Horner's syndrome in dogs – symptoms and recovery

Overview

The syndrome indicates nerve pathway dysfunction

Horner's syndrome does not name one particular disease: it describes impaired sympathetic nerve supply to the eye. Sympathetic nerves help dilate the pupil. When their activity is reduced, the pupil remains small. The tone of smooth muscles around the eye also changes.

The eye appears more deeply sunken in its socket even though the eyeball has not become smaller. Drooping of the upper eyelid narrows the eyelid opening. In dogs, protrusion of the third eyelid partly results from the eyeball moving backwards.

Signs are usually one-sided. A bilateral form is possible but less common. The pupillary light reflex and vision itself are generally preserved in Horner's syndrome.

Causes

The nerve pathway also travels far from the eye

The eye's sympathetic pathway consists of a chain of 3 nerve cells. Its first part travels from the brain through the brainstem and cervical spinal cord to the beginning of the thoracic spinal cord. The next part passes through the chest and neck towards the head. The final part passes near the middle ear to the eye. The cause of an eye sign is therefore not always in the eye itself.

Damage can result from a bite, bruising or another neck or chest injury. A tumour can also damage the pathway. Cervical disc disease can cause the same set of signs. A stretching injury or avulsion of the forelimb's brachial plexus can be associated with Horner's syndrome.

Ear infection or an unknown cause

Middle or inner ear disease can disrupt the sympathetic nerves. Middle ear infection may also involve facial paralysis. Horner's syndrome and facial paralysis are therefore different findings, although they can coexist.

If investigations find no underlying cause, the condition is called idiopathic. This conclusion follows investigation of other possible causes. Horner's signs can also appear after surgery around the ear. Loss of sympathetic nerve supply can additionally dilate local blood vessels.

Around 50% of canine Horner's cases have been described as idiopathic. An unknown cause does not, however, mean that every new case of unequal pupils belongs to this group.

Risk factors

Horner's syndrome can affect any breed. Predisposition has been described in Golden and Labrador Retrievers, Collies, Shetland Sheepdogs, Weimaraners and Dobermanns. Breed predisposition has not been consistently demonstrated. The available information does not establish overall breed-specific disease risks.

The reported mean age at onset of the idiopathic form is 5–8 years. Canine cases involving all causes have ranged from 5 weeks to 17 years of age. An average age therefore does not rule out signs in a young or old dog. Ear disease, a neck injury or a chest lesion can make investigation of the underlying cause particularly important.

Typical symptoms

Horner's signs often start suddenly. Typical observations include:

• One eye has a smaller pupil.
• The upper eyelid droops and the eye looks smaller than usual.
• The eye appears sunken.
• The third eyelid is more visible at the inner corner.
• The difference in pupil size is more obvious in dim light.
• The ear or muzzle skin may feel warmer than usual or look red.

A small pupil is the most frequently recognised component of Horner's syndrome. In a partial syndrome associated with brachial plexus injury, a small pupil may be the only finding. Pupil asymmetry can be difficult to notice in a bright room. In dim light, the affected pupil does not dilate like the normal one. Your dog may nevertheless see normally. Some dogs also have profuse drooling or difficulty eating on the affected side.

Head shaking, ear rubbing and ear discharge may suggest middle ear disease. Inner ear signs include a head tilt and nystagmus. These additional signs are not considered part of a change in eye appearance alone.

Complications and consequences

The effects depend on the underlying cause

Even permanent changes from Horner's syndrome alone are generally mainly cosmetic. A markedly protruding third eyelid may nevertheless cover part of the field of vision. Clear vision loss is not considered typical of Horner's syndrome alone.

Brainstem or spinal cord damage can also involve changes in consciousness, proprioception or limb movement. Middle ear disease can involve hearing problems or dry eye. Neurological deficits and impaired hearing may remain after an ear infection settles.

The idiopathic form often recovers

Improvement in idiopathic Horner's syndrome may be seen as early as 4 weeks. In some dogs, improvement starts only at 15 weeks. The idiopathic form in Golden Retrievers usually resolves within 16 weeks. Recovery can take around 4 months. These are times for spontaneous recovery, rather than medication course lengths.

Some changes may resolve while the small pupil remains. Repeated episodes of the idiopathic form are unusual. After brachial plexus injury, improvement of eye signs does not mean forelimb function has returned.

Diagnosis at the vet

First distinguishing eye disease from impaired nerve supply

Your vet compares pupil sizes in bright and dim light. It is also necessary to establish which of the unequal pupils is abnormal. A large pupil can be associated with glaucoma, for example, requiring different investigation and treatment.

A thorough eye examination looks for iris inflammation, a corneal ulcer and other causes of a small pupil. A tear test measures tear production. Fluorescein staining reveals ulcers or scratches on the eye surface. Intraocular pressure is measured with a tonometer where needed. In uveitis, inflammation of the eye's uveal tract, pressure is often lower than usual. Examination may also reveal cloudiness in the anterior chamber, supporting eye inflammation.

Corneal swelling, blood vessel growth and scarring can also suggest keratitis.

