August 30, 2025
Horner’s Syndrome in Dogs
When your dog’s eyes reflect more than emotion.
Horner’s Syndrome is a neurological condition in dogs that affects the eye and facial muscles, often appearing suddenly and causing noticeable changes to one side of the face. While it may look alarming, the condition is usually painless—but it can be a sign of a more serious underlying issue.
At Riri Vet Unit, we believe in educating pet parents so you can spot issues early and seek proper care. Here’s everything you need to know about Horner’s Syndrome in dogs.
What is Horner’s Syndrome?
Horner’s Syndrome occurs when there is damage or disruption to the sympathetic nerve pathway that controls many automatic functions around the eye and face. These include pupil dilation, eyelid positioning, tear production, and the position of the eyeball.
This pathway is long and complex—it starts in the brain and passes through the spinal cord, chest, neck, and finally reaches the eye. Damage at any point along this route can lead to the classic symptoms of Horner’s Syndrome.
Classic Symptoms of Horner’s Syndrome in Dogs
Horner’s Syndrome typically affects one side of the face (unilateral), though it can occasionally occur on both sides. The hallmark symptoms include:
| Symptom | Description |
|---|---|
| 🟤 Ptosis | Drooping of the upper eyelid due to loss of nerve stimulation |
| ⚫ Miosis | Abnormally small pupil that doesn’t dilate well in dim light |
| 👁️ Enophthalmos | The eyeball appears sunken into the socket |
| 🧿 Prolapsed Third Eyelid | The “nictitating membrane” becomes visible and covers part of the eye |
| 🧊 Reduced Sweating or Vasoconstriction (in some species, more notable in horses or humans) | Not usually observed in dogs but may affect skin temperature or moisture in some cases |
These signs appear suddenly and are often more noticeable in short-haired or light-colored breeds.
Classification of Horner’s Syndrome (Types)
Veterinarians classify Horner’s Syndrome into three categories, based on where the nerve damage occurs:
1. First-Order Neuron Lesions (Central Nervous System)
Origin: Brainstem or cervical spinal cord
Causes: Trauma, brain tumors, intervertebral disc disease (IVDD), strokes, spinal cord injury
2. Second-Order Neuron Lesions (Chest/Neck Area)
Origin: From the spinal cord to the base of the skull
Causes: Neck trauma, chest tumors, injuries from collars, brachial plexus injuries
3. Third-Order Neuron Lesions (Behind the Eye or Middle Ear)
Origin: Middle ear to the eye
Causes: Otitis media (middle ear infection), orbital (eye socket) trauma, post-surgical complications
Idiopathic Horner’s Syndrome is the term used when no cause can be found—this is the most common form and usually has a good prognosis.
Is Horner’s Syndrome Contagious?
No. Horner’s Syndrome is not contagious. It cannot be transmitted between animals or from pets to humans. However, underlying causes like bacterial ear infections might need to be managed to prevent spread to other pets.
Prevention of Horner’s Syndrome
While Horner’s Syndrome itself may not be entirely preventable due to its diverse causes, you can lower the risk by:
✅ Promptly treating ear infections and ear mites
✅ Avoiding excessive neck pressure from collars or rough play
✅ Seeking veterinary help for neurological signs like balance issues or abnormal gait
✅ Keeping your pet’s vaccinations and parasite control up to date
✅ Scheduling regular wellness exams to catch early signs of inner ear or spinal problems
Diagnosis of Horner’s Syndrome
Your vet will perform a thorough physical and neurological exam, and may use the following diagnostic tools:
Eye exam to confirm Horner’s signs
Phenylephrine test (eye drops to localize the lesion site)
Ear examination (otoscope or imaging for middle ear disease)
X-rays or CT scans for chest or neck abnormalities
MRI for brain or spinal cord lesions
Blood tests to check for infections or systemic illness
At Riri Vet Unit, we use evidence-based veterinary diagnostics to determine the cause of your pet’s condition and formulate a tailored treatment plan.
Prognosis
The outlook depends on the underlying cause:
Idiopathic cases – Excellent prognosis; most recover completely
Infections – Good prognosis if treated early
Trauma or tumors – Variable prognosis depending on severity and treatment
When Should You See a Vet?
Contact your vet immediately if you notice:
Sudden changes in your dog’s eyes
Drooping eyelid or uneven face
Visible third eyelid or change in pupil size
Quick action can prevent complications and allow for faster recovery.
In Summary
| Topic | Key Details |
|---|---|
| Condition | Horner’s Syndrome |
| Affects | Eye and facial muscles |
| Symptoms | Drooping eyelid, small pupil, third eyelid protrusion, sunken eye |
| Causes | Ear infections, spinal injuries, tumors, trauma, or unknown |
| Contagious? | ❌ No |
| Treatable? | ✅ Yes, depending on the cause |
| Prognosis | Excellent in idiopathic cases |
