The amazing thing about horses is how such large, powerful animals can be exquisitely aware of the slightest sensation, such as a small insect on its back or face. Rippling of skin or an occasional head shake is a normal response to the tickling trigger of nerve endings. But, there are times when a horse can't stop shaking or tossing its head to a seemingly inapparent sensation; such incessant behavior is known as headshaking.
Even when a human understands a source of discomfort or pain, it is hard to ignore it or stop natural aversion reflexes. In the case of headshaking, a horse doesn't understand why his muzzle or face persistently feels a tingling or painful sensation. Headshaking behavior is considered to be caused by overactivity of branches of the trigeminal nerve that supply sensation to the face and muzzle. A horse's behavioral reflex causes him to flip his head, snort or sneeze, rub his head, or take evasive action. Most headshaking horses (89% of them) flip their head vertically, according to research findings.
In general, the horse behaves like you might expect if a bee flew up his nose, making it difficult or dangerous for him to be ridden or handled.
Researchers agree this syndrome involves abnormal firing of the trigeminal nerve. Research (Knottenbelt, 2009) evaluated the response achieved by placing a coil into the infraorbital canal to put pressure on the infraorbital nerve, one branch of the trigeminal nerve. Continuous feedback induced by the coil was able to stop nerve firing, thereby corroborating the trigeminal nerve as a source of irritation.
Derek Knottenbelt, BVM&S, DVM&S, MRCVS, Dipl. ECEIM, of the University of Liverpool, calls headshaking a "loaded gun disease" in that it is ready and waiting for something to pull the trigger. He observes, "The specific trigger for an individual horse is variable--triggers include heat, cold, wind, dust, particles, irritant gases, moisture, dryness, pressure, or anything that stimulates and sensitizes the respiratory surface, such as allergies."
Cases elicited by an obvious pathologic lesion respond to specific treatment, such as addressing a bad tooth or an inflamed sinus. Still, Knottenbelt feels the disease itself lowers the threshold of perception so a small signal can result in a large reflex response to pain perception. "That's why headshaking is so awful, since the horse perceives pain similar to that caused by inflammation, yet painkillers won't resolve the true condition."
John Madigan, DVM, MS, Dipl. ACVIM, a professor of veterinary medicine at the University of California, Davis, is recognized as the leading expert on equine headshaking syndrome. Years of examining difficult cases have led him to question why this phenomenon is seasonal-- as many as two-thirds of headshaking cases occur in spring.
Madigan has identified a correlation of gender to this behavior, finding 66-75% of cases are geldings. "In some studies," he reports, "90% are geldings. The prototype is 9 to 11 years old, although the age range is wide."
Madigan says there is a rise in reproductive hormones called gonadotropins in the spring, and this is normally associated with the breeding season typical of horses. He explains, "This surge occurs as days lengthen. Geldings have no testosterone feedback, so they have higher levels of gonadotropins, like FSH (follicle-stimulating hormone) and LH (luteinizing hormone), for a period of time."
In evaluating if these particular reproductive hormones play a role in the pathophysiology of trigeminal nerve irritation, he describes research done on ferrets that were found to have an amplified adrenal response following neutering procedures--the researchers identified the presence of LH receptors on the adrenal glands of these neutered animals.
Madigan says, "This demonstrates that reproductive trophic (pertaining to a nutritive effect on or quality of cellular activity) hormones can have target tissues outside the reproductive tract, especially when levels of gonadotropins are higher due to lack of normal feedback, similar to what occurs in testosterone-deficient geldings."
How do gonadotropins influence the trigeminal nerve? Madigan explains, "The hormones seem to affect some part of the trigeminal ganglia, altering its chemistry and rendering it unstable. (Ganglia provide relay points and connections between nerves.) As normal sensory 'traffic' comes into the trigeminal ganglia, then additional triggers like light, sound, smell, or exercise may stimulate this nerve to 'fire.'
"The horse then receives neuropathic pain such as burning, itching, tingling, or electriclike sensations, any of which are manifestations of horses with headshaking," he adds.
A New Zealand mare provided evidence to the role of gonadotropins. Madigan describes the case: "In the spring, exam of a 4-year-old headshaking mare found small ovaries along with sky-high LH levels. She was given an anti-GnRH (gonadotropin-releasing-hormone) vaccine (Equity) licensed in New Zealand; this dropped her LH levels and the headshaking behavior went away, and stayed away."
By altering gondadotropin access to the trigeminal nerve, he says the adverse behavior can be modified in other horses, too. Madigan notes, "There have been cases of headshaking in stallions, but 50% have responded to gonadotropin therapy (described on page 24)."
