Research has uncovered clues to the mystery of headshaking, but for some horse owners the condition remains frustratingly difficult to manage
By Heather Smith Thomas
Reprinted from the November/December 2021 issue of USDF Connection magazine
After she retired from her job as an aerospace-industry systems engineer, dressage rider Christine Stevens looked forward to more time in the saddle. Formerly a “weekend warrior” who kept her horses in training 90 miles away, she eagerly moved her older gelding and her young Vivaldi mare, Fiona, closer to her home in California.
One April evening, “I got a call from the barn telling me something was wrong with Fiona,” Stevens recalls. “When I arrived, she was violently shaking her head up and down, rubbing her face on stall walls and striking out with her foreleg.”
Stevens’ veterinarian diagnosed Fiona’s condition as headshaking syndrome, but the problem was far from solved. That’s because this puzzling condition is a sign of disease rather than a disease itself. Multiple factors can be causes, and treating headshaking often involves trial and error, not always with complete success. In this article, we’ll untangle the web of symptoms and triggers, and we’ll look at the latest research, treatments, and equipment that may allow the headshaking horse to continue in its dressage career.
A Nervous Disorder
Headshaking syndrome is the behavioral manifestation of a disorder involving the trigeminal nerve, which runs from the top of the horse’s skull down along its face, says John Madigan, DVM, professor emeritus of medicine and epidemiology at the University of California, Davis, School of Veterinary Medicine.
A “headshaker” “shows signs of neuropathic pain—a tingling, itching sensation, like a bug inside the nose,” says Madigan, who has studied the condition for many years. “The horse may snort and rub the nose.” The unpleasant sensation triggers repetitive, involuntary movements, with affected horses typically flicking their noses up and down rapidly and repeatedly.
Horsemen once regarded the condition as a behavioral or training issue, using running martingales, elevator bits, and other devices in an attempt to eliminate the behavior, says Pamela Wilkins, DVM, PhD, DACVIM (LA-IM), DACVECC, professor of veterinary clinical medicine at the University of Illinois College of Veterinary Medicine. After Wilkins, a former hunter/jumper professional, became a veterinarian, she realized that headshakers are physically disturbed, not naughty.
“Rubbing the head or muzzle may be constant and vigorous, like a horrible itch that has to be scratched,” she says.
Determining the triggers of a horse’s headshaking can be challenging because “episodes may be intermittent and seasonal,” says Madigan. “Some horses headshake continuously while at rest, and others mainly when exercised.” Diagnosis can also be tricky because the site of the nerve pain or disturbance usually isn’t the actual location of the source of the problem, Wilkins explains.
About 30 percent of headshaking horses are light-sensitive, with the horse’s optic nerve also involved, the result being that “sunlight is the primary trigger” for those animals, Wilkins says. A comparative disorder in humans is “sun sneezing,” with exposure to light—not to a nasal irritant—triggering the achoos. The light causes sensory stimulation in the brain that makes the trigeminal nerve fire excessively. In humans, “a trigeminal-nerve problem can cause facial pain with nasal discharge, tears, sneezing, et cetera,” she says.
Wilkins says that many headshaking horses start seasonally—which often proves a red herring, with owners incorrectly suspecting allergies because the behavior rears its head in the spring—but eventually exhibit headshaking year-round. The actual trigger in such cases may be spring’s increasing hours of daylight, says Madigan, who adds that there may also be “an endocrine or hormonal component,” with geldings more likely than mares to be affected.
In fact, according to the results of Madigan’s research group at UC Davis, there is no link between headshaking and environmental causes. Madigan explains that in a headshaking horse, “the nerve itself is abnormal, but there’s no damage to the nerve. It’s a functional disorder. If we could figure out the functional disturbance and resolve it, the nerve would go back to normal function. Our evolving knowledge shows that it has something to do with the channel movement of ions that changes the threshold for the nerve, and it fires when it’s not supposed to. In some horses, the trigger is sound. Clap your hands and there’s an immediate reaction, as if you hit them in the face with a hammer.”
Madigan’s research team uncovered another possible factor: diet.
In a two-year study of six headshakers and six controls, researchers wanted to see whether changes in horses’ dietary pH affected headshaking behavior. “When we fed the horses something to make them a little acidified, headshaking became worse. When we changed the diet to slightly more alkalized, their headshaking diminished” by 48% but was not eliminated, he says.
Audrey DeClue, DVM, a Minnesota-based veterinarian in private practice, says she’s encountered headshakers in every breed and equestrian discipline, with higher prevalence in horses worked in high collection with lots of compression and poll flexion (we’re looking at you, dressage). Some of the cases she sees involve previous head and neck injuries, such as from pulling back while tied or striking the head on the roof of a trailer. Whether caused by riding and training or an accident, she says, the resulting damage to the nuchal ligament can lead to headshaking behavior.
