There’s a nerve-tingling explanation behind many headshaking cases
A toss of the head. A flip of the nose. It seems so benign, right? A horse that shakes his head continually, almost viciously, however, is no trivial matter. This type of headshaking can be frustrating for owners, detrimental to horse welfare, and pose diagnostic and treatment challenges for veterinarians.
Equine headshaking has long been a research interest, but scientists continue to hone in on what causes this vexing condition and how to best manage it. Let’s take a look at what they’re learning.
Hitting a Nerve
Sure, it’s possible your horse is shaking his head to deter flies or it’s simply a repetitive behavior he’s developed, but the truly uncontrollable and chronic headshaking that poses a danger to both horse and human is typically due to nerve pain. Researchers call it trigeminal-mediated headshaking.
“These horses suddenly start very violently shaking their head, especially vertically,” says Monica Aleman, MVZ, PhD, Dipl. ACVIM, an associate professor of medicine and epidemiology at the University of California, Davis (UC Davis), School of Veterinary Medicine. “It looks almost like a bee or a fly is in their nose or someone electrocuted the horse, because it’s a very violent headshake. It can be so bad it can become dangerous for the rider and the horse itself.”
With these horses, the trigeminal nerve that supplies all sensation to the face is on the verge of firing at all times. The horse might snort, rub, and shake his head constantly or intermittently in response to the burning, tingling, itching, or electric sensations this neuropathic (nerve) pain causes. Many horses display the behavior seasonally or in response to a seemingly innocuous trigger such as wind or light.
“Approximately one-third of horses are seasonally affected, usually in the spring and summer,” says Veronica Roberts, MA, VetMB, a senior clinical fellow in equine medicine at the University of Bristol Veterinary School, in the U.K.
She says the headshaking can come on either suddenly or gradually, most often in horses ages 6 to 12, and seems to be more common in geldings. Interestingly, says Roberts, the trigeminal nerve in affected horses appears normal when examined under a microscope post-mortem.
“We do not know why this neuropathy occurs or, indeed, what it is which goes wrong in the nerve,” she says. “We do know, however, that the nerve is sensitized, firing at too low a threshold.”
There’s a Test for That
Before you jump to the conclusion that your headshaker has trigeminal neuropathy, work with your veterinarian to rule out other medical or environmental reasons for the behavior. Additional causes of headshaking, our sources say, include:
- Abscessed or fractured teeth;
- Dental disease;
- Nerve irritation secondary to a viral infection;
- Ear infections;
- Ear mites, ticks, or foreign bodies;
- Neck issues;
- Temporohyoid osteoarthopathy (a disorder of the hyoid apparatus, which supports the larynx and tongue, and associated structures);
- Guttural pouch abnormalities; and
- Ethmoid hematomas (masses that originate from the mucous lining of the ethmoid turbinate, located in the back part of the nasal cavity).
Your veterinarian might recommend performing a CT scan of the head, radiography (X rays), endoscopy, dental and ear exams, and more to diagnose or rule out any of these conditions.
In 2013, however, UC Davis researchers developed a test to definitively diagnose trigeminal-mediated headshaking. This involves stimulating a nerve in the horse’s gum and measuring its electrical activity. The downside, says Aleman, is it causes so much pain that horses must be anesthetized.
“In that sense it’s not that practical,” she says, adding that her team still diagnoses most cases based on clinical signs and ruling out other conditions. “But this test was a major discovery in the sense that we now know the nerve is in a ‘close to fire’ mode. We do not definitively know what makes the nerve so sensitive or what exactly triggers the nerve to fire.”
Using the test, Aleman and her team have compared seasonal headshakers and healthy controls during the “on” and “off” seasons. During the off-season the headshakers responded just like the controls.
“So we know now that it’s not (literally) something in their heads, not a structural abnormality,” she says. “It’s a functional abnormality where the nerve just fires and, therefore, causes pain.”
Currently, there’s no cure for trigeminal-mediated headshaking. Many researchers have drawn comparisons between these horses and humans with a similar condition called trigeminal neuralgia. For this reason scientists have tried applying therapies used to treat neuropathic pain in people to headshaking horses, with mixed or only short-term results, says Roberts.
Many of the available therapies and procedures aim to relieve the horse’s discomfort, first and foremost. These might include:
Veterinarians have prescribed a number of drugs in an attempt to control the nerve pain associated with headshaking, with mixed results.
“Successes tend to be short-term,” says Roberts. “Furthermore, the drugs are expensive, carry side effects of drowsiness, and are prohibited in competition.”
