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Horner's Syndrome in Dogs: Causes & Treatments

Horner's syndrome is a neurological disorder that can affect your dog's eye and facial muscles. Here, our Renton veterinarians discuss potential causes and symptoms, diagnosis, and more.

What is Horner's Syndrome?

Some people wonder whether Horner's syndrome is classified as a disorder or a disease in dogs. It's officially known as a common neurological disorder that can affect a dog's eye and facial muscles and involves dysfunction in the sympathetic and parasympathetic nervous systems that make up your dog's autonomic nervous system. 

What are the Symptoms of Horner's Syndrome?

Horner's syndrome in dogs is an eye condition characterized by a collection of symptoms rather than an official diagnosis. Damage to your dog's sympathetic nervous system, whose nerves supply the eye and facial muscles on the affected side, can result in symptoms such as: 

  • Constricted pupil 
  • Drooping eyelid 
  • Sunken eyeball 
  • Elevated third eyelid 
  • Eyeball retraction (i.e. the eye appears shrunken) 
  • Impairment of vision 

Having a constricted pupil, a droopy upper eyelid and a sunken eyeball (which can cause the third eyelid to become elevated) confirms Horner's syndrome. 

Horner's syndrome can come on suddenly and without warning. In some cases, you may notice ocular symptoms in addition to excessive salivation and/or difficulty eating on the affected side. 

This disorder typically only affects one eye; it's extremely rare for dogs to experience the disorder in both eyes. If this were the case, brainstem diseases would likely be the underlying cause. 

What is the Sympathetic Nervous System?

Your dog's nervous system is composed of different components, including the somatic nervous system that sends signals to activate movement in skin and muscles and is involved in conscious activity. 

The autonomic nervous system controls bodily functions not continuously directed, such as pupil dilation and constriction, blinking, muscle tone, heart and respiratory rates, and blood circulation. This system is composed of two nerve sets, including: 

  • Parasympathetic Nervous System - This system maintains the body during a normal state. In the eye area, this system retracts the eye for protection, constricts the pupil, and raises the third eyelid. 
  • Sympathetic Nervous System - This system prepares the body for fight or flight. In the eye area, the sympathetic nervous system dilates the pupil, drops the third eyelid, widens the eyelids, and prevents the eye from moving from its forward position in the socket. 

The sympathetic and parasympathetic nervous systems usually work together to maintain balance in your dog's body, with one system slightly dominating the other depending on the circumstances. 

What Causes Horner's Syndrome in Dogs?

Sometimes the sympathetic nerves supplying the eye area can become damaged due to tumors, trauma, diseases such as cancer or intervertebral disc disease, middle or inner ear infections, lesions, or other causes, leaving only the parasympathetic nerves at full functioning.

This can result in the ocular symptoms we know as Horner's Syndrome. However, this syndrome is often classified as idiopathic, which means it has no known cause. 

Which breeds and age groups are most often affected by Horner's Syndrome?

While any dog can develop Horner's syndrome, Golden retrievers, Labrador retrievers,, Shetland sheepdogs, Weimaraners, Doberman pinschers, and and Collies have a somewhat higher incidence. The condition happens most often in dogs between five and eight years old. 

How is Horner's Syndrome Diagnosed?

If you notice any symptoms of Horner's syndrome in your dog, we recommend contacting your primary care veterinarian immediately, so they can conduct a thorough physical examination. 

A veterinarian may perform several diagnostic tests to determine whether an underlying cause may be triggering symptoms, including:

  • Ear and eye exams 
  • Blood work
  • Radiographs (X-rays) of the chest and skull 
  • Advanced imaging such as MRIs or CT scans 
  • Pharmacologic tests such as phenylephrine drops placed in the affected eye to help localize the source of the issue

Your vet may also refer you to our board-certified veterinary ophthalmologist at Northwest Animal Eye Specialists for Horner's syndrome diagnosis and treatment. 

We partner with your primary care vet to provide the most comprehensive and compassionate eye care possible for your pet. This includes diagnosing and treating virtually any eye disease or condition. 

How is Horner's Syndrome Treated?

There is no definitive treatment for Horner's syndrome in dogs and many cases resolve spontaneously. However, your veterinarian may recommend options to address the underlying cause and its prognosis. 

What are Recovery Rates and Prognosis for Dogs with Horner's Syndrome?

If no pathological cause is discovered, the prognosis is typically very good, although some pets only recover partially. Though Horner's syndrome tends to resolve by itself, this may take anywhere from several weeks to four months depending on the severity of the condition. It's rare for Horner's syndrome to reoccur. 

Note: The advice provided in this post is intended for informational purposes and does not constitute medical or behavioral advice regarding pets. Please make an appointment with your vet for an accurate diagnosis of your pet's condition.

Have you noticed concerning symptoms of an eye disease or condition in your dog? Contact our Renton vets to book an appointment with our veterinary ophthalmologist.

New Patients Welcome By Referral

Northwest Animal Eye Specialists is accepting new patients! Our experienced vets are passionate about the eye health of animals. Talk to your vet today about getting a referral to our Renton or Kirkland clinics.

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