A drooping eyelid, a visible third eyelid and a smaller pupil — this combination of asymmetrical eye signs can be very alarming for an owner. It often appears suddenly and is a clear neurological signal. The condition is known as Horner’s syndrome and, while it is not usually painful in itself and does not threaten sight, it is the clinical reflection of another, sometimes serious, underlying disease. Prompt, precise diagnosis is essential to establish the primary cause, start the right treatment and give your pet the best possible care.
Below we explain what Horner’s syndrome is, why it develops, how advanced diagnostic work-up proceeds and what the treatment options are. The aim is to provide sound, expert information that will help you understand the problem and work through the diagnostic process under a veterinarian’s guidance, with a sense of calm and control over the situation.
What is Horner’s syndrome?
Horner’s syndrome (also called Claude Bernard-Horner syndrome) is a set of ocular signs that appear as a result of damage to or interruption of the sympathetic nerve fibres supplying the eyeball and the surrounding muscles.
A simple definition:
Horner’s syndrome is a set of characteristic, asymmetrical signs around the eye (a constricted pupil and a drooping eyelid among them) caused by the loss of tone in muscles controlled by the damaged sympathetic system. It is a neurological disorder.
Briefly, on the role of the sympathetic system in the eye. The sympathetic system is part of the autonomic nervous system and works outside conscious control. In the eye, its fibres govern several key functions:
- Dilating the pupil.
- Holding the upper eyelid in its normal, raised position.
- Holding the eyeball in its proper position within the orbit.
When the sympathetic fibres are damaged, their action stops and the intact parasympathetic system takes over. The muscles slacken and the clinical signs appear, among them a constricted pupil (miosis).
What do the signs of Horner’s syndrome look like in dogs and cats?
The signs of Horner’s syndrome are one-sided and very distinctive.
The main ocular signs, plus the additional ones:
- Constricted pupil (miosis): the pupil in the affected eye is smaller (anisocoria). This is the most noticeable of the signs of Horner’s syndrome in a dog or cat.
- Drooping upper eyelid (ptosis): the lid hangs down, so the eye looks half closed.
- Sunken eyeball (enophthalmos): the eye appears drawn back into the orbit.
- Protrusion of the third eyelid: because the eyeball has receded, the nictitating membrane becomes more visible.
The signs in brief:
- Miosis: a constricted pupil.
- Ptosis: a drooping upper eyelid.
- Enophthalmos: a sunken eyeball and a protruding third eyelid.
Additional vascular and neurological signs:
- Dilation of the skin’s blood vessels (vasodilatatio): a local rise in skin temperature on the face.
- Others: a temporary drop in intraocular pressure, or accompanying paresis of a forelimb when the problem lies in the spinal cord.
Why does Horner’s syndrome develop? The commonest causes
Horner’s syndrome is acquired. It results from damage along the long and complicated route taken by the sympathetic fibres, which runs from the brain through the spinal cord, the chest, the neck and the middle ear before finally reaching the eye. That extended path leaves the fibres vulnerable to injury and to pressure from neighbouring structures such as tumours or abscesses.
Damage anywhere along this route interrupts the nerve signal. The muscles that depend on the sympathetic system are then left without stimulation, they slacken, and the signs appear.
Classification by site of the lesion
Where the sympathetic neuron is damaged determines how severe the condition is and how it progresses.
- Central (first-order neuron):
- Site: covers the first neuron, from the hypothalamus through the brainstem to the spinal cord (T1-T3).
- Example causes: intracranial disease, damage to the cervical spinal cord, endocrine disorders such as diabetes mellitus and hypothyroidism.
- Preganglionic (second-order neuron):
- Site: covers the second neuron, running through the chest and neck as far as the cranial cervical ganglion.
- Example causes: tumours in the chest (mediastinal lymphosarcoma in cats, for instance), changes in the mediastinal lymph nodes, injuries to the brachial plexus.
- Postganglionic (third-order neuron):
- Site: covers the third neuron, which leaves the cranial cervical ganglion and passes through the middle ear to the eyeball.
- Example causes: this is the commonest form. Middle or inner ear infections. Injuries and bite wounds to the head and neck. Retrobulbar abscesses.
Idiopathic cases
In a large proportion of dogs and cats no specific primary cause can be found. These cases are classified as idiopathic Horner’s syndrome.
- It is the commonest cause of all (particularly in Golden Retrievers and Cocker Spaniels).
- The prognosis is usually good.
Is Horner’s syndrome dangerous?
Horner’s syndrome in itself poses no danger to the eye. What is dangerous is the underlying disease it may be signalling. It tells us that nerve conduction has been interrupted somewhere, and that can be linked to serious conditions such as tumours or spinal cord injuries.
Checklist: when not to wait. An urgent veterinary appointment is essential, especially where the eye signs are accompanied by:
- Neurological signs such as paresis or paralysis of the limbs.
- Difficulty walking or keeping balance, or a head tilt.
- Respiratory signs (laboured breathing) associated with tumours in the chest.
- Fresh wounds, injuries or obvious pain in the head or neck.
- General signs pointing to endocrine disease.
Horner’s syndrome in cats and dogs — what does the work-up involve?
The aim of the work-up is not merely to confirm the set of signs, but above all to locate the lesion and identify its primary cause.
