Headshaking

What is Headshaking?

Headshaking is a repetitious behavior that appears unexpectedly in adult life between 4 and 17 years of age. The nature of the headshaking varies, but generally it appears to be an uncontrollable urge to shake or toss the head up and down, left to right, or in circles. The neck is included in the movement in many cases, and the horse is difficult to ride.

There is no age, sex, breed or functional predisposition to the headshaking syndrome in horses. Headshaking behavior to a greater or lesser extent is dependent on various triggering factors, so the behavior may not be apparent in a veterinarian's clinic because the appropriate trigger factors may not be present. The behavior might also be suppressed by the psychological factors attributable to the unfamiliar environment.

Symptoms

Some affected horses manifest headshaking at rest and others during exercise. The problem is exacerbated during exercise and thus renders some horses useless for riding. Interestingly, there is little mention of headshaking in the old veterinary literature, implying that the condition is more prevalent now. There really isn't any consistent pattern in how the problem begins, but in some cases the onset coincides with allergy season. For these horses, headshaking can become progressively worse, and extend into the "off" seasons. In others, headshaking arises spontaneously with no apparent change of season, ownership, barn, country, or discipline.

In horses with a seasonal affliction there may be symptoms suggestive of an allergy: nose rubbing, a watery ocular and nasal discharge, low head carriage, sneezing, snorting, head pressing, pressing the muzzle into the ground. This behavior is so dramatic you'll think your horse is possessed. Some horses get so irritated or frustrated, that they will actually endangering the rider.

In addition to the effects of seasonal change, headshaking has also been reported to worsen in bright sunlight, on warm and humid days or when the wind blows into the affected horse's face. Some affected horses will actively seek out the shade. In a few, the clinical signs appear to affect one side of the horse's head to a greater extent than the other.

The headshaker appears to be having a reaction to an unusually irritating stimulus in the nasal passages. In some instances, affected horses act as if an insect has flown up their nose, although there is none present. Horses affected with mild clinical signs of headshaking may simply exhibit mild, periodic head movements and low-grade twitching of muscles in the face, but he may be rideable. Moderately affected horses may be rideable with some difficulty under special circumstances (cooler weather, protected from bright sunlight, and on non-windy days). More severely affected horses are difficult to control, uncontrollable or unrideable. Very severely affected horses exhibit dangerous fitful head tossing and cannot be ridden; consideration of euthanasia in these cases is not uncommon.

Causes of Headshaking Syndrome

Headshaking is only a symptom, and there can be many different causes. In most cases, a definite cause cannot be found, even with an extensive workup.
It appears to be similar to trigeminal neuralgia, a human condition in which the trigeminal nerves become painful. Both trigeminal nerves (left and right) are large, extensive nerves that are responsible for most of the physical sensation of touch for the whole head. In a recent study, some horses were found to respond to surgical scarring or local anesthesia of a major branch of the trigeminal nerve. This suggests that the nerve is indeed part of an important pathway that transmits the irritating sensation to the horse's brain, which responds by shaking or spasming the head. The reason for trigeminal nerve pain in horses and humans is unknown.

The manner by which the trigeminal nerves are diseased in horses affected with the headshaking is unknown. In human patients, trigeminal neuralgia is usually attributed to a neurovascular compressive lesion close to the brain that might be relieved by decompressive surgical treatment. Although MRI is not widely available for horses, human neurovascular compression of the trigeminal nerves is identified using this imaging modality.

Triggers

Certainly, the most convincing theories at the present time support the idea that the headshaking behavior results from trigeminal neuralgia. It appears that once trigeminal neuralgia has set in, the condition can be clinically silent until "activated" by one or more of several "trigger" factors. The recognized trigger factors, which are similar in horses and human beings, include:

• Sunlight. In much the same manner that bright sunlight triggers sneezing in people (photic sneezing), it has been suggested that bright sunlight is also able to trigger trigeminal pain in horses affected with headshaking syndrome. This feature is evident when affected horses are moved from bright sunlight into a dark stable or when the eyes of affected horses are protected from the bright sunlight using either tinted contact lenses or by being covered with a towel. In many (but not all) cases, the headshaking behavior is completely obviated by blocking the sunlight.
• Air movement. Strategies aimed at inhibiting air flow movements within the nasal passages usually reduces the incidence of headshaking. The two strategies used to test this theory in individual horses include:

The use of a facemask. Covering the nose and mouth with a facemask that allows the horse to breathe through the nose seems to reduce the incidence of headshaking. The mask simply prevents wind on the nose.

