Horners Syndrome in Dogs
If the side of the face that looks affected is warm this is a real warning sign

Horners Syndrome in dogs is a very unusual condition that can affect any dog, at any age, and has no one single cause.

However, when and if it does occur in your dog, it can be a very frightening experience.

But as bad as it can appear, it is not actually painful to your dog. The most frightening aspect of this syndrome is the changing of your dog’s eyes.


Horners Syndrome is not a single symptom, but rather a grouping of signs that occur when very specific muscle in your dog’s face lose control or the ability to be stimulated by nerves.

In most all cases, it will involve the sympathetic nerves, which are part of your dog’s sympathetic nervous system and are critical for contracting blood vessels in your pet.

A syndrome is almost always a collection of symptoms that have very little meaning or impact until they all start to fit together.

A syndrome is often confused with a diagnosis, or an answer to an actual cause, but it is not an actual diagnosis.

It is, however, a situation where the actual potential number of causes become limited to the extent that your veterinarian has a much better chance at actually developing a diagnosis, and as such, can find a treatment.

Your dog’s body has several functions that are controlled by their nervous system that act without your dog’s knowledge or control.

Examples of this would be the heart as well as the respiratory system, sweat and other secretions, and the amounts that are released.

It also includes pupil dilation and constriction. This is referred to as the automatic system and it is part of your dog’s nervous system.

It is divided into two parts; the sympathetic nervous system and the parasympathetic system.

The parasympathetic system maintains normalcy, while the sympathetic system will prepare your dog’s body for a fight of some kind or an escape from something that is threatening them.

There are some changes that may occur or become stimulated by the sympathetic system.

They will include an increase in sweating, dilated pupils, as well as an increase in the flow of blood to your dog’s muscles.

In some cases it can also cause an increased heart rate.

These systems work in complete harmony in your dog until the sympathetic system becomes damaged, either by an injury or an underlying condition.

Once this happens, only the parasympathetic system remains active, and the result is referred to as Horners Syndrome in dogs.


Finding and diagnosing the actual cause of Horners syndrome in dogs can be very difficult, simply because there may be several potential causes.

The first cause is very common with syndromes, and is called Idiopathic, meaning that the cause will never be known.

It may also be the result of a car accident that has caused trauma in your dog’s head, neck, or their chest.

Horners Syndrome in dogsThe good news is that it can be treated

It may not be a severe trauma and you may never know that it actually occurred.

However it is just enough to throw off this very delicate harmony that exists between the two systems that causes Horners Syndrome in dogs.

It may also be caused by some type of a bite wound from a wild animal or another dog, or it may be the result of an infection in your dog’s middle ear.

However, even though it is not actually painful for your dog, it may have very sinister underlying causes.

These would include an Inter-vertebral disk disease that is starting to form, a disease in your dog’s orbit, which is the area behind their eye, or it may be the result of cancer.

Brain cancer or chest cancer can very easily throw off this balance.


As troubling as Horners syndrome in dogs is to actually diagnose or find the actual cause, it has very distinct symptoms that will occur in almost every case.

When more than a couple of these symptoms surface at the same time, the rest should follow very quickly as they are all classic symptoms of this syndrome.

The first is referred to as Miosis, which is where your dog’s pupil constricts to a much smaller size.

This constriction is a normal reaction to increased light and will return to full size when they adjust. However, in this case they do not adjust as the automatic nerve messages have become disturbed.

Once this occurs, there will be a protrusion of your dog’s third eyelid.

This is also common in pink eye or red eye, but is also one of the five classic symptoms of Horners Syndrome in dogs.

The next symptom is called ptosis, which is drooping of the upper eyelid of your dog.

But perhaps the most familiar symptom, called enophthalmos, is the best known of all the symptoms of Horners syndrome.

With this symptom, your dog’s eye will sink into their socket and can be very frightening the first time you witness it.

The final of the classic five symptoms is a dilation of the blood vessels in your dog’s face.

This syndrome will almost exclusively be on only one side of your dog’s face, and all of the symptoms, including this one, will stay on the affected side.

To confirm Horners syndrome in your dog, touch the effected side of their face; if it is feels much warmer than the other side, it is confirmed.


The answer is yes.

If your dog exhibits all five of these symptoms, you have just diagnosed it yourself. The difficult part is now up to your veterinarian in trying to find the actual cause of this syndrome.

They will have to do a complete neurological exam, perform x-rays, chemistry panels, as well as a complete blood count.

In some cases they may also have to do a CAT scan or a MRI before they can get a complete answer, if they can.

It most every case, your veterinarian will also have to administer Epinephrine into your dog’s eyes to assist them in finding the actual location of the injury.

This is done by measuring the time between the first dosage and the actual dilation of the pupil. If it is an injury to your dog’s nerves that is outside of their brain, it will cause dilation within 20 minutes.

If it is lesion or the injury that is in the brain or the spinal column, it will take much longer for dilation to occur.

This is a very fascinating testing process and should be one of the first tests you ask your veterinarian to perform if your dog does indeed show all five symptoms.

What is even more amazing about Horners syndrome in dogs is that unless your veterinarian can actually find the actual cause, such as a bite wound or middle ear infection, there is no treatment needed.

Eye drops will generally be given to relieve the clinical signs, but it will not help the pain because there is no pain. In most cases, it will simply go away on its own.


Horners syndrome in dogs in most every case looks much more severe than it actually is. It will usually resolve itself between six to eight weeks and your dog will appear as normal as ever.

But it is still recommend having the Epinephrine tests just for your peace of mind.

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