two weeks of the drug, then two weeks of the placebo. Roberts is still looking for subjects and says anyone with a trigeminal-mediated headshaker who may be interested should contact the Equine Centre at Bristol University.

For all the latest equestrian news and reports, don’t miss Horse & Hound magazine, out every Thursday, plus don’t miss the latest issue, out today (18 April 2019), for our vet special