Whole Lotta Shakin Going On
- Apr 1, 2006
Your horse is doing it again: Inexplicably tossing his head and sometimes charging off. Your trainer has tried everything--fly spray, changing tack, new bits, negative reinforcement--and while your horse goes through periods where he never flips his head, sooner or later, he always reverts back to his head-tossing and unruliness, sometimes acting that way for months. The vet can't find anything wrong either. He's otherwise a great horse, but his behavior is more than just an annoyance--it's unsettling and sometimes makes for risky riding.
"I get e-mails about this almost every day," says John E. Madigan, DVM, MS, Dipl. ACVIM, a professor in the Department of Medicine and Epidemiology at the University of California, Davis, School of Veterinary Medicine. "Somebody is always struggling with this. Owners and horses are really suffering. It's very, very sad."
Madigan, who has been researching this problem for 14 years, says most of these cases do not involve ill-fitting tack, flies, or bad behavior, but are physiological conditions that he named "trigeminal-mediated headshaking syndrome."
The important distinction of physiological headshaking is that this is an abnormal, involuntary behavior.
For sure, there are times when head-tossing is deliberate: When flies are irritating a horse about the face and ears, when a restrained horse is impatient prior to exercise, when a horse is angry or upset, and/or when tack is irritating. When the handler applies the fly spray, removes the tack, releases the horse, or addresses the cause of upset (i.e., a rider with grabby hands, bouncy seat, or thumping heels; an unpleasant veterinary or farrier procedure; or an incompatible herd member or stall neighbor), the headshaking ceases.
But with headshaking syndrome, the horse shakes its head for no apparent reason. Sometimes these episodes are almost violent and out of control, occurring with alarming frequency. At times, the horse runs off or runs through a jump, strikes at its face, and snorts. Some rub their noses on objects or exhibit odd behaviors. Madigan recalls one horse that kept sticking his head in a 50-gallon drum.
The syndrome is not completely understood, with experts differing on cause and treatments.
Unraveling the Mystery
Headshaking syndrome is most likely due to neuropathic pain caused by a problem with the trigeminal nerves, Madigan reports. These nerves supply motor and sensory fibers to the face, jaw, and head region. For some reason in affected horses, a stimulating factor--light, feed, smells, exercise, etc.--results in increased nerve traffic in and around the trigeminal ganglia (a mass of nerve tissue associated with the face and head). This causes inappropriate nerve firing that in turn results in persistent or intermittent burning, tingling, itching, or electric-like sensation in the nose and face.
"This has been described in human literature," says Madigan. "People that have trigeminal nerve pain will tell you it's the most excruciating, difficult situation they've ever encountered. Trigeminal nerve pain in the horse is very similar to trigeminal neuralgia in humans, which does not have a known cause, either."
There are other possible causes than trigeminal neuralgia. Professor Derek C. Knottenbelt, MRCVS, Dipl. ECEIM, of Philip Leverhulme Hospital at the University of Liverpool, has been studying headshaking syndrome for 15 years, working on developing treatment methods. In addition to the trigeminal neuralgia, he believes the problem could also be a lowering of the threshold of perception or pain with a facial nerve reflex response. "The pathology, I think, lies in the brain stem and may be similar to post-herpetic neuralgia (nerve pain caused by a herpesvirus) or multiple sclerosis in people."
Equine herpesvirus type 1, in particular, has been thought by some to be a risk factor in headshaking. Explains Knottenbelt, "The theory is the herpesvirus lies dormant in the horse's trigeminal ganglia, where it sets up an immunological response that 'blocks' the damping down of nerves. When this occurs, even a small signal applied to a trigger zone in the nose or facial region triggers a massive response that the horse reacts to in typical fashion. If the condition is one that involves the sensory perception, either the signal has to be over threshold or the threshold has to be reduced so that a smaller, localized signal (normally ignored) causes a big response. This pathology is recognized in other species, but we are not at all sure if herpesvirus is involved."
Knottenbelt feels that vaccinating the horse's dam for herpesvirus seems to be protective--he has seen no cases of headshaking in horses born to vaccinated mares. Of course, this does not mean that every horse with herpes will be a headshaker--there is clearly something else involved, and Knottenbelt feels that this might have an immunological basis similar to the post-herpetic neuralgia in humans.
Madigan, who says almost all horses have latent herpesvirus type 1, but few horses have headshaking, does not share that viewpoint.
Regardless of cause, horses respond to these sensations with involuntary, sudden, quick flicks of the head; usually these are up-down, but can be side-to-side if the trigger zone is in the ear or eye regions, Knottenbelt states. They rub their noses or faces on objects, the rider's leg, or the ground, often while moving. They snort, sneeze, or strike at their faces. Some horses become frantic, posing danger to themselves and their riders by running through jumps or into something.
