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Horner’s Syndrome in Dogs — Symptoms, Diagnosis and Treatment

Is your dog’s eye suddenly looking strange? Maybe your dog’s eye is drooping, his third eyelid is protruding or his pupils appear to be different sizes. Could it be Horner’s syndrome? Let’s learn more about Horner’s syndrome in dogs.

Jackie Brown  |  May 21st 2018


A sudden and drastic change to your dog’s face is alarming: a sunken eye, droopy eyelid, exposed third eyelid or a pupil that looks smaller than the other could mean your dog has Horner’s syndrome, a complex neurological disorder that affects the eyes and muscles of your dog’s face. Let’s learn more about the symptoms, diagnosis and treatment for Horner’s Syndrome in dogs.

What is Horner’s syndrome?

Dog eye closeup. Photography ©Vulkanov | iStock / Getty Images Plus.

It all starts with the nervous system. “The nervous system has a sympathetic and a parasympathetic component,” explains Michelle Murray, DVM, MS, Dipl. ACVIM (Neurology), CCRT, owner of NEST Veterinary Neurology in San Clemente, California. “The sympathetic component takes over when an animal is scared, frightened or anxious (the ‘flight or fight response’) and the parasympathetic component takes over during times of rest, relaxation and sleep.”

With Horner’s syndrome, damage to the sympathetic nervous system causes changes to the appearance of a dog’s eye and face. “When the sympathetic pathway to the eye is working properly, when stimulated, the muscles behind the eyeball will cause the eye to be forward in the socket, the eyelids open wide and the pupil dilated,” Dr. Murray says. “The sympathetic pathway can be disrupted if there is an injury or disease disrupting the nerves anywhere along this pathway, including the brainstem, spinal cord anywhere down to the third thoracic vertebra, a disease outside the vertebral column in the neck, middle ear disease, or disease in the tissues behind the eye.”

Symptoms of Horner’s syndrome in dogs

With Horner’s syndrome in dogs, you might notice a few distinctive signs. Usually, all of these signs occur on the same side of the body.

  1. A slightly drooping upper eyelid, which causes the eye on that side to appear slightly smaller than the other eye.
  2. An eyeball that looks like it’s slightly sunken in the eye socket.
  3. A protruding third eyelid that partially covers the side of the eye that’s closest to the nose.
  4. One pupil that appears to be smaller than the pupil in the other eye and doesn’t dilate completely in dim/dark lighting.
  5. Rarely, the affected side might be warmer to the touch, and the skin might look “pinker” when compared to the unaffected side.

How is Horner’s syndrome in dogs diagnosed?

“A diagnosis is usually made in the exam room, based on the findings discussed above,” Dr. Murray explains. Sometimes, a dog might only have a few symptoms, so the veterinarian might need to confirm the diagnosis by trying to dilate the pupil with special eye drops. This cannot only determine if Horner’s syndrome is present, it can also help the vet pinpoint what might be causing it.

“Some dogs have other symptoms along with the Horner’s syndrome, which helps us with diagnosis,” Dr. Murray says. “For example, a dog with severe middle ear disease may have other symptoms (shaking head, scratching and discharge) along with the Horner’s syndrome.”

Similarly, a dog with a spinal injury who is exhibiting Horner’s syndrome symptoms might have other neurologic signs like severe pain, visible wounds, internal injuries or paralysis. A dog with a brain tumor might exhibit other neurologic signs that can help the vet narrow down what’s causing the problem.

What is the prognosis of Horner’s syndrome in dogs?

The prognosis for Horner’s syndrome in dogs is entirely dependent on the cause. “There is no specific treatment for Horner’s syndrome, as it is only a collection of symptoms, not an actual disease itself,” Dr. Murray explains. “If the underlying cause is treatable, then the prognosis is good. If the underlying cause is a severe neurologic injury, cancer or other serious disease, then the prognosis may be poor.”

Thumbnail: Photography ©Leppert | Getty Images.

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