A neurological examination assesses movement, reflexes, proprioception and cranial nerves. Central nervous system Horner's syndrome rarely occurs without other neurological deficits. Eye signs can nevertheless look the same regardless of the site of pathway damage.

The damage location guides further tests

A pharmacological eye drop test can help assess the site of nerve pathway damage. Phenylephrine's effect relates to increased sensitivity after loss of nerve supply. A test performed very early can localise the lesion incorrectly.

Development of this hypersensitivity has been described over both 2–10 days and 2–3 weeks. Interpretation therefore takes the onset date into account. Pharmacological testing does not reliably distinguish first-order from second-order pathway lesions. The apraclonidine test used in humans has not been validated for diagnosing Horner's syndrome in animal patients.

The final portion of the pathway to the eye is the most common lesion level. The ears are examined with an otoscope if the lesion fits this final portion. A normal-looking or intact eardrum does not, however, exclude middle ear infection. A middle ear sample differs from an outer ear canal discharge sample. An outer ear sample may contain microbes misleading for treatment. Blood tests can also assess other diseases, such as diabetes.

Neck and chest X-rays can help investigate damage to the middle portion of the pathway. Suspected brain or spinal cord damage may require MRI. CT or MRI can also examine ear, neck, chest and orbital structures in greater detail. An identified mass may require biopsy to confirm the cause.

Treatment

Treating an identified disease

Your vet chooses treatment according to the cause of Horner's syndrome. Ear infection can be treated using bacterial culture and susceptibility results.

Successful ear infection treatment may also resolve Horner's signs. Tumour-related nerve pathway damage may instead be permanent. Treatment associated with trauma or disc disease is determined by the identified lesion.

Eye appearance alone does not always need treatment

There is no specific cause-directed treatment for the idiopathic form. Symptomatic phenylephrine eye drops can temporarily reduce third eyelid protrusion. This may help if the third eyelid obscures vision. A medicine-induced improvement in appearance does not itself mean the nerve damage has healed.

Eye drops and the need for them are determined at the consultation. Permanent pupil asymmetry may be mainly cosmetic if a more serious underlying disease has been excluded.

Eye signs after a procedure are assessed separately

After surgery on the ear's bony bulla, Horner's signs have been described for around 2 weeks in dogs. This observed duration concerns that particular surgical complication. The speed of eye recovery alone does not establish recovery of another nervous system or limb injury.

Prevention

The original cause of idiopathic Horner's syndrome is unknown, so there is no precise preventive measure. An appropriate harness can reduce forceful jerking of the neck. Avoiding bites and other neck or chest injuries removes recognised causes of nerve damage.

Ear signs need investigation and appropriate treatment because middle ear disease can damage sympathetic nerve supply. One Golden Retriever breed organisation's guidance states that Horner's syndrome alone does not restrict breeding from the dog or their relatives. This is that breed's own guidance and does not remove the significance of another diagnosed disease.

Important for dog owners

Record when signs began and photograph both eyes in the same lighting. Pupil asymmetry is often more visible in dim light. Tell your vet about ear signs, a neck injury, changes in forelimb function and recent procedures.

Monitor eating, walking, balance and any eye pain or squinting. Horner's syndrome alone should not generally cause pain. Do not use human eye drops or test pupil dilation at home. Attend the agreed follow-up even if the eye's appearance starts returning to normal.

Keep a record of whether the condition was considered idiopathic or an ear, nervous system or other organ-related cause was found. New limb weakness or altered consciousness may involve more extensive nervous system damage. Do not simply observe such a change on the strength of a previous Horner's diagnosis.

Frequently asked questions

Is Horner's syndrome painful for a dog?

Horner's syndrome alone is usually painless. Marked squinting or pain therefore needs reassessment. Iris inflammation or a corneal ulcer can also cause a small pupil. Labelling the signs Horner's syndrome does not replace a thorough eye examination.

Can Horner's syndrome make a dog blind?

Vision itself and the pupillary light reflex are generally preserved in Horner's syndrome alone. A raised third eyelid may, however, physically obscure vision. An underlying ear disease, tumour or nervous system disease is assessed separately. Clear vision loss must not be dismissed as a familiar Horner's sign.

How long does Horner's syndrome last?

Improvement in the idiopathic form can be seen as early as 4 weeks. In a dog recovering more slowly, improvement may begin only after 15 weeks. The idiopathic form in Golden Retrievers usually resolves within 16 weeks. Some signs, such as a small pupil, may nevertheless be permanent.

Why image the neck or chest for an eye sign?

The eye's sympathetic pathway passes through the chest and neck as well as the brain and spinal cord. A tumour or injury in these areas can therefore appear as an eye sign. The imaging target is selected according to clinical examination and the estimated lesion site.

Does improvement with eye drops mean the nerve has healed?

Not automatically: symptomatic phenylephrine can improve appearance only temporarily. The cause of nerve damage is assessed and treated separately. Eye signs may also improve spontaneously without specific treatment. A single change after a drop therefore does not determine the final prognosis for the whole condition.