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Canine Horner's Syndrome

Horner's syndrome is a common disorder of a sudden loss of stimulation of the nerves--of the eyes and area surrounding the eyes.

This syndrome is just a cosmetic issue--it does not cause pain and does not affect vision.


Horner's syndrome is caused when the nervous system gets damaged and stops functioning in the eye, ear, or neck area.

About half of cases are idiopathic (cause is unknown). Other reasons for damage may include:

  • Eye disease
  • Ear infection
  • Trauma (in the head, neck, or chest)
  • Tumor in the brain or chest
  • Bite wounds
  • Blood clot


The affected side of the face will show the following signs suddenly:

  • Eyelids drooping
  • Shrunken pupil
  • The eye is sunken in
  • The third eyelid is protruding
  • Blood vessels constrict, making the area pink and warm to the touch

Sometimes, the dog will also have excessive salivation and difficulty eating on the affected side.


Your veterinarian will do tests to determine the cause of the syndrome, including:

  • A neurologic (brain) exam
  • X-rays: to rule out cancer
  • Blood tests
  • MRI
  • Special eye drops: causes the pupils to widen, showing if the nerve damage is inside or outside the brain.


Treatment is not necessary for horner's syndrome because it usually resolves on its own in a few weeks. However, any underlying diseases must be treated.

There are eye drops available for cosmetic purposes.


The best way to prevent this condition is to avoid trauma, bite wounds, eye disease, and ear infections.


There is a very good prognosis if there is no serious underlying cause.

Additionally, the prognosis is even better if the damaged nerves are outside of the brain and spinal cord.

Medically Reviewed by Sara Ochoa, DVM

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