"A few cases present with a 'glass-wall syndrome,' in which they refuse to go forward at all--preferring to rear and even fall over rather than respond to riders' encouragement to move on," says Knottenbelt. "Many cases clamp their nostrils closed even after heavy exercise. This suggests that the trigger factors may be acting on a trigger zone within the nasal cavity or sinuses."
Clinical signs vary in intermittence and persistence, says Knottenbelt. "Intermittent signs are shown only under defined conditions such as exercise, bright light, wind, rain, inside or outside, etc.," he notes. "Persistent signs occur all the time under all circumstances. Signs can appear seasonally or year-round. The worst type of headshaking is the non-seasonal, persistent headshaker."
Experts disagree on how common the headshaking syndrome is. Knottenbelt believes the disorder is rare (but one of the most under-diagnosed conditions in its early stages). Madigan differs to some extent: "Data does not exist on the incidence in the population, but in my opinion, it's frequently undiagnosed or misdiagnosed. However, it appears that a significant number of horses suffer from this condition. Most practitioners will have one or two headshakers in their practice; I have dozens and dozens of clients whose horses suffer from this often-debilitating and painful condition. The German FEI organization had 200 separate requests in the past few years for horses to wear protective devices on their noses (to control clinical signs of this problem)."
Headshaking is seen more commonly in geldings. "Typical age of onset is about nine years," Madigan says, "although the range varies from two years to 20-plus years of age. Often, the disease is seasonal and affects horses that are overweight and in poor athletic condition. It occurs worldwide and manifests itself in many breeds and disciplines, but the occurrence seems less common in horses with heavy exercise such as endurance, polo, and racehorses. But I've seen it in racing horses after they've been retired and they lose condition."
About two-thirds of affected horses have a seasonal component to their clinical signs, with little or no headshaking activity occurring in the winter; spring and summer months are when their headshaking problems are at their worst.
"For some horses, their signs appear like clockwork--the same month, every year," says Madigan.
In fewer cases, some horses have increased problems in the autumn. A minority of affected horses experience problems year-round. In the latter, it's been theorized the condition began as a seasonal problem that regressed.
Seasonal changes might involve alterations in serotonin and melatonin levels, but this remains to be studied and proven. Says Knottenbelt, "The role of serotonin and melatonin may be important, but it seems possible that the trigger factors are seasonal, also. Thus a horse whose trigger factor is a particular type of pollen or insect irritation in the nose will only show the signs when those factors are present and the horse has access to them. This has given rise to the concept of an allergic disease, but I consider it more likely that the trigger factor for the trigenminal nerve activation may occasionally be related to a primary allergic inflammation in the nose."
Another theory supported by Madigan (but not by Knottenbelt) is that intense spring and summer sunlight is a trigger in some horses.
Sunlight stimulates the trigeminal nerve that produces facial neuropathic pain or sensation, similarly to the photic (light-involving) sneeze described in some humans. Affected humans experience a sudden urge to sneeze, followed by one or more sneezes, when exposed to strong light. The estimated incidence in the human population varies, ranging from 18-35%, with the condition believed to be genetic.
"Outdoor behavior of horses that suffer from this condition suggests that horses seek to diminish their exposure to sunlight naturally," Madigan reports. "For example, the horse will seek shady areas or put his face right under the tail of other horses." Or, in the case of one horse, putting his head inside a drum when a closed gate prevented the horse from going inside.
"He figured it out," Madigan adds.
Madigan observed that placing a light-reducing covering over that horse's head or tinted lenses over the horse's eyes stopped the headshaking.
Knottenbelt's position on sunlight being a trigger is that, "We subject the horse to artificial, extremely bright light with strobes and continuous light, then there is nothing seen and no change--no induced headshaking. Most of the headshaking cases we see will do it on the treadmill whether light is there or not!
"The role of light needs to be clarified," Knottenbelt continues, "because clearly there are circumstances when bright light (e.g., sunlight) is combined with dust, warmth, wind, pollen, and many other things that are present when the sun is shining. Covering the eye also protects it from other possible trigger factors apart from simply the light. Application of contact lenses can help, but the simple application of local anesthetic to the surface of the eye also helps. It is very complicated."
Other triggers could include wind, exercise, rain, loud sounds, dry conditions, warmth, cold, or physical factors such as insects, pollen, and dust particles. Other trigger zones could include the surface of the eye, the nasal cavity, sinuses, or any of the facial skin. "We have studied individual horses with sound-, smell-, and coarse forage-induced neuropathic pain," says Madigan. "So it's a real finding in some horses."
Some cases have been ascribed to a trigger factor as simple as intranasal air turbulence, Knottenbelt says. That could explain the worsening as the horse exercises and the amount of air increases in the nasal cavity. "This corresponds with an increased blood supply, faster heart rate, and exposure to increasing cold, warm, dirty, or damp or dry air," Knottenbelt says.
He adds, "It is also true that some headshaking horses have defined disease such as head trauma, dental disease, sinus or nasal inflammation, or even middle ear disease. However, there is little data to support those theories because there are so few researchers interested in the condition. I have just had a case that has come up with an atypical form of headshaking that is due to a pinched nerve in its neck! There are so many forms of the disease and the term is commonly applied to cases that have pain or resentment at other places."
