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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.
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