Unpacking Horner’s Syndrome in Dogs

Kaylee Ferreira

Published by Dr. Kaylee Ferreira

Updated on

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What is Horner’s Syndrome in Dogs?

Horner’s Syndrome in dogs is a central nervous system disorder that commonly affects the eye and specific facial muscles. The condition’s etiology is complex, and its onset can be sudden. It typically only influences one side of the face, but it can, in rare cases, affect both sides of the face. To understand Horner’s in dogs, we need to explore a few neurological basics. 

Sick dog with the vet

Nervous System Break Down 

The eye has two essential sources of innervation: 

  • The sympathetic nerve supply.
  • The parasympathetic nerve supply.

The sympathetic nerves control input and output for several bodily functions as a part of the autonomic nervous system. This nerve supply to the eye starts at the base of the brain and splits into three separate pathways. 

The first pathway runs down the neck and exits at the level of the shoulders into the thorax. The second section runs up the neck into the skull base. The last pathway has an intricate route up into the skull, closely associating with the middle ear until it reaches the eye. The nerves branch out and supply many structures within the eye.

Inflammation, trauma, or infection can result in sympathetic nerve impulse interruption. If the sympathetic nerve impulses are blocked, it causes an overcompensation from the parasympathetic nerve supply. 

The overcompensation results in the tell-tale Horner’s Syndrome clinical signs.  

What are the Clinical Signs of Horner’s Syndrome in Dogs?

Specific signs that indicate Horner’s Syndrome symptoms in dogs are pathognomonic by veterinarians. Horner’s Syndrome is also known as oculosympathetic paresis because the symptoms affect the sympathetic innervation to the structures closely associated with the eyes.

These clinical signs are due to a disrupted sympathetic nerve supply and can include the following symptoms:

  • A prominent or prolapsed third eyelid.
  • A droopy upper eyelid.
  • The appearance of a sunken eye.
  • A constricted pupil that does not respond to light. 

The signs mentioned above are not painful, but they can cause visual impairment. 

What Causes Horner’s Syndrome in Dogs?

The causes of Horner’s Syndrome in dogs depend on the etiology of the sympathetic nerve dysfunction. Therefore, localizing the dysfunction’s origin helps determine the syndrome’s exact cause. 

Damage within the oculosympathetic pathway gets divided into three orders:

  • First-order Horner’s syndrome occurs within the brain and spinal cord. Patients may have a history of brain or spinal cord trauma.
  • Second-order Horner’s occurs between the thorax and the base of the skull. Predisposing factors may include intra-thoracic pathology or cervical trauma.
  • Third-order Horner’s occurs between the skull and the eye. This is the most common form diagnosed and is mainly associated with middle or outer ear canal infections.

The problem with finding the root of the problem is that there are several non-specific symptoms and no easy way to diagnose them. Specialists will try to group the causes into either postganglionic, preganglionic, or central lesions.

Below is a list of possible causes of Horner’s Syndrome according to the suspected origin of the lesions:

Postganglionic Causes of Horner’s Syndrome 

  • Neoplasms include Neuroblastomas or Carotid body paraganglioma.
  • Idiopathic.
  • Iatrogenic.
  • Post-operative complications from a TECA-LBO (total ear canal ablation and lateral bulla osteotomy).
  • Infections of the ear, including Otitis media or Otitis interna.

Preganglionic Causes of Horner’s Syndrome

  • Idiopathic.
  • Iatrogenic.
  • Brachial plexus nerve block.
  • Epidural ropivacaine.
  • Vagus nerve stimulator placement.
  • Thoracic surgery.
  • Traumatic causes during birth or from accidents that result in brachial plexus avulsions.
  • Neoplasias include Mediastinal lymphoma or a Peripheral Nerve Sheath Tumor of the vagus nerve.
  • Infectious causes like tick paralysis caused by the tick species, Ixodes holocyclus.

Central Causes of Horner’s Syndrome

  • Traumatic incidents include being shot by an air pellet gun in the spinal cord.
  • Infectious causes such as Neospora or Tetanus.
  • Fibrocartilaginous embolism in the cervical spine.
  • Diabetic polyneuropathy.

Is any Dog Breed or Dog Age Predisposed to Horner’s Syndrome?

The predisposed dog demographic diagnosed with Horner’s Syndrome include middle-aged Golden Retrievers, Cocker Spaniels, and Collies. Golden Retrievers represent over 90% of the cases reported.