Tracheostomy tube. This method is considerable more invasive, and involves the surgical placement of a tracheostomy tube, which is a bypass device that allows the horse to breathe directly through the windpipe, so little if any air goes in and out of the nasal passages. Tracheostomy is a very effective method for controlling headshaking in horses bothered by air which brushes on the inner surface of the nasal passages.

• Allergic reactions. It has been suggested that an immune-mediated or allergic reaction to certain dusts and pollen in the nasal cavities may also act to trigger the behavioral signs of headshaking.
• Inflammation of the middle and inner ear
• Ear mites
• Dental pain
• Allergic rhinitis associated with summer pasture associated heaves
• Protozoal myeloencephalitis (EPM)
• Injuries to the neck and head
• Disease of the guttural pouches
• Jaw tumors
• Sinusitis
• Brain disease
• Problems with the eyes
• Nasal fold collapse
• Vices

When presented with a new case of headshaking behavior, your veterinarian will endeavor to rule out these other diseases using special diagnostic procedures including endoscopy and radiography. If a specific disease cannot be identified, the headshaking behavior is generally attributed to trigeminal neuralgia by process of elimination.

Diagnosis

The headshaking syndrome is largely diagnosed based on observation of the abnormal behavior and reviewing the horse's medical background. Other possible causes must be considered and ruled out using appropriate diagnostic tests. However, the headshaking syndrome, an irritation of the trigeminal nerve branches, appears to be the most common cause of headshaking in horses in North America and Western Europe.

Treatment

Unfortunately, many affected horses fail to respond to any and all of the available treatments and there is not a single treatment option that is universally effective.

• Medication. Some drugs that may reduce the effects of trigeminal neuralgia, such as cyproheptadine and carbamazepine, have been used to good effect in some horses affected with headshaking.
• Eye shading. Specially developed tinted contact lenses and eye shades (sun glasses) have been useful in some of those horses in which bright light appears to aggravate the headshaking behavior.
• Scarring of the trigeminal nerve. If anesthesia of a part of the trigeminal nerve is shown to ameliorate headshaking behavior, some veterinarians have elected to either cut or permanently scar (injection if a chemical sclerosing agent) those parts of the trigeminal nerve, although these procedures have not been universally successful. Scarring of the deeper portions of the trigeminal nerve must be performed under general anesthesia.

Surgical cutting of the trigeminal nerve should not be undertaken lightly; a common complication (apart from the possibility that the procedure may fail to stop the headshaking problem) is development of a new abnormal nerve sensation (neuroma) at the site of the surgery. Trigeminal neuroma formation may, in some cases, lead to abnormal facial sensation and pain when the horse's head is handled. The last thing you want is a horse that is worse, and possibly disfigured.

• Facial mask. Being fitted with a special mask (nasal mask) that either reduces the intake of dust or attenuates the airflow through the nasal passages helps some affected horses. You can even try something as simple as a nylon stocking at first as a test, although this obviously does not block the wind completely.
• Tracheostomy. A treatment of relatively last resort is the permanent tracheostomy; many severely affected horses can be managed by providing the alternative airway and abrogating the triggering effect of airflow in the nasal passages.
• Acupuncture. There are many claims that acupuncture reduces the signs of headshaking but there have been no controlled studies in horses or humans with the similar disease to date.
• Euthanasia. Euthanasia is elected for some of the severely affected horses for which attempted treatment options have failed. As a testament to the mental anguish with which horses affected with the headshaking problem are subjected, a common consequence of trigeminal neuralgia in human patients is suicide.

Other Consequences

In many cases, the affected horses are sold during the early stages of the disease while the headshaking behavior is episodic and the horse might be seen to be unaffected. In some cases, insurance companies are asked to pay for the cost of "loss of use" (depending on the insurance policy.) A number of horses affected with severe signs of headshaking are euthanatized.