Horner’s Syndrome in Dogs: Causes, Symptoms and Treatment

Horner's syndrome in dogs

Horner’s syndrome in dogs occurs when there is damage to the sympathetic nerve supply on one side of the face. The sympathetic nerves control functions like pupil size, eyelid position, and sweating. With Horner’s syndrome, the nerve supply to one eye is disrupted, causing changes in the eye’s appearance and function.

The affected eye may have a drooping eyelid, small pupil, and lack of sweating. The third eyelid may also protrude slightly. These changes occur because the sympathetic nerves control the muscles and glands in the eye area. Without proper nerve signals, the eyelid droops, the pupil constricts, and sweating decreases.

The most common cause of Horner’s syndrome in dogs is damage or injury to the sympathetic trunk or nerves. This includes nerve damage from trauma, tumors, or even insect stings. Sometimes, the cause of the nerve damage is unknown. Other potential causes include:

  • Middle ear infections that put pressure on the sympathetic nerves.
  • Tumors in the chest cavity that compress the sympathetic trunk.
  • Blood clots that disrupt sympathetic nerve function.
  • Spinal cord injuries at the level of the upper chest.

To determine the cause of Horner’s syndrome, your vet will perform a physical exam, reflex tests, and imaging like CT scans or MRIs. They may also do blood work or fluid analysis to check for infections. Identifying the underlying cause is important, especially if it’s something serious like a tumor. In some cases, the cause remains unknown, even after testing.

The good news is Horner’s syndrome itself is usually not painful or dangerous. However, the underlying condition causing it may require treatment. With treatment of the underlying problem, the signs of Horner’s syndrome often resolve over time. In mild cases with no known cause, your vet may recommend just monitoring your dog for changes.

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Horner's syndrome in dogs

Common Causes of Horner’s Syndrome

Horner’s syndrome in dogs has several potential causes, ranging from minor injuries to more serious medical conditions. The most common causes of Horner’s syndrome include:

Nerve Injuries or Tumors

The nerves that control eye movement and facial muscles can become damaged or compressed, often due to trauma or injury. For example, a dog may get a cut or puncture wound that severs or pinches the sympathetic nerves in the neck. Tumors growing in the neck region or chest cavity may also put pressure on these nerves. If caught early, nerve injuries and minor tumors are often treatable.

Middle Ear Infection

An infection of the middle ear, known as otitis media, can cause inflammation that reaches the nearby nerves controlling facial muscles and eye movement. While uncomfortable, most ear infections are not life-threatening and can be treated with medication. However, repeated or chronic infections require further diagnostics to determine the underlying cause.

Growth Disorders

In rare cases, Horner’s syndrome in puppies can be caused by a birth defect or damage during development in the womb. For example, a cyst in the neck region may compress sympathetic nerves. Most congenital causes of Horner’s syndrome are not preventable, though corrective surgeries or treatments may be options depending on the specifics.

Lyme Disease

Lyme disease, transmitted by deer tick bites, can sometimes cause Horner’s syndrome in dogs. The bacteria that causes Lyme disease may damage or inflame the nerves responsible for eye movement and facial control. A diagnosis of Lyme disease is made through blood testing, and the condition is usually treated successfully with a course of antibiotics.

To determine the underlying cause of your dog’s Horner’s syndrome, your vet will first obtain a medical history, perform a physical exam, and run some diagnostic tests such as blood work, imaging scans, or nerve function testing. In many cases, the cause can be identified and addressed to prevent permanent damage or disability. With prompt diagnosis and treatment, most dogs recover full nerve function and control of their eye and facial movements.

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Recognizing the Symptoms of Horner’s Syndrome

When a dog develops Horner’s syndrome, you may notice some distinct symptoms that affect the eye area. The earlier you recognize these signs, the sooner you can get your pup checked out by the vet.

Drooping Eyelid

One of the hallmark signs of Horner’s syndrome is a drooping eyelid, known as ptosis. This occurs because the nerves that control the eyelid muscles are impacted. At first, you may just notice your dog’s eyelid looks a bit sleepy or half-closed. Over time, the drooping can become more severe if the underlying cause isn’t addressed.

Constricted Pupil

You may also observe that your dog’s pupil appears constricted or smaller than normal. The nerves that control pupil dilation are affected in Horner’s syndrome, preventing the pupil from opening as wide as it should in dim light. Your vet can test pupil response to confirm this symptom.

Protruding Third Eyelid

In some cases of Horner’s syndrome, you’ll notice the third eyelid (nictitating membrane) protruding across part of the eye. This is due to muscle weakness caused by the nerve impairment. The third eyelid may cover just a portion of the eye or in severe cases, most of the eyeball.

Lack of Sweating

Since Horner’s syndrome impacts the sympathetic nervous system, your dog may exhibit decreased sweating on the affected side of the face. You can check for this by gently wiping the area around your dog’s eye with a damp cloth to see if one side produces less sweat. Lack of sweating, combined with a droopy eyelid and constricted pupil, strongly indicate Horner’s syndrome.

If your faithful companion shows any symptoms of Horner’s syndrome, schedule a vet exam right away. Early diagnosis and treatment are key to successfully managing this condition and preventing permanent damage. Your vet can determine the underlying cause and recommend appropriate treatment to get your pup back to perfect health.

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Treatment Options for Horner’s Syndrome

Medical Treatment

Unfortunately, there’s no way to reverse nerve damage that has already occurred. However, your vet may prescribe eye drops, such as cyclopentolate, to keep the affected eye dilated and prevent vision loss or amblyopia (“lazy eye”). Corticosteroids may also help reduce inflammation around the nerve. If an underlying condition like a tumor or blood clot is found, your vet will treat that to prevent further nerve damage.


In some cases, surgery may be recommended to remove a tumor or relieve pressure on the nerves. The specific procedure will depend on the underlying cause of your dog’s Horner’s syndrome. For example, if a tumor is present, your vet may surgically remove it. If a neck injury or blood clot is involved, your vet can perform a procedure to relieve pressure on the nerves or improve blood flow.

Recovery and Aftercare

Most dogs recover well after treatment for Horner’s syndrome, especially if an underlying condition was found and addressed. However, recovery depends on the severity of nerve damage. Nerves can regenerate over time, but it may take weeks, months or even up to a year. Follow-up vet exams will check your dog’s pupil size and symmetry, as well as eye pressure and vision.

It’s important to keep your dog as comfortable as possible during recovery. Provide a stress-free environment, soft bedding, warmth, and any prescribed medications as directed by your vet. Gently massage the area around the affected eye and ear to stimulate nerve function and circulation. Be very careful not to put any pressure directly on the eye itself.

With treatment and time, many dogs experience a full recovery from Horner’s syndrome. However, some may be left with minor nerve damage that results in a slightly droopy eyelid or pupil. As long as vision remains normal and the eye is otherwise healthy, this should not cause any issues for your dog. It’s a good idea to have your vet examine the eye periodically to monitor for any changes, even after recovery.