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.
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.
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:
This form of photic sneezing in horses was found to be the cause of headshaking in 60 percent of horses in one study from California, but rarely the cause in a large study in Liverpool, England. So there may be regional differences in the cause of headshaking.
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.
The beneficial effects of these methods clearly supports the role of the trigeminal nerve to these areas in headshaking. This notion is further supported by the observation that headshaking behavior can often be eliminated temporarily by anesthetizing or destroying the trigeminal nerves of affected horses.
Other Possible Causes
Headshaking has been attributed to numerous other disease processes (either positively identified or generally advocated) in horses. On careful observation of the affected horse's headshaking behavior, however, the characteristic and complete clinical expression of the headshaking syndrome is rarely so evident in these other diseases. Other disease processes that sometimes include a headshaking behavior component include:
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.
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.
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.
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.
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.