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Horner’s Syndrome

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Horner’s Syndrome involves a number of symptoms that affect your pet’s eyes. The result of damage to the sympathetic nervous system, Horner’s Syndrome is more specifically related to damage to nerves that lead to the eye. Because this nerve path is quite long, damage can occur anywhere along the route between the spinal cord and the eye. The sympathetic nervous system is the part of the autonomic nervous system that is responsible for the “fight or flight” response in animals and humans.

Symptoms and Diagnosis

There are 5 main symptoms of Horner’s Syndrome and they are all related to eye function. Typically, an animal with Horner’s Syndrome will see the effects in only one eye. The symptoms of Horner’s syndrome are as follows: constricted pupil, elevation of the third eyelid, the eye is retracted into the head, some droopiness of the eyelid, and an increase in the pink color of the ear and nose on the side that has been affected.

Diagnosing Horner’s Syndrome relies on an identification of these symptoms. If more than one of the symptoms is present a veterinarian is likely to diagnose your pet with Horner’s Syndrome. In addition, your vet will probably want to identify which area of the sympathetic nervous system has been affected. Sometimes your vet will use special eye drops to stimulate different sections of the sympathetic nervous system in order to identify the location of the lesion. Damage usually occurs in either the first or second nerve segment. With damage to the second nerve segment the syndrome can usually resolve itself. If however the damage occurs to the first segment of the nerve then the chest or spinal cord is usually implicated and the syndrome is much more serious. Chest radiographs are often ordered so that other diseases, such as cancer, can be ruled out.


Treating Horner’s syndrome is not really necessary. This syndrome does not cause any pain and will not actually interfere with your pet’s vision. The main issue with Horner’s Syndrome is that it makes us aware that some kind of nerve damage has been sustained and may be associated with more serious problems. Nonetheless, if your veterinarian has determined that there are no serious underlying issues he/she will likely not insist on any particular course of treatment. However, many pet owners might be concerned about the cosmetic implications of Horner’s Syndrome. Your pet’s eye may be droopy or retracted and this may not appear very attractive. The most common treatment for the cosmetic symptoms of Horner’s Syndrome is phenylephrine. Phenylephrine is an eye drop that can be prescribed by your veterinarian to treat the clinical symptoms of this syndrome. These drops are safe and painless so constitute a popular treatment method for Horner’s Syndrome.

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