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.
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.
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.
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:
Inflammation of the middle and inner ear
Allergic rhinitis associated with summer pasture associated heaves
Protozoal myeloencephalitis (EPM)
Injuries to the neck and head
Disease of the guttural pouches
Problems with the eyes
Nasal fold collapse
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.