“In horses, the opening where the trigeminal nerve comes out of the base of the skull is bigger than in any other species,” DeClue says, “with more chance for that nerve to be compressed if there is damage to the tissue layers that pull on it.” The nuchal ligament, which runs from tail to poll and attaches at the top of the skull, is one of those tissue layers, and riding in overflexion can lead to “severe displacement of the nuchal ligament at C2,” the second cervical vertebra. She adds that of nearly 100 cases of headshaking she has treated, “in 95% of those horses the nuchal ligament is displaced.” The displacement causes compression of the trigeminal nerve at the base of the affected horse’s skull, which triggers the problem, she says.
Most treatments for headshaking are aimed at eliminating or reducing the behavior. Some are intended to reduce exposure to triggers or to minimize the aggravating sensations, while others actually purport to address underlying factors, calming triggers so that the irritated trigeminal nerve can regain normal function.
For a horse that headshakes when exposed to bright light, spending days indoors and being turned out at night may help. There are fly-mask-type face masks with eye shields or “sunglasses” lenses to reduce glare while an affected horse is turned out or ridden outside during daylight hours.
Some horses headshake only while under saddle. Face masks and ear nets help to block sunlight and wind. Other horses’ headshaking worsens on windy days or if there’s a fan blowing on them in the barn. Some people tie dangling pom-poms to the bridle noseband, to bounce around and distract the horse from the sensation in the nose.
“Most of these tricks only work for a while,” says Wilkins. “Some horses respond to a nose mask. I used to make these out of pantyhose. It should fit firmly around the muzzle, attached to the noseband. Its purpose is not to filter out bugs or dust; I cut openings for the nostrils. It helps because of the pressure it puts over that part of the face—like pushing against your nose when you have to sneeze.”
A nose net “alters the cycle of the nerve firing and provides an overriding impact,” explains Madigan. “A study in England showed that a high percentage of headshakers respond to this method. A nose net made in Germany with heavy braided nylon jiggles on the muzzle, and some horses do well with it.”
It may take experimentation to discover what works best for a particular horse. Insect control is also important, as headshakers tend to be very sensitive to anything flying around the facial area.
A number of drugs have been tried. Twenty years ago, Madigan gave the first horses in his studies cyproheptadine, a first-generation antihistamine. About two-thirds of the horses responded to the drug, he reports, but inasmuch as it’s not permitted in competition, its usefulness is limited.
After the more recent UC Davis study showing that an alkaline diet could help to reduce symptoms, Madigan and his team began trying various magnesium supplements. The horses that were given magnesium and boron showed the greatest reduction in headshaking. Platinum Performance, the manufacturer that helped the researchers formulate the supplement for the trial, now sells the product, called Platinum Steady, as a nutritional supplement to aid in the reduction of headshaking, Madigan says.
In DeClue’s experience, “about 80 percent” of headshaking horses have allergies as a side effect of the irritated trigeminal nerve. She’ll recommend having an affected horse tested for allergies and put on a regimen of allergy shots if findings warrant. “Once we get the allergies under control, then I treat the horse for the headshaking.”
If the horse has a displaced nuchal ligament, DeClue will do shockwave therapy from the poll to the third cervical joint (C3). “This will actually create headshaking while I am shockwaving the horse,” she says. “When I get to the spot on the ligament that’s affected, the horse starts headshaking. I put injections into the facets from C1 to C3, to calm down that nerve pathway.”
DeClue says that “most horses need a couple shockwave treatments. I usually only inject them once, though in two cases I had to inject them twice. The rest is managed by shockwave therapy.”
Challenges with Dressage Horses
It’s impossible to ride with a steady, soft, elastic contact if your horse is flipping his head. Nose nets help many of these horses, and they are now permitted in USEF-licensed/USDF-recognized dressage competition if the entry is accompanied by a letter from a veterinarian affirming a diagnosis of headshaking syndrome that is improved by the use of a nose net (read the full rule at USEF DR 121.13). The letter must state the brand of nose net to be used, and it must be one that is USEF-approved for dressage. (Find the USEF rule book and the list of approved nose nets at usef.org.)
“Requests to allow use of nose nets were the number-one request in eventing and dressage for two years,” says USDF president Lisa Gorretta, an FEI chief dressage steward and USEF “R” dressage technical delegate who co-chairs the USEF Dressage Sport Committee, which makes the rules for US national-level dressage competition. “People with headshaking horses said nose nets helped, and veterinarians agreed.”
After repeated competitor requests for presidential modifications to permit nose nets, the Dressage Sport Committee decided to proceed with a rule change to allow them. “It was a rule change of expediency. It did not create unfair advantage, so there was no reason to not make it part of the fabric of dressage and eventing,” Gorretta says.