The mean age of affected individuals varies between 5-8 years. Even though golden retrievers and collies can be affected more often, any dog breed can be diagnosed with Horner’s at any age.

Woman with Boston Terrier Dog at Vet

How is the Location of Damage Caused by Horner’s Syndrome Determined?

Localization of the lesion within the oculosympathetic pathway requires extensive knowledge and expertise. A veterinary specialist in internal medicine or neurology is often necessary to diagnose the disorder’s origin. 

A veterinarian will administer various eye drops to determine the nerve damage origin. The drugs used can include the topical application of a 10 % Cocaine solution or direct and indirect sympathomimetic drugs like phenylephrine or hydroxyamphetamine. Different sections of the nerve respond by dilating the pupil affected with Horner’s syndrome. 

A 5 or 10 percent solution of cocaine is an ingredient in the standard gold test for Horner’s syndrome in humans. Cocaine prevents the reuptake of norepinephrine by postganglionic neurons. This leads to pupillary dilation unless a lesion affects the oculosympathetic pathway. Then, the affected pupil will not dilate, and an increase in anisocoria will get noted.

Cocaine is a strictly controlled substance, so it is not readily available or used in veterinary medicine. Most specialists will assume that the minimal dilation of the miotic pupil to a parasympatholytic drug will confirm Horner’s Syndrome. A thorough ophthalmologic examination is also required to rule out subtle uveitis and keratitis.

Unilateral third-order Horner’s syndrome is the most common presentation, and applying a dilute direct sympathomimetic phenylephrine will dilate the pupil and resolve most clinical symptoms. However, phenylephrine will not dilate an average pupil or a first or second-order Horner’s pupil.

If idiopathic Horner’s syndrome is suspected, further diagnostic imaging is required. These tests include x-rays, CT scans, or an MRI. 

What is the Recovery and Outcome of Horner’s Syndrome in Dogs?

Is Horner’s Syndrome life-threatening in dogs? Patients who suffer from Horner’s are not in immediate danger, but some potential underlying causes can be severe and need to be assessed by a veterinarian.

The long-term effects of Horner’s Syndrome depend on the cause of the disease. Recovery can be slow and sometimes frustrating because it can take several weeks to several months. 

Most cases recover spontaneously if the underlying condition is treated, but Horner’s Syndrome in dogs treatment involves an extensive medical workup. This is quite costly. If the cause is determined to be due to cancer or benign growth, the prognosis and potential for recovery get guarded.

If no underlying condition gets found, then idiopathic Horner’s Syndrome is diagnosed. Unfortunately, Idiopathic Horner’s syndrome is the most common diagnosis, which can be frustrating for pet owners, especially after an extensive medical investigation. 

It can take many months to see a small amount of progress, and only symptomatic treatment will be available to alleviate, not cure, symptoms.  

Topical treatments of phenylephrine eye drops can improve impaired vision and the eye’s aesthetic as many owners are bothered by the “droopy eye dog” appearance.

How Does Horner’s Syndrome in Dogs Affect the Third Eyelid?

The third eyelid is also known as the nictitating membrane. Nictare means “to blink” in Latin. This structure is a transparent structure that moistens and protects the eye. Cats have third eyelids, whereas dogs have a less prominent third eyelid. 

Horner’s most commonly reported symptom is that a dog’s third eyelid won’t retract or that the dog’s inner eyelid is covering a significant portion of the cornea. The degree of third eyelid protrusion varies with each affected individual. 

It appears that the dog’s eye sunk into the socket and that the eyelid is pulling over the eye. The cause of eyelid protrusion in dogs is passive protrusion because of the primary enophthalmos.

Cats have smooth muscle fibers within the third eyelid, which get sympathetically mediated. If the cat has Horner’s, that nerve supply is interrupted, and the eyelid remains retracted.  

Sick dog in bed


Horner’s syndrome can get caused by several diverse lesions, including trauma, inflammation, neoplasia, or systemic disease processes. The best way to diagnose the cause is to consult a specialist in internal medicine or neurology. 

Most dogs can lead a comfortable, good quality of life after they get diagnosed, and, depending on the original cause, they may even make a full recovery. 

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Kaylee Ferreira


Dr. Kaylee Ferreira, a South African vet from Johannesburg, excels in diverse veterinary roles. Founder of Kubuntu Veterinary Services, she's a dedicated animal lover and adventurer.


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