Sarah brought her new horse home in December. Westley, an 8-year-old Thoroughbred cross, was well- behaved and showed real potential as the pair spent the winter months training in the indoor arena. It was only when warmer weather arrived that their troubles began. The first time Westley flipped his nose up, jerking the reins from her hands, Ongoing Sarah thought he had been bothered research into a by an insect she didn’t see. But neurologic cause then he did it again. And again. of headshaking Within minutes, Westley was having may soon lead a full-on meltdown: tossing his head up and down frantically, snorting, to more effective and rubbing his nose on his front treatments legs. He became so agitated that he for the most was unsafe to ride. Sarah hopped off frustrating and wondered what had happened cases. to her easy-going horse. It would be a weeks of veterinary visits before she By Anna Sochocky had the devastating answer: Westley was a headshaker. very horse shakes his head on occasion---after all, it’s an effective method for evading flies and other pests. In NEW some cases, however, Eheadshaking is signal that a horse is uncomfortable or in pain. Repetitive headshaking can be caused by poorly fitted tack, a heavy-handed rider, tooth HOPE FOR abscesses, tumors, ear mites, cysts or infections around the eyes or in the sinuses. The behavior is disruptive, but once the source of trouble is identified and addressed, it stops. Far more frustrating are cases of headshaking that have no identifiable 46 EQUUS 499 HEAD SHAKERS NEW HOPE FOR HEAD SHAKERS EQUUS 499 47 cause. When tack fits, the rider is euthanatized because they were cases and a similar condition in human skilled and the veterinary exam in a constant state of agitation or neurology, and consensus emerged on reveals no physical issues. What became a danger to work around. the source of the pain: the trigeminal happens then? Idiopathic headshaking cases were nerve, which runs along each side of These cases were once called often heartbreaking, for owners and a horse’s head and carries sensations “idiopathic headshaking”---meaning veterinarians alike. from the face to the brain. Dysfunction that the cause is unknown---and About two decades ago, however, of this nerve, researchers now believe, there was little that could be done. veterinarians who shared their causes the horse to feel chronic Owners often experimented for years experiences in veterinary journals neuropathic pain, which is sensation with fly masks, supplements and and at professional meetings began originating from the nerve itself, rather management techniques (see “What to notice patterns among cases than an external cause. Can Help,” page 51) to provide their of idiopathic headshakers. The “It’s very, very rare that these horses with relief. Sometimes they headshaking itself, for instance, headshaking cases are related to tooth would succeed in halting chronic was nearly always quick flips, up and abscesses, tumors or other problems,” headshaking but more often they down, of the nose, rather than a side- says John Madigan, DVM, DACVIM, didn’t. Persistent headshaking can to-side motion. The affected horses of the University of California, Davis. have dire consequences. In the often seemed anxious or surprised, “These do occur, but by and large the most extreme cases, horses were not willful or stubborn. They would problem is caused by changes in the snort repeatedly and rub their way the trigeminal nerve functions.” muzzles on their forelegs or other Knowing this means that objects. For many horses, exercise researchers are one step closer to Triggers for trigeminal- seemed to be a trigger. Some horses finding more effective treatments to mediated headshaking avoided sunlight, even hiding their ease, or even to stop, the neuropathic heads against a herdmate’s flank pain that leads to what is now called are still not fully when shade wasn’t available. “trigeminal-mediated headshaking.” understood but may Eventually, researchers were able to Researchers in California and in include exercise, as connect the dots between the equine England are both working on different well as exposure to treatment approaches that are already sunlight or wind. offering at least some horses relief from this maddening condition. PROMISING RESEARCH FINDINGS POINT THE WAY Persistent head- shaking in horses was once thought to be similar to a human condition called trigeminal neuralgia (also called tic douloureux), which is pain stemming from the trigeminal nerve. However, Madigan and the UC Davis group found many AUTUMN 2019 NEURAL ANATOMY A horse has two trigeminal nerves, one on each side of his head, which carry sensory information back to the brain. Each one divides into three main branches: the ophthalmic nerve, which covers the region around the eye; the maxillary nerve, which extends down over the muzzle; and the mandibular nerve, which runs down over the lower jaw. differences between the equine and observed. This suggests human disorders. In people, the