Triggers of Headshaking
Bright sunlight seems to trigger headshaking behavior in half of affected individuals, termed photic headshakers. A similar phenomenon in humans, called photic sneezing, is triggered by light stimulation of the eyes that, in turn, induces facial and nasal muscles to sneeze. Madigan says when light is the primary stimulus to headshaking, there are no symptoms at night or when the horse wears goggles.
"It's not a different disease, it's just that the principle stimulus is light," he explains. "Some horses may be wired for more input into the trigeminal nerve just as are people with photic sneeze."
In another example he describes a horse that displayed headshaking behavior only when eating long-stem hay; the behavior resolved with feeding soaked pellets.
Knottenbelt does not find light stimulation to be a primary trigger of European headshakers, noting, "Lots of other things happen when the sun is out, making it difficult to attribute this syndrome simply to optico-trigeminal summation."
He says covering the eyes of a horse does more than just block light; it also protects the surfaces of the eyes and face, thereby removing common trigger zones. He notes, "In bright sunlight, the eye surface and face get warm, insects emerge, with dust adding to these stimulations."
Madigan explains how exercise might trigger headshaking behavior: "The autonomic activity of exercise initiates a lot of activity in the nasal area, such as engorgement of the nasal turbinates and snorting that is normal to clear the nostrils so the horse doesn't inhale dust or debris. Higher speeds and more effort ramp up stimulation within the head, each stimuli increasing 'traffic' in the trigeminal area."
Madigan notes two other primary risk factors for headshaking:
- A horse that has gained weight, as, for example, during a layoff;
- A horse that does not regularly perform aerobic exercise.
The author of an 1897 paper said that this is "a disease of overfed and under-worked horses." Madigan feels there must be some chemical set point for the trigeminal nerve. He has not seen headshaking in endurance, polo, or racehorses except in fit athletes that have retired. Changes in hormonal feedback related to being sedentary, such as leptin in fat, might factor into these cases. He says we have assumed that removal of an animal's gonadal tissues might be performed without side effects; however, the previously mentioned ferret study illustrates this is not the case.
For horses with trigeminal nerve- mediated headshaking, Madigan says, "It doesn't matter what gear is on their head. With or without a rider they exhibit the same behavior with stimuli. In some, if you clap your hands, the horse acts like it receives an electric shock to the face, clearly indicating a connection between the auditory (eighth) cranial nerve and the trigeminal (fifth) nerve. If a horse only displays headshaking when wearing a bit or another piece of equipment, then you need to look for some other physical cause."
Other causes include insect irritation, tooth or mouth discomfort, ear or head discomfort, nasal allergies, eye pain or vision problems, or neurologic issues.
An Alternate Theory
Knottenbelt believes there might be a different cause for equine headshaking in U.K. horses. He theorizes that equine herpesvirus (EHV) plays a contributing role. Knottenbelt explains, "While the role of herpesvirus is purely conjectural, it is based on the clinical similarity between equine headshaking and human facial pain diseases caused by herpesvirus, as well as the fact that in horses, the syndrome relates specifically to the trigeminal nerve where EHV often localizes.
"The progressive nature of headshaking without evidence of any external pathology suggests that herpesvirus may affect the brainstem through immune stimulation," he says. His investigations into an immune basis for headshaking are ongoing.
This theory has been met with controversy, as Madigan says, "We've been checking for herpesvirus in the trigeminal nerve in all horses that are euthanized at UC Davis, and so far we've not made the correlation. However, all of us researching headshaking syndrome believe that something is wrong with the trigeminal nerve."
Madigan feels that a thorough history provided by the owner, along with a physical exam and videotape of the horse headshaking, are the most important aspects to consider when diagnosing a horse. It is important to identify or rule out any pathology that might contribute to this behavior. He says, "Neuropathic pain can come and go and can be overridden by other sensory input; one feature of headshaking is that it varies from day to day."
Possible diagnostic tools include head radiographs, nasopharyngeal and guttural pouch endoscopy, and anesthetic nerve blocks of branches of the trigeminal nerve and/or infraorbital nerve. (Madigan does not recommend blocks of the infraorbital nerve, however, as attempting them can be very dangerous to the handler. "If the nerve is hit with a needle the horse can explode in a severe fashion," he notes, adding that "many vets use sedation prior to this block and some horses stop headshaking just due to the short-term sedation, which then alters results even further."). Other noninvasive diagnostic strategies include temporary use of contact lenses, goggles, or eye shades to reduce light, as well as the use of nasal occlusion masks.
Madigan has implemented every possible diagnostic tool, usually with disappointing results. "Dental exams, eye exams, radiography, endoscopy, anesthetic blocking of the cornea, nasal cavity, or infraorbital nerve--none of these tests reveals much, resulting in less than 1% chance of finding an actual lesion.