Several well-known riders have encountered headshaking horses. British Olympic gold medalist Charlotte Dujardin’s superstar mount Valegro suffered from headshaking and related allergies as a young horse, according to her memoir, The Girl on the Dancing Horse. In Valegro’s case, diligent insect control helped, as did an earlier-than-usual introduction to the double bridle, as the curb chain lies on an acupuncture point and wearing it calmed the headshaking behavior, Dujardin writes.
Closer to home, North Carolina-based USEF “S” dressage judge and equestrian journalist Margaret Freeman has had two headshakers of her own. In the 1990s, during her tenure at the equestrian newsletter Horse Journal, Freeman wrote an article about her experiences.
The first, a gelding, “was 15 years old and an FEI-level dressage horse when his headshaking started,” Freeman recalls. “The other was a four-year-old Hanoverian mare I imported from Germany. She’d had a lot of stress, but the veterinarians and I thought hers was related to allergies.” The mare gradually got over the headshaking.
Freeman believes that her gelding’s headshaking was indeed stress-related. “I had had him all his life and was in the process of selling him, and several people tried him. At that point, he started headshaking. It got worse, and he was almost unridable. I finally took him to my trainer for a change of scenery, and the headshaking stopped.”
“With both horses,” Freeman continues, “what I learned is that you should not react to the headshaking by pulling on the reins or trying to discipline the horse in any way because this makes it worse.” After she began riding her gelding again, the headshaking continued “but I just chilled and didn’t react; I kept my arms soft and ignored it. Within a week, his headshaking was gone.”
If a horse starts headshaking, “stop training for a while and see if the horse still does it,” Freeman advises. “Does it get worse when the horse goes on the bit? Does it get worse if his stress level goes up? In my experience, this problem can be helped by doing the right thing early on and trying to discover the trigger.”
DeClue and her approach have helped a mare owned by dressage rider Karen Turner of Washington state. The mare, Halle, had become “quite violent in headshaking,” Turner says. She “rubbed her nose on her legs and on the sides of walls in the barn, and ticked her head when cross-tied. Nothing seemed to help, and it was getting worse.”
After five agonizing months, Turner discovered DeClue and learned that “she was coming to this area, and agreed to see my horse. My mare had two sessions of shockwave therapy, which took care of the problem and made a huge difference. But 18 months later, the headshaking started again. I will probably have Dr. DeClue do these treatments again because this was the only thing that’s given my mare relief,” she says.
The veterinarian and the owner believe that Halle’s trigger is “extreme sensitivity at the poll. She’s not light-sensitive or sensitive around her face or nose area. But if you put a bridle, any kind of crownpiece or halter behind her ears, or even a fly mask that touches her poll, she goes nuts.” Turner has found that wrapping the crownpiece with foam to alleviate poll pressure “seems to give relief…but we can’t show her with an adjustment to a bridle. I could have a custom-made bridle created for her, but then I’d have to get it approved for competition. I’m trying to figure out how to build something like this and get approval to use it. We tried many different bridles and bits, and none made any difference except to remove pressure at the poll.”
As for Christine Stevens, the California rider, the quest to get to the bottom of her mare’s headshaking is ongoing.
“I keep a notebook to document observations about Fiona’s behavior and her environment, and together with my vet, we’re working through each possibility,” she says.
Last November, Fiona had surgery to remove a sinus cyst. “When headshaking began, we did a CT scan of her head,” says Stevens, “but it didn’t show anything that could be the cause of the headshaking.”
They’ve tried steroids, which reduced the behavior immediately, but the headshaking came back once Fiona was off the meds. Then, “at the next vet visit, we put Fiona on a lunge line and could see she was getting very little air flow through her left nostril.” Nearby cottonwood trees were shedding fluff, so the vet suspected allergies. “We started antihistamines and Fiona’s breathing improved, but the headshaking, while reduced, continues,” says Stevens.
Allergy testing showed that the cottonwoods weren’t the culprit. The tests indicated that Fiona is allergic to alfalfa, “but she’s been fed alfalfa for four years—much longer than she’s been headshaking,” says her stymied owner. “There’s too much headshaking to ride her, so we’re still lungeing for exercise. In the meantime, we’ve added a magnesium supplement to Fiona’s buckets, and I keep updating my log that I share with my vet, hoping to eventually find a solution.”
Although for some horse owners headshaking is a transient issue, others find the process of pinpointing triggers and finding effective solutions maddeningly difficult. For more information about this still-mysterious condition, see researcher Dr. John Madigan’s website, headshakerinfo.org.
Idaho-based cattle rancher and freelance writer Heather Smith Thomas has been writing about horses and cattle, and raising and training horses, for 50 years.