Horner's syndrome in dogs

Written by Dr Andrew Miller MRCVSDr Andrew Miller MRCVS is an expert veterinary working in the field for over 10 years after graduating from Bristol University. Andy fact checks and writes for Pure Pet Food while also working as a full time veterinarian. Pure Pet FoodPure Pet Food are the experts in healthy dog food and healthy dogs featured in media outlets such as BBC, Good Housekeeping and The Telegraph. Working with high profile veterinary professionals and nutritionists, Pure Pet Food are changing dog food for the better. - Our editorial process

Horner’s syndrome is a common, but complicated neurological disorder that impacts the eye and facial muscles. It affects many species of animal, including dogs, cats, horses and more. With the disorder, you’ll notice that your dog’s pupil looks shrunken and their eyelids will droop, and although this typically only impacts one of the eyes, it can occasionally be bilateral, meaning that it affects both eyes.

Let’s take a look into Horner’s syndrome and get a grasp of this complex disorder, understand how the neurological system operates, how the disorder comes to be and how you and your dog can manage the issue effectively.

What is Horner’s syndrome in dogs?

To put it simply, Horner’s syndrome is when the nerves that automatically control and manage parts of the eye and nearby facial muscles are disrupted in some way. As a result, this has an impact on how the eye looks and operates.

However, to totally understand Horner’s syndrome and how it will impact your pooch, it’s essential to try and gain an understanding of how the neurological system works so we can comprehend how neurological problems can disrupt the proper functioning of the eye.

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Understanding the neurological system

In both humans and hounds, the nervous system operates in relatively similar ways, both having two central divisions that break them apart.

One section of the system is the one that you’re conscious of and have voluntary control over (somatic system) and then there’s the other half that you have no control over, it just operates automatically, called the autonomic system.

So, an example of how this operates could be when you’re feeling cold, your somatic system will consciously try and combat the cold by instigating the movements that you know will warm you up, such as reaching for and putting on your coat.

However, the autonomic system works to help fight the cold through automatic bodily responses that you don’t consciously initiate, such as shivering and getting goosebumps. You don’t even have to think about it, it just happens involuntarily, similarly, you don’t need to consciously think to make yourself blink or get your heart to beat.

Both yours and your dog’s autonomic nervous system also has two separate components, the parasympathetic and the sympathetic nervous system which operate simultaneously with a fine balance.

The parasympathetic system controls your dog’s ordinary state and maintains routine, whereas the sympathetic system is in control of your dog’s fight or flight responses, such as increasing heart rate and dilating the pupils. As a result, the sympathetic responses are in control when your dog is experiencing a sense of heightened emotion, like being stressed or anxious, while the parasympathetic controls the body’s responses to being in a relaxed or even sleeping state.

Usually, these two systems have a happy balance, but when an external trigger disrupts the sympathetic innervation to the eye, there will be an excess amount of parasympathetic impulses. As a result, the eye’s normal functioning is disrupted due to the imbalance, causing Horner’s syndrome.

What causes Horner’s syndrome in dogs?

So, as we know, Horner’s syndrome is triggered due to some disturbance to the normal operations of the sympathetic nerve, but what could these disturbances be?

The sympathetic nerve has quite a journey before it reaches the eye, travelling down the spinal cord from the brain, springing up in the chest, and then migrating up the neck through to the middle ear and finally meets the eye.

Due to its lengthy course through the body, there are a few things along the way that could pose a problem and disrupt the sympathetic nerve’s usual process. For example:

  • Bite wound

  • Trauma to the nerves

  • Brain, chest and other types of tumour

  • Intervertebral disc disease

  • Middle or inner ear infections (otitis)

  • Spinal cord injuries

However, a lot of the time Horner’s syndrome is classed as being idiopathic. This is the medical term for when a disease has no identifiable cause, it seems to have just happened out of nowhere.

Other causes for an elevated or protruding eyelid gland include tetanus, facial nerve paralysis, facial muscle atrophy and dehydration.

What are the signs and symptoms of Horner’s syndrome in dogs?

The signs and symptoms of Horner’s syndrome can sometimes present similarly to uveitis, which is another common canine eye disorder.

The main symptoms to look out for are:

  • The upper eyelid starting to droop (ptosis)

  • Constricted, shrunken pupils (miosis)

  • Sunken eyes (enophthalmos)

  • Impaired vision

  • Visible third eyelid that looks red and raised

Did you know that dogs actually have a third eyelid? Well, it usually wouldn’t be visible to us, so if you see it, this could be a big tell-tale sign of Horner’s syndrome.

Although the symptoms of the disorder are the most apparent in the eye, you might also notice that your dog’s face looks sunken on the affected side, alongside excessive drooling and they might even find it difficult to eat on that side.

Are any dog breeds more prone to Horner’s syndrome?

Many vets theorise that there are some dog breeds that seem to have a higher chance of developing Horner’s syndrome, such as Golden Retrievers, Collies and Cocker Spaniels.

However, one study reports that their results were inconclusive on breed disposition, stating that there didn’t seem to be any specific breed, age or size of dog that seemed more likely to develop Horner’s syndrome.

How is Horner’s syndrome in dogs diagnosed?

The symptoms of Horner’s syndrome will probably be quite obvious to your vet, so diagnosis of the disorder can usually be totally done based on a full eye examination.

Sometimes, phenylephrine eyedrops might be dropped into the affected eye so your vet can monitor how the pupils respond and ideally identify the location of the problem. The most important part of diagnosis is identifying what, if anything, caused the eye disorder in the first place.

So, numerous diagnostic tests will be carried out to detect the underlying cause, such as ear examinations, X-rays and potentially MRI or CT scans to receive advanced images. However, more often than not, the results will come back with nothing, suggesting that the reason behind why Horner’s syndrome developed is unidentifiable (idiopathic).

How is Horner’s syndrome in dogs treated?

Horner’s syndrome doesn’t really require any treatment, it will usually clear up on its own if it’s idiopathic. However, this recovery is slow, possibly taking anywhere from a few weeks to around 4 months to totally heal.

Although, if Horner’s syndrome has been triggered by another underlying issue, such as a middle ear infection, then this problem will need to be treated however the vet sees fit before Horner’s syndrome will begin to resolve itself.

What is the outlook for dogs with Horner’s syndrome?

Luckily, the outlook is excellent for dogs suffering from Horner’s syndrome if there’s no underlying cause present, how the disease presents itself is probably far worse than what it actually is. Just remember that it might take a fair bit of time to fully resolve depending on how severe the condition is.


All in all, if your dog is suffering with Horner’s syndrome, try not to worry too much and your dog will be back to normal before you know it.

  1. Horner's syndrome in dogs and cats: 100 cases (1975-1985) Journal of the American Veterinary Association, 195, (3), Aug 1989, 369-373
  2. A review of Horner’s syndrome in small animals The Canadian Veterinary Journal, 60, (1), Jan 2019, 81-88, PMID:30651655