Horner’s syndrome is a collection of symptoms involving the eye and the face on one side. It often comes on suddenly; so it can be a bit alarming to see if you don’t know what’s happening! Let’s take a look at what Horner’s Syndrome is in pets, and what can be done. 

What is Horner’s Syndrome? 

Horner’s syndrome is a neurological condition affecting the eye and the muscles of the face. It usually comes on suddenly and only affects one side of the face. In very rare cases, it may affect both sides. 

Any dog or cat can develop Horner’s, although some breeds seem to be more prone, including: 

  • Labrador retrievers 
  • Golden retrievers 
  • Collies 
  • Doberman pinschers 
  • Weimeraners 

Horner’s syndrome can either be ‘idiopathic’ (no known cause), or secondary to another medical issue. 

What are the signs of Horner’s Syndrome in pets? 

The symptoms of Horner’s syndrome (on the affected side) include: 

  • A droopy upper eyelid (‘ptosis’) 
  • A sunken appearance to the eye (‘enophthalmos’) 
  • A constricted (small) pupil (‘miosis’) 
  • The third eyelid comes across (‘protrudes’) 

There may be other symptoms relating to the underlying cause, but these are the symptoms of Horner’s syndrome itself. 

What causes Horner’s Syndrome? 

The autonomic nervous system is made up of the parasympathetic and the sympathetic nervous system. They work together to control the bodily functions that happen automatically, without your pet thinking about them.  

The sympathetic nervous system is the part of the nervous system that prepares the body for ‘flight or fright’. Horner’s syndrome happens when the sympathetic nervous system isn’t working properly.  

In relation to Horner’s syndrome, the sympathetic nervous system widens the eyelids, dilates the pupil (makes it bigger) and drops the third eyelid. The parasympathetic nervous system retracts the eye when needed (to protect it), constricts the pupil (makes it smaller) and brings the third eyelid across.  

Normally, the systems work together. So, when the sympathetic nervous system isn’t doing its job properly, we see signs of parasympathetic dominance.  

Horner’s syndrome can happen when the sympathetic nervous syndrome is damaged in the relevant area. This could be as it runs through the neck or chest, or from damage to the ear or eye. For example, by: 

  • Trauma, such as being hit by a car, bitten by another animal, or even pulling too hard on the lead 
  • Diseases within or behind the eye 
  • Tumours (in the brain, neck or chest) 
  • Blood clots 
  • Inflammatory lesions 
  • Middle or inner ear disease 
  • Intervertebral disc disease. 

In dogs, Horner’s syndrome is often (about 50% of the time) idiopathic, meaning there is no known cause. In cats, it is more often secondary (for example, to trauma). 

What can be mistaken for Horner’s syndrome in pets? 

If your pet has signs of Horner’s syndrome, it’s important that you take them to a vet for a diagnosis. You should never self-diagnose Horner’s syndrome in your pet. 

Although it sounds obvious, it can actually be hard to tell which eye is affected! It may well be that the eye with the larger pupil is actually abnormal (for example, with glaucoma), which would need a very different diagnostic and treatment plan. 

Important differential diagnoses for Horner’s syndrome include uveitis (inflammation of the uvea, which is essentially the middle layer of the eye) and focal ulcerative keratitis (inflammation of the cornea, the clear outer layer of the eye, covering the pupil and iris). In cats, the third eyelid can protrude in response to illness, especially involving the gastrointestinal tract (guts).  

There is also a rare dysautonomia in cats (Key-Gaskell syndrome) that presents in a similar way, but with symptoms affecting other areas of the body too (such as constipation).  

Your vet will examine your pet to determine if it truly is Horner’s. 

How is Horner’s Syndrome diagnosed? 

If you notice any signs of Horner’s syndrome in your dog or cat, contact your veterinary clinic straight away. 

Your vet will start by examining your pet and asking you some questions about their recent health. The physical exam may include putting some drops into the eye. They may then advise some tests to look for an underlying cause. This could include bloods, urine tests and / or imaging.  

It’s important to note that there is no test to diagnose Horner’s syndrome itself. Horner’s syndrome is a collection of signs that your vet can diagnose by examining your pet. The further tests are to look for possible causes. 

What is the treatment for Horner’s Syndrome? 

There is no treatment for Horner’s syndrome in dogs and cats. Horner’s syndrome is not painful. If the third eyelid is protruding enough to cover a significant portion of your pet’s eye, their vision may be affected. In this case, your vet may prescribe eye drops in the short term. Similarly, if your pet cannot blink, their eyes will become dry. Your vet may suggest using false tears to protect the cornea, while your pet recovers. 

Most cases resolve spontaneously, meaning they get better by themselves with time. If an underlying cause can be found, your vet will recommend treatment depending on the cause. 

Can a dog or cat recover from Horner’s Syndrome? 

The prognosis (outlook) is usually good for idiopathic Horner’s syndrome, and most pets make a full recovery over time (weeks to months). Some pets recover partially. It is unusual for a dog to have a recurrence of Horner’s syndrome once they have recovered. 

For other causes of Horner’s syndrome, the recovery will depend on the cause. 

Key Points 

Horner’s Syndrome in pets is a neurological condition causing a collection of symptoms that happen simultaneously, rather than being a medical condition itself.  

The signs of Horner’s are a droopy upper eyelid, a constricted pupil, third eyelid protrusion and a sunken eye. The symptoms usually come on very quickly, and only affect one side of the face. 

In cats, there is usually an underlying cause that needs diagnosing and treating. If the cause is trauma, Horner’s can resolve by itself with time. However, your cat may still need treatment for other damage caused by the trauma. 

In dogs, most cases are idiopathic, meaning there is no known cause. These cases usually recover without treatment within a few weeks to months. However, it’s still important for you to take your dog to the vet if you notice any signs of Horner’s syndrome. They will check that they truly have Horner’s, and for underlying causes.