Horner's Syndrome in Dogs

Horner’s Syndrome in Dogs

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Horner’s syndrome is a neurological disorder that causes abnormalities in the eyes, eyelids, and eyeballs. It may be caused by trauma such as a traffic accident or choke chain, but in many cases the cause is unknown. Veterinarian Sato explains the symptoms, possible diseases, and treatment methods.

What is Horner Syndrome in Dogs?

What is Horner Syndrome in Dogs
What is Horner Syndrome in Dogs

Horner’s syndrome is an eye disorder caused by some kind of disorder in the sympathetic nerve that connects the eyes and the brain . Any dog ​​can be affected regardless of breed, age or sex.

  1. Miosis :pupils become smaller
  2. Protrusion of the nictitating membrane : A white membrane covers the eyes
  3. Blepharoptosis : Drooping upper eyelids
  4. Sinking eyeballs: Eyes appear to be sunken in

One or more of these symptoms occur in one eye and very rarely in both eyes . All of them are characterized by the eyes appearing smaller, and are generally not accompanied by pain or itching (they may be accompanied by pain or itching depending on the cause of the symptoms) .

Horner syndrome has many causes, but most resolve on their own within days to months. However, some of them may be caused by life-threatening diseases, so please do not leave them unattended and have them examined at a veterinary hospital.

* Named after the Swiss ophthalmologist Johann Friedrick Horner (1831-1886). Also called Horner syndrome.

Symptoms of Horner Syndrome in Dogs

Symptoms of Horner Syndrome in Dogs
Symptoms of Horner Syndrome in Dogs

In Horner’s syndrome, the four symptoms mentioned earlier are seen, but there is another cause, and the symptoms appear as a result of nerve damage. Therefore, depending on the cause, in addition to the four symptoms, anorexia, drooling due to excessive salivary secretion, paralysis, and convulsions may also be seen.

1. Miosis

The pupil normally becomes smaller in bright light (miosis) and larger in dark light (mydriasis), but in Horner syndrome, one pupil is always small. Therefore, the size of the left and right pupils is different in bright places, and the difference becomes even more pronounced in dark places. It may be easier to bump into things by making it difficult to see.

2. Protrusion of the nictitating membrane

The nictitating membrane is the membrane that comes out from the inner corner of the eye, and it is called “nictating membrane” because it comes out momentarily. Birds and reptiles protect their eyes by blinking with nictitating membranes or covering them like goggles, but dogs do not come out when their eyes are open, so they have a “third eyelid”. ) is also called (*).

In Horner’s syndrome, the nictitating membrane protrudes without being closed even when the eyes are opened. Even with the same protrusion of the nictitating membrane, the condition in which the nictitating membrane (more precisely, the nictitating membrane gland behind the nictitating membrane that produces tears) swells like cherry is called “cherry eye (third eyelid prolapse) “.

*Many mammals, including humans, have degenerated nictitating membranes and become vestigial organs.

3. Ptosis

The eyelid is the eyelid, and the upper eyelid is lowered more than usual, and even if you try to open your eyes, you will not be able to open them. Although it may look sleepy, Horner’s syndrome is characterized by it occurring in only one eye.

4. Sinking eyes

Eyeball retraction (recessed eyeball) is a condition in which the eyeball appears to be sunken due to retraction into the orbit where the eyeball is located. The eyeball itself does not collapse, and there is no abnormality in vision. Horner’s syndrome causes one eye to recede and appear smaller.

Causes of Horner Syndrome in Dogs

Causes of Horner Syndrome in Dogs
Causes of Horner Syndrome in Dogs

The causes of Horner’s syndrome are many, difficult to identify, and often unexplained (idiopathic) . In any case, it is thought that there is a problem somewhere in the sympathetic nerves from the brain through the spinal cord, chest, neck, etc. to the eyes .

In order to identify the cause, the sympathetic nerve is roughly divided into the following three pathways.

  • Primary (Central): From the brain (hypothalamus) to the spinal cord
  • Secondary (preganglionic): From the spinal cord to the neck ganglia ( where nerves converge)
  • Tertiary (postganglionic): Peripheral nerves from the neck ganglion to the eye

Possible Causes of Primary Horner Syndrome

Neurologic symptoms other than Horner’s syndrome may be seen.

  • cerebral infarction
  • tumor
  • cervical fracture
  • Encephalomyelitis
  • intervertebral disc disease
  • inflammation

Possible Causes of Secondary Horner Syndrome

It can be caused by traffic accidents, choke chains, and injuries from leashes.

  • Trauma
  • tumor
  • inflammation

Possible Causes of Tertiary Horner Syndrome

  • otitis media
  • tumor
  • abscess
  • inflammation

Read Also: Nystagmus in dogs | Symptoms, causes, and remedies explained by a veterinarian

Treatment of Horner Syndrome in Dogs

Treatment of Horner Syndrome in Dogs
Treatment of Horner Syndrome in Dogs

If a doctor suspects Horner syndrome because of an eye problem, tests to check for other physical abnormalities are done, such as the following:

  • palpation
  • behavioral observation
  • Blood test
  • ophthalmic examination
  • neurological examination
  • X-ray (roentgen)inspection
  • Ultrasound examination
  • CT/MRI examination
  • Cerebrospinal fluid test
  • Phenylephrine test

If the cause is found, its treatment leads to the treatment of Horner syndrome. By timing how long the phenylephrine dilation eye drops work, you may be able to determine which pathway is causing the problem.

Although the cause of Horner syndrome is often unknown, most people resolve on their own within a few months. However, if the cause is a tumor or progressive myelomalacia caused by a herniated disc, it can lead to death if left untreated.


  • Horner’s Syndrome Causes Nerve Problems
  • Easy to notice because the eyes look smaller
  • Can happen with a choke chain
  • Although there are many natural cures, the examination is necessary

Horner’s syndrome is an eye disorder caused by nerve damage. Most cases occur in one eye and are characterized by the eye appearing smaller. Many resolve on their own, but they can also be caused by a more serious illness. Be sure to have your pet examined at a veterinary hospital.

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