Because there is no definitive laboratory test to diagnose headshaking syndrome, arriving at a presumptive diagnosis is largely based on clinical signs, history, and elimination of other problems.
Start out by taking the rider off the horse's back, removing the horse's tack, and applying fly spray or determining that flies are not around. If the horse still continues headshaking activity, look further.
"Review the clinical signs and do a physical exam that includes endoscopy and skull radiographs," suggests Madigan. "That's usually enough to make a determination that it's a trigeminal nerve and not something else that's causing the horse some discomfort. It is very, very, very
uncommon to find another problem that produces these clinical signs."
Knottenbelt suggests also doing tests to see if there are any organ abnormalities or lameness that could produce the horse's clinical signs. "Response to nerve blocks or sodium channel-blocking drugs such as carbamazepine can help confirm a diagnosis of headshaking; these drugs suppress nerve excitation and their effect is greatest on the hyperexcitable nerve," he says.
Although headshaking syndrome cannot be cured, various techniques can help mitigate it; but again, opinions on efficacy differ.
Identify and remove the trigger--Try to identify triggers by keeping a diary of circumstances in which headshaking appears and disappears--day/night-time, rainy/clear weather, time of day/year, and activities.
Light reduction--If light is a factor, keep the horse in a dark environment, ride inside, and/or have the horse wear light-blocking eye protectors, masks, shields, goggles, or tinted lenses. Madigan says, "This has been useful in many cases." Knottenbelt says, "In my experience, they are only effective if they protect the surface of the eye and the facial skin."
Nose pressure--"In published studies, about 30% of affected horses will respond to pressure on the nose, which interrupts the nerve firing just like when you interrupt a sneeze by putting your finger under your nose," Madigan says. "This sends interference up the trigeminal nerve and blocks the sensation." This can be achieved with a nose net, a device that attaches to the bridle and puts pressure on the nose.
Explains Knottenbelt, "Nose nets may simply touch the nose and give the horse a reference point for its sensation. This corresponds to the reason we rub our heads when we have a headache--you don't massage your brain, you give the brain something it can measure the discomfort against. Horses sometimes rub because they are numb. The nose net may simply provide relieving stimulus for a numb patch. They may work by filtering out particulate matter or by altering the turbulence of the air in the nose itself."
Drug therapy--Various drugs have been tried to block nerve transmission, with mixed success. One is cyproheptadine, which is an anti-serotonergic, antihistamine, and anticholinergic (partially blocks parasympathetic nervous system response) drug. Others include anti-seizure medications such as phenobarbital, carbamazepine, and fluphenazine (a tranquilizer used to treat psychotic disorders). "Studies in the U.S. and elsewhere show that two-thirds of owners of horses which are headshakers report improvement with cyproheptadine," Madigan says.
Surgical treatment--An infraorbital neurectomy (destruction of a branch of the trigeminal nerve) eliminates sensation in the upper lip and cheek. A study done in 1993 found the procedure was, at best, 30-40% effective. "I strongly recommend not attempting any surgical options," Madigan states. "Cutting nerves is never a good idea, and it won't work."
Alternative therapies--So far, complementary therapies have not shown to be of much help. A survey and small trial conducted by Madigan found that chiropractic and acupuncture produced no improvement, while a 2002 study reported that only 6% of owners felt homeopathy produced helpful results.
"If you identify and remove the triggering factor, then the condition may settle down," says Knottenbelt. "A few cases resolve eventually, but most get worse over time. The prognosis is very poor."
Adds Madigan, "I think the prognosis is not good for the horse getting over the condition. The horse will need light-blocking conditions or medications most of its life. Some horses are refractory to all those attempts; unfortunately, humane euthanasia is the only option for these severely affected horses in constant, unremitting pain."
Hopefully, definitive answers are forthcoming as research continues on this disorder.
Says Knottenbelt, "The cause of headshaking syndrome may only involve a single or few nerve fibers and a minute focus in the brain stem, but finding that may be a needle in a haystack. We are looking for intelligent ways of 'searching for the needle' that do not involve looking at every single hay strand until we find it!"
Madigan is conducting a clinical trial in which he interrupts gonadotrophins to see how seasonal changes in gonadotropins might affect trigeminal nerve activity. (Gonadotrophins are the hormones that stimulate the testes in the male and ovaries in the female.) His laboratory is also in the process of analyzing results relating to neuro-pharmacological changes with the seasons; he hopes to publish the results later this year.
Until more answers and better techniques emerge, Madigan encourages owners of affected horses to not give up. "It's not always the bridle, saddle, or trainer," he says. "If your horse has a combination of headshaking signs, that means there's a medical problem. The horse does not need to be punished because it's doing this, or have its head tied down or some other intervention. If you can correct the headshaking pain, that behavior stops."
About the Author
Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.
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