Clinical and ophthalmic examination
The process starts with a detailed history and a clinical and neuro-ophthalmic examination.
- Assessing the signs: confirming by eye that the classic signs are present.
- Distinguishing it from other eye conditions: anterior uveitis (uveitis anterior) has to be considered.
Pharmacological testing
Pharmacological tests using drops (sympathomimetics) are the principal tool for localising the lesion.
- Phenylephrine 10%: the pupil of the affected eye dilates within 5-8 minutes if the lesion involves the postganglionic neuron (third-order). If dilation takes around 40 minutes, that points to damage to a preganglionic neuron (first- or second-order).
- Adrenaline 0.1%: dilates the pupil of the affected eye within 20 minutes if the postganglionic neuron is the one damaged.
Further investigations
Imaging and laboratory tests are needed to rule out serious disease and pinpoint the lesion.
Which tests are run depends on what is suspected:
- Otoscopy: to assess the middle ear.
- Neurological examination: particularly where a first- or second-order neuron lesion is suspected.
- Radiographs of the chest and cervical spine: needed to rule out tumours and bony changes.
- In cats, chest radiographs are recommended as a matter of routine.
- Magnetic resonance imaging (MRI) or computed tomography (CT): advanced imaging, recommended in complicated cases.
- Blood tests (haematology, biochemistry, thyroid hormones): these rule out a metabolic or endocrine background.
Treating Horner’s syndrome
Treatment of Horner’s syndrome depends entirely on the cause that is found.
Treating the cause
This means dealing with the underlying disease:
- Treating an ear infection (with antibiotics, for example).
- Surgery (removing a tumour, dealing with retrobulbar abscesses).
- Treating endocrine disease.
Where the problem lies in the preganglionic neurons, treatment has to address the cause.
Symptomatic treatment (when no cause can be found)
This is used in idiopathic cases, or where the cause cannot be removed.
- Phenylephrine drops: used for symptomatic relief. As a sympathomimetic, phenylephrine supports the local action of the sympathetic supply.
- Its effect lifts the drooping eyelid, draws back the third eyelid and dilates the pupil.
- The drops are usually given 1-2 times a day for 6-8 weeks.
- Acupuncture: there are reports of a beneficial effect in idiopathic cases.
- Ophthalmic monitoring: essential for keeping track of the patient’s progress.
Prognosis — when does Horner’s syndrome resolve?
The prognosis depends entirely on where the lesion sits and on whether the underlying disease can be treated successfully.
- Postganglionic lesions (third-order neuron): the prognosis is generally good. In idiopathic cases the signs resolve on their own within 4 to 8 weeks, occasionally taking up to 4 months.
- Preganglionic lesions (first- and second-order neurons): the prognosis is guarded to poor, because these are often bound up with serious disease of the brain or spinal cord, or with tumours.
What matters most is getting to a veterinarian quickly to establish whether Horner’s syndrome is signalling something more serious.
If you notice anything worrying in your pet, or you would like to book a vaccination appointment, please contact one of the Zviropolis veterinary practices without delay:
Contact number (all locations): +48 602 525 050
In summary
Horner’s syndrome in dogs and cats is a neurological phenomenon that gives a clear, if asymmetrical, signal around the eyes. Although in many cases it turns out to be idiopathic and resolves on its own, a full and thorough work-up is still necessary to rule out primary conditions serious enough to threaten a pet’s health, and even its life. Locating the nerve lesion and identifying the cause is the only route to effective treatment.
If you notice your animal suddenly develop a drooping eyelid, a constricted pupil or a protruding third eyelid, do not put it off. The best step you can take is to get quickly to a professional veterinary clinic, where specialists in ophthalmology, neurology and internal medicine can carry out a full clinical examination, pharmacological tests and any imaging required, so the problem can be located quickly and the right treatment begun.
Owners’ most common questions (FAQ)
Does Horner’s syndrome hurt? No, the set of eye signs itself (drooping lid, smaller pupil) is not painful. Pain may, however, come from the primary cause — an ear infection or an abscess, for example.
Can the dog or cat still see normally? As a rule, Horner’s syndrome does not affect vision. The exception is when the third eyelid covers a substantial part of the eye.
Can the signs come back? If the Horner’s syndrome was idiopathic, recurrence is rare. If the primary cause (a tumour or a chronic endocrine disease, say) has not been fully resolved, however, the signs can return.
Can it be treated at home? Home treatment is possible only after a thorough work-up and under close veterinary supervision. Symptomatic treatment (drops) should not be used until serious causes have been ruled out.
Is Horner’s syndrome dangerous? The signs themselves pose no danger to the eye, but the syndrome is potentially dangerous as a signal of a more serious disease that may threaten the animal’s life.
How long does treatment take? Treatment is lengthy and depends on the cause. With idiopathic Horner’s syndrome the signs resolve on their own within 6-8 weeks. Full recovery may take several months.
Which breeds are most at risk? Among dogs, a higher incidence is seen in breeds such as the Golden Retriever and the Cocker Spaniel.
Can Horner’s syndrome affect both eyes? Although it happens very rarely, the signs of Horner’s syndrome are usually one-sided. There is, however, a separate condition — Haw’s syndrome — in which cats develop protrusion of both third eyelids.