One of those is the oral antihistamine (and seratonin blocker) cyproheptadine, which has shown a response rate ranging from 48% to 70% in various studies. Aleman says about 60% of the headshakers they administer this drug to at UC Davis show improvement. Other oral antihistamines have produced positive responses in some horses, but at lower rates and in conjunction with drowsiness and side effects such as lethargy and colic.
Masks and Nets
In horses for which sunlight is a clear trigger, these products might provide relief. Your everyday fly mask, however, won’t work; the mask must be specially made with ultraviolet light protection. “It doesn’t completely stop it, but reduces (reactivity) to the point the horse can be rideable again,” says Aleman.
The same goes for devices such as nose nets that dangle from the horse’s bridle over his muzzle. Roberts attributes their occasional success to gate-control theory: Essentially, a nonpainful stimulus closes the “gates” to the painful one, changing that sensory input from something that hurts to something innocuous.
Recently scientists have been studying headshaking horses’ diets. Specifically, says Aleman, they are looking into the effects of feeding alfalfa, which has a higher magnesium content than grass hay. Magnesium can help balance the body’s pH level, which is important for nerve firing as well as calming.
“When we change the diet of these horses, we may actually decrease or slow down the headshaking,” Aleman says of her team’s early observations of diet changes. “When we go back to diets that are lower in magnesium, horses start shaking a little more.
“Can you imagine if by just changing the diet or providing something to change the pH, you can completely avoid (headshaking)?” she adds. “That’d be a major, major finding.”
Researchers have studied or observed nutritional supplements’ effects on headshaking, as well. The hormone melatonin, for instance, might help reduce the seasonal headshaking of light-sensitive horses when administered daily. Basically, it tricks the horse’s internal seasonal clock into thinking it’s winter.
For the aforementioned reasons Aleman also recommends supplementing affected horses’ diets with oral magnesium daily. Based on the results of an owner survey she conducted in 2014, supplementing with both melatonin and magnesium yielded positive results in 55% of treated horses.
Aleman cautions owners to use products containing only magnesium, and not multiple other ingredients, and to have the veterinarian monitor horses’ blood magnesium levels to avoid overdose.
“We have six horses in our center (at UC Davis) that were donated because they were unrideable and completely unsafe that we have been able to manage with diet and magnesium daily,” she says. “It has not stopped the headshaking completely, but we can now do things with the horses without getting injured.”
Researchers have recently taken a page from human medicine and applied neuromodulation therapy to affected horses. Roberts says this safe and minimally invasive procedure involves using a probe to electrically stimulate the trigeminal nerve for a short period, with the goal of “resetting” its normal threshold level for firing.
As of August 2017 Roberts says her team has performed approximately 500 neuromodulation procedures on standing sedated horses with no significant adverse effects. She says about a quarter of these produced long-term success, a quarter short-term success, and the remaining half no response.
“Some horses have a promising start but then fail to respond to later treatments,” she says. “It is my early impression that where horses respond, but for insufficient time, length of remission may increase with repeated procedures. It is still very early days and we have a lot to learn and refine.”
Veterinarians have not yet devised a surgical procedure that consistently corrects headshaking. Roberts, however, participated in the development of one in which surgeons implanted platinum coils into the infraorbital nerve in an attempt to interrupt nerve conduction.
“It carries a long-term approximate success rate of 50%, but there is a significant risk of severe side effects, which may require euthanasia, and about a quarter of cases relapse,” she warns. “We therefore only recommend this procedure where euthanasia is the only other option.”
Aleman says potential complications associated with this surgery include aggravating pain and neuromas (tumors arising from a nerve), and many horses require more surgery or euthanasia. “Currently, I am not recommending surgery for this specific condition for any horse,” she says.
Determining which treatment or combination of treatments works best for your horse is likely going to require some trial and error, as each individual responds differently.
“That’s what’s been so frustrating about this disorder,” says Aleman. “You talk to several people, and they are going to tell you, ‘Yes, that works,’ or ‘No, that didn’t work.’ But what worked for one horse might not work for another. It’s very variable.”
Trigeminal-mediated headshaking is clearly a multifactorial problem, meaning a number of components play a role; environment, diet, hormones (since it primarily affects geldings), and pH levels likely all have an effect on its severity. And when no combination of treatments seems to help, sometimes euthanasia is the most humane option.
“This is such a frustrating disorder,” says Aleman. “We hope one day we’ll be able to help these horses, because it’s terrifying for the owner and devastating to see these animals going through that.”