"Even if you find something and fix it, headshaking is not guaranteed to go away because, in most cases, this is a disorder of the trigeminal ganglia related to chemical imbalance, and it will appear again with an appropriate trigger," he concludes.
Madigan proposes a therapeutic protocol that he has found successful in treating headshaking:
- Measure gonadotropins (LH) and lower their levels by administering daily melatonin (12-16 mg) at 5 p.m. This "tells" the horse's body that it is winter. He reports good results for seasonal headshakers.
- Feed the label dose (4 ounces) of magnesium oxide (Quiessence) to raise the threshold for "firing" of the trigeminal nerve.
- Feed the label dose of spirulina (a homeopathic product) to raise the threshold of for firing of the trigeminal nerve.
Madigan stresses that it may take four to eight weeks until results are appreciated with this combination of therapies.
Once the problem is solved, it is best to continue this treatment indefinitely. In addition, the anti-GnRH vaccine (Equity), licensed in New Zealand, might become available in the United States in the future. Although researchers have not studied its long-term use for headshaking, Madigan expects that an annual booster should be effective in lowering gonadotropin levels.
Nerve Suppression Therapy
Drugs that suppress nerve firing, such as cyproheptadine, carbamazepine, hydroxyzine, and phenylbarbital, have been used effectively in headshakers. Cyproheptadine helps 40% of cases, but there are downsides to its regular use.
Madigan says an owner must consider cyproheptadine's cost, the need for twice-daily administration, that it is not allowed for competition horses, and that it has the potential of causing gas colic.
A treatment for headshaking with a favorable success rate in 40% of cases is a face mask with a nose net. Madigan explains: "The nose net provides a mechanical stimulus, like sticking a finger on your nose, which sends afferent (conducting) impulses up the nerve so it quits firing. Not all horses are improved with nose nets."
"Allergies have been looked at as a cause," Madigan comments, "but there is a lot of indirect evidence to suggest that allergies are not the cause. For example, allergy testing and hyposensitization injections along with antihistamine and/or steroid therapy are strategies that typically control allergies for the short term, yet these approaches still do not control most cases of headshaking."
While allergies are not a common cause of headshaking, nasal allergies or rhinitis could induce headshaking behavior.
Dennis Brooks, DVM, PhD, Dipl. ACVO, a professor of ophthalmology at the University of Florida veterinary school, explains his approach: "Some headshakers suffer from allergies centered in the nasal passages with mast cell release of histamine creating an irritating allergic response. Preventing the breakdown of mast cell membranes by using cromolyn sodium eye drops may reduce headshaking--if no histamine is released, then there's no irritation or headshaking."
Brooks qualifies this by saying, "Not all headshakers suffer from an environmental allergy. When mast cells are not involved, cromolyn drops won't work."
For difficult allergy-affected cases, Brooks suggests the use of a corticosteroid, such as dexamethasone, given systemically in pulsed doses (on several consecutive days each month) as recommended by a veterinarian.
Certain headshaking cases could be surgical candidates. For example, Madigan describes one horse with substantially large corpora nigra (growths of the iris tissue) that stimulated the visual field. Removal of these iris growths with a diode laser along with behavioral modification averted headshaking behavior that occurred during bridling. While this is not a typical case, it does point out other unusual causes that might be considered and could be solved surgically.
Previously it was thought that blocking or cutting the trigeminal nerve could be a relevant treatment. However, Madigan emphasizes, "Trigeminal neurectomy doesn't help since the irritation is not in the branch of the infraorbital nerve, but rather the problem is in the central controlling area when the nerve fires, far higher up in the head. The symptoms can only be blocked with anesthetic injected just behind the eye."
At UC Davis Madigan and colleagues are working on an electrophysiology study in donated horses that haven't responded to any form of treatment. They examine the trigeminal nerves of these horses under general anesthesia in a process approved by animal use protocol. (Anyone wishing to contribute to this study is welcome to send donations to Headshaking Research c/o John Madigan at University of California, Davis, VMTH, One Shields Avenue, Davis, CA, 95616.)
Madigan emphasizes that an owner should realize headshaking comes and goes, and once gone, it's likely to come back. "With a seasonal problem," he says, "if headshaking goes away following the last thing you do, that doesn't mean that strategy corrected it."
That said, he has heard of as many as 20% of cases resolving spontaneously.
Madigan emphasizes the importance of working with your veterinarian to identify if your horse's headshaking behavior is mediated by the trigeminal nerve or is caused by something else. Knowing this means you'll more likely achieve therapeutic results. He stresses, "Season certainly plays a role in headshaking syndrome. It's usually not the owner, bridle, bit, rider, gender, technique, or athletic pursuit. Because this is a medical condition and is not necessarily behavioral, it is inappropriate to administer punishment to a horse with trigeminal-mediated pain."
About the Author
Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.