nerve that headshaking is is oversensitive, firing in response to functional and could minor stimuli so that even mild contact, then be reversible, such as a brush on the cheek, can which explains the cause a jolt of pain. People describe off-and-on signs sensations ranging from severe often seen in stabbing pain to constant itching, horses with burning, tingling or aching. Horses headshaking, with persistent headshaking have says Madigan. signs of neuropathic pain, burning, In people, tingling, itching-like sensations, but trigeminal Physical damage to the triggers are different. neuralgia the nerve would have been Researchers over the past few is caused by difficult, if not impossible, to treat, years have sought to identify the physical damage explains Roberts. “The fact that the role of the trigeminal nerve in to the myelin sheath, nerve is only functionally abnormal equine headshaking. In a 2013 the outer surface of the nerve cell. suggests that somehow a switch has study from UC Davis, veterinary Aleman in 2013 published detailed been flicked to make it work badly, and neurologist Monica Aleman, DVM, histopathology of the trigeminal nerve maybe we can flick that switch back,” PhD, put 12 mature geldings under of six horses with headshaking and she says. “What we don’t know is what anesthesia and placed electrodes on confirmed there was no abnormal the switch is and how it got flicked in their faces to measure the reactions activity of the nerve and found no the first place.” of their sensory nerves. Six of geldings damage to its myelin sheath or other These findings about the had idiopathic headshaking and six physical changes. hypersensitivity of the trigeminal were non-headshaking “control” The group in England repeated nerve are opening new avenues of geldings. The researchers found these studies and reached the same research toward potential treatment that when electrical stimulation was conclusion. “I worked with a human options. “What we are trying to do is applied to the gums behind the pathologist who did a lot of work in make the nerve behave as normally upper canine teeth of the study similar conditions in people,” as we can,” says Madigan. “So we horses, the nerves of the headshakers Roberts says. “We looked at the are looking to identify specific fired at a much lower threshold than nerve from front to back in normal abnormalities in the pathways that did those in the control group. This horses and compared them to regulate the change in threshold for was the first proof that the trigeminal headshaking horses, and the nerve firing the nerve.” Once researchers nerve was functioning differently in is completely normal in the understand why the nerve misfires, headshaking horses. headshaking horses. So, what it is hoped, they will be able to find In the same study, the structures you are looking at is a nerve that is a way to fix the headshaking at its of the trigeminal nerves were also structurally normal but functionally source. So far, the following three analyzed using detailed histopathology, abnormal. And that’s areas of research have produced the and no changes in the nerve were very encouraging.” most promising results. AUTUMN 2019 EQUUS 499 79 In extreme cases, persistent headshaking can put a horse in a state of nearly constant agitation and make him dangerous to be around. POTENTIAL THERAPY: effects, however, lasted only about an but their ionized magnesium was hour because the body’s regulatory low. The ionized magnesium test is MAGNESIUM mechanisms tend to “normalize” its relatively new. Before, there was only SUPPLEMENTS blood levels of salts and minerals. a total magnesium test. We thought Madigan and other researchers in Nevertheless, says Madigan, “this ionized magnesium was important the Equine and Comparative Neurology study showed that altering a horse’s because that is the active form in the Research Group (ECNRG) at UC, Davis chemical balance, such as might be blood that can work on nerves firing.” have been investigating the role of achieved with diet, can make the nerve To investigate further, in a follow- nutrition in headshaking. Specifically, behave more normally.” up study published in January 2019, they have been looking into how This study also yielded a surprise the research team administered varying blood levels of electrolytes finding: All six of the headshaking intravenous magnesium sulfate to ---ionized minerals that play a role in horses had low levels of ionized six horses with trigeminal-mediated many functions throughout the body--- magnesium in their blood before the headshaking. Several blood electrolyte might affect neural function. trial started. “We were not specifically levels changed in response to the For a study published in the journal looking at ionized magnesium at treatment and headshaking behavior Animals in June 2018, the UC Davis this point,” says UC Davis equine was reduced by 29 percent, compared researchers looked into the effects of nutritionist Shara Sheldon, PhD, to horses that were not treated.