Horner’s Syndrome


A syndrome is defined as either a) a group of symptoms that consistently occur together or b) a condition characterized by a set of associated symptoms. In the case of the commonly observed Horner’s Syndrome, either definition works.

Dogs and cats who experience this very specific cluster of signs will display the following five signs:

  • One constricted pupil

  • Elevation of this eye’s third eyelid

  • Retraction of this eyeball into the head

  • Slight drooping of the affected eye’s lid

  • Slightly pinker color and increased warmth of the ear and nose on the affected side

The causes of Horner’s Syndrome are many and various, but they can all be traced to damage sustained by the nerves that innervate the eye on the affected side of the head. In the case of Horner’s Syndrome, the affected nerves are always part of the autonomic nervous system. In other words, these are nerves that govern the body’s automatic functions. They are not part of the nervous system that helps manage sight or conscious movement of the eyes or lids.

The damage to this particular section of the nervous system occurs commonly perhaps because it’s made up of very long nerve fibers that pass through the neck and take a long, arcing course through the chest before returning to the middle ear and only then coursing towards the eye.

The cause of nerve damage that leads to Horner’s Syndrome may be the result of an insult to the nerve as it passes through any of the aforementioned anatomical locales. Trauma, a blood clot (infarction), tumors, infection, or diseases at the end of the nerve’s path –– at the eye itself –– can all lead to this cluster of findings.

Some specific causes of Horner’s Syndrome include Intervertebral Disk Disease, nerve trauma sustained after straining against a leash, middle ear infection, and damage to the nerves in the underarm area (typically after an injury to the limb has led to stretching of the nerve bundles there).

Theoretically, Horner’s Syndrome may also be classified as idiopathic, meaning that it does not have a discernible cause.

Symptoms and Identification

The five signs listed above will typically be in evidence, though to varying degrees. In fact, even when present, some may be more difficult to discern than others. For example, the pinker color and increased warmth of the face on the affected size can be difficult to appreciate. Of these signs, uneven pupil size is the sign most pet owners are likely to observe –– at least initially.

Technically, Horner’s Syndrome is not a diagnosis, per se. As such, it’s identified only after the above findings are observed in conjunction.

Discerning the cause of the Horner’s Syndrome is typically undertaken by a) attempting to localize the exact section of the nervous system affected and then b) honing in on that area with imaging techniques (X-rays, CT scans, MRI) that will elucidate the area of nerve damage, if possible.

Affected Breeds

Any breed of dog or cat may be diagnosed with Horner’s Syndrome. However, some of the causes of the kind of nerve damage that can lead to Horner’s Syndrome are more specific to certain breeds. For example, certain breeds are predisposed to Intervertebral Disk Disease.


Since it’s not considered uncomfortable in and of itself, this condition tends not to require treatment. The underlying cause of the nerve damage, however, may or may not require intervention.

Veterinary Cost

The cost of veterinary care for Horner’s Syndrome depends on a) the cost of basic diagnostics and b) whether the animal is perceived to be suffering from an underlying condition that requires definitive diagnosis and treatment.

Diagnosis of Horner’s Syndrome runs between $50 and $5,000. The cost disparity depends on whether or not a specialist is elected and on the degree to which the exact lesion is localized. In cases for which no severe, progressive or quality of life-affecting disease is perceived to be in play, expensive imaging is unlikely to be warranted and the entire cost of diagnosis and care may prove significantly lower than $250.


Horner’s Syndrome is not considered a preventable condition. However, limiting the possibility of trauma to this area and treating skin diseases before they lead to middle ear infections are possible routes of prevention, nonetheless.


Carpenter JL, King NW Jr, Abrams KL. Bilateral trigeminal nerve paralysis and Horner's syndrome associated with myelomonocytic neoplasia in a dog. Journal of the American Veterinary Medical Association [1987, 191(12):1594-1596]

Kern TJ, Aromando MC, Erb HN. Horner's syndrome in dogs and cats: 100 cases (1975-1985). J Am Vet Med Assoc. 1989 Aug 1;195(3):369-73.

Morgan RV, Zanotti SW. Horner's syndrome in dogs and cats: 49 cases (1980-1986). J Am Vet Med Assoc. 1989 Apr 15;194(8):1096-9.