Equine Internal Medicine
Headshaking is a common behavior in horses characterized by head tossing in the absence of obvious stimulation. Some horses will headshake with such violence that they are dangerous to the handler and rider. Prior to the 1990's, the cause of headshaking was rarely identified and the condition was largely unresponsive to therapy. In 1995, it was noted that headshaking was triggered by natural sunlight and darkness provided relief from the condition (photic headshaking). Horses with photic headshaking are suspected to experience a burning sensation or tingling of the muzzle (neuropathic pain) in response to bright sunlight. The mechanism may be similar to photic sneezing in humans, in which staring into sunlight triggers sneezing episodes. Photic sneezing in humans is a heritable, non-allergic disorder. Some veterinarians suspect inflammation of the trigeminal nerve due to latent viral infection (rhinopneumonitis) may contribute to irritability of the infraorbital nerve, a branch of the trigeminal nerve. Bright sunlight is the most common trigger for headshakers, but other stimuli, including specific feeds (gustatory headshaking), may also serve as a trigger. In some horses, the triggering stimulus is not sunlight and cannot be identified. However, their headshaking behavior appears characteristic for neuropathic pain and they respond to appropriate medical therapy. Many other disorders besides neuropathic pain may induce headshaking behavior in horses (dental disease, ear infection, foreign body). A thorough diagnostic evaluation is indicated to eliminate other etiologies of headshaking behavior.
Headshakers demonstrate sudden, violent jerking movements of the head in the absence of obvious external stimuli. Quick vertical flips or jerking movements (as if stung by a bee on the end of the nose) are characteristic, but some horses may intersperse horizontal and rotary activity. Affected horses often snort, sneeze, and rub their nose. Horses that demonstrate headshaking behavior due to dental disease or ear infections usually perform intentional, head tossing behavior, rather than rapid, bee-sting vertical flips characteristic of photic headshaking. Photo-induced horses attempt to avoid direct sunlight by seeking shade or hiding their heads in unusual places. Clinical signs are often seasonal, abating during winter months and returning in spring. There is no breed or gender predilection, and the condition affects adult horses. The behavior may be exhibited at rest or during exercise. Violent headshaking may render a horse unusable for riding. Some horses will cease headshaking with a change in rider or environment, failing to exhibit the behavior on the day of examination. Therefore, the owner should obtain a videotape of the headshaking behavior prior to their appointment.
There are numerous differential diagnoses for headshaking behavior. Inner ear infection, dental disease, and foreign body may also produce headshaking behavior. Endoscopic examination of the upper respiratory tract (including guttural pouches) and radiographic examination of the skull may be performed in horses demonstrating headshaking behavior to rule-out other causes of this behavior.
The response of photic headshakers to medical therapy is variable. Favorable, transient, and non-response to therapy are commonly reported outcomes of medical management. If sunlight has been identified to be the stimulus for infraorbital pain, reduced sunlight exposure by providing shelter or a mask is indicated. The following drugs may be prescribed along or in combination for horses with classic headshaking: cyproheptidine, carbamazepine, melatonin, and topical anesthetic cream. The ideal combination of drug therapy is tailored to the individual horse, based on trial and error response to therapy. Surgical intervention should be considered for horses that cannot be controlled with medical therapy. Resection or chemical destruction of the infraorbital nerve is a salvage procedure for refractory cases.