New Bolton Lecture: Headshaking Syndrome in Horses
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New Bolton Lecture: Headshaking Syndrome in Horses
February 2013

Headshaking syndrome in horses was the subject of a presentation at Penn Vet New Bolton Center on November 6, by Joy Tomlinson, DVM and Large Animal Internal Medicine Resident. The lecture was part of the First Tuesday Lecture series at New Bolton Center, offering the public open lectures on equine topics, at no charge, the first Tuesday of each month.

What’s shaking?
Idiopathic headshaking syndrome is defined as a horse that shakes or jerks its head uncontrollably and without any apparent external stimulus. This is generally a vertical shake or flip of the head, but can also be horizontal or rotary. Headshaking is usually an adult-onset disease, starting at around seven to nine years old. Geldings are most commonly affected, and horses that are over-conditioned seem to be at higher risk. Horses can be seasonal headshakers or shake all year round, and some will progress from seasonal to year-round. Common triggers for headshaking are bright light, heat, spring/summer season, and exercise. The severity of headshaking is extremely variable between horses and may progress over time.

Why is he doing that?
There is a lot to be learned about what exactly causes headshaking, but what we understand so far is that there are two types of headshakers recognized: photic headshakers, and trigeminal headshakers. Photic headshakers are horses that have light as a trigger factor. These horses may be seen seeking shade and improve immediately when blindfolded. This reaction is thought to be like the photic sneeze in people, where light signals to the eye are transmitted to the facial sensation nerves and cause people to sneeze or horses to shake their heads. Trigeminal headshakers are thought to have a facial pain syndrome similar to trigeminal neuralgia in people. In this syndrome, the trigeminal nerve (the nerve supplying all the sensation to the face) is too reactive. Normal sensations such as cold, touch, or wind are felt as severe pain.  Many theories have been proposed as to what causes the trigeminal nerve to go haywire in horses, such as equine herpes virus infection, reproductive hormone changes, or trauma. However, none of these has been confirmed when investigated.

At this time, there is no single test to diagnose a horse with headshaking syndrome. Because there can be many reasons why a horse might shake its head, veterinarians often recommend a battery of diagnostic tests to rule out other conditions before declaring a horse a headshaker. This might include blood work; a thorough physical examination focusing on the head, neck, and possible sources of pain; endoscopy of the upper airways and guttural pouches; radiographs of the skull; and occasionally otoscopy (looking in the ear). These tests are important to rule out potentially treatable diseases before concluding that a horse is a headshaker.

Minimizing shaking
There is no cure for headshaking. Treatment is aimed at minimizing triggers and reducing the horse’s pain response. If the horse is a photic headshaker, management changes such as stabling during the day with turnout at night and riding in an indoor arena might help minimize signs. A facemask that has a UV light filter may also be helpful. For trigeminal headshakers, a nose net (such as a fly mask with nose extension, panty hose covering the nose, or a specialized nose net for headshakers) may reduce signs and can be used during many competitions. For seasonal headshakers, supplementing with melatonin may trick the body into responding as if it were winter and may decrease headshaking. To have a chance to be effective, melatonin must be administered at 5 pm every day, year round. Magnesium is another supplement that helps some headshakers. To be effective, it sometimes needs to be given at higher doses than recommended by the manufacturer. Overdosage can cause death; therefore any use above the labeled recommendation should be under the guidance of a veterinarian with routine blood level monitoring.

If none of the above treatments help, medication may be recommended. Current medications used for head shaking in horses are cyproheptadine and carbamazepine. These drugs both act in the brain and can have side effects of lethargy and, occasionally, colic. They can be used alone or in combination and have variable success. Some horses respond initially, but will then become refractory to treatment. Neither medication can be used in competitions and, unfortunately, neither has a lasting effect, so withdrawing the medication right before a show generally results in the clinical signs of headshaking returning during the competition.

As a last treatment option, a couple of surgeries have been developed. All current surgical treatments have the potential to make a horse worse to the point of necessitating euthanasia. Therefore, surgical treatment is reserved as a last resort in most cases.

Headshaking in horses is a frustrating disease for horses, owners, and veterinarians. Take the time to work with your veterinarian to rule out treatable causes of head shaking before identifying your horse as an idiopathic headshaker. When starting management for your headshaker, start treatments one at a time to see what works for your horse.  Be careful when you evaluate remedies marketed on the internet; anecdotal reports of success are frequently confounded by other medications that a horse might receive. The seasonality of the disease may also make it appear as though the treatment caused the signs to resolve.