Horner's Syndrome: Page 2 of 3

More Info

Any dog can develop Horner’s syndrome. However, of all dogs diagnosed with Horner's syndrome, Golden Retrievers and Cocker Spaniels are the most commonly affected breed because of chronic ear infections.

Damage to the nerve can occur anywhere along its pathway from the brain, to the spinal cord, and back up to the eye.  The damage can be classified as either central (in the brain), pre-ganglionic (between the brain and the ganglion), or post-ganglionic (after the ganglion)., The damage can be temporary or permanent, and can be caused by brain or spinal cord tumors, blood clots, trauma, or idiopathic (unknown cause).

In a central lesion, the nerve is interrupted somewhere before it exits the spinal cord. Animals with this form usually exhibit other neurologic signs (head tilt, stumbling, and reduced coordination).

Preganglionic lesions are the result of an interruption of the nerve between the spinal cord and the synapse in the ganglion, and are generally due to tumors in the chest or neck (such as thyroid tumors or mediastinal lymphoma), or trauma to the neck cause by a strong jerk or pulling on the collar or leash.

In post-ganglionic lesions, the nerve has been interrupted between the synapse and the eye itself. Middle ear disease (inflammation, masses, or polyps) or vigorous ear cleaning can damage the nerve. However, the cause of the majority (42-55%) of post-ganglionic Horner’s lesions is unknown.

However, in most cases of post-ganglionic Horner’s syndrome, the cause is unknown. In these cases, patients usually spontaneously recover within an average of 16 weeks. It is important to note that if it is concluded that Horner’s syndrome does have an identifiable underlying cause, you should have your dog treated for that condition to also alleviate the symptoms associated with Horner’s syndrome.

Clinical Signs

Horner’s syndrome usually occurs suddenly, and within a few hours you will observe that your dog has the 3 hallmark findings of Horner’s syndrome: drooping to the upper eyelid, (ptosis), constriction of the pupil of the affected eye (miosis), a sunken appearance in the affected eye (enophthalmos).  As mentioned earlier, the sunken globe allows the 3rd eyelid to become elevated, which is often times the most noticeable clinical sign.


Horner’s syndrome is usually diagnosed based on the symptoms, including a constricted pupil (miosis), drooping upper eyelid (ptosis), sunken globe (enophthalmos), and protrusion of the third eyelid. However, localization of the lesion is important, as treatment will depend on where the damage has occurred. While there are several initial diagnostic tests that will be performed to determine if there is an underlying cause of Horner’s syndrome in your pet, a thorough physical examination is the first step.

A pharmacologic test should be performed by a veterinary ophthalmologist to further localize the site of the nerve abnormality and determine if it is pre- or post-ganglionic Horner's syndrome.  A phenylephrine test is used to both localize and confirm the diagnosis of Horner’s syndrome. In this test, the ophthalmologist will place an eye drop consisting of 1% phenylephrine in both eyes. This drug stimulates the sympathetic nervous system - if the lesion is post-ganglionic, the symptoms in the affected eye will resolve, generally within 15 minutes.  If the damage is in the central nervous system or in the pre-ganglionic nerve, the clinical signs will not resolve quickly, and further diagnostics will be required.  Causes arising from problems within the brain, spinal cord, and chest may require further testing, such as a CT or MRI scan, and a consultation with a veterinary neurologist should be pursued.  Blood tests and chest x-rays may also be needed.  A correlation has been shown between Horner’s syndrome and hypothyroidism in dogs, so it may be wise to also have your pet tested for hypothyroidism to be sure that all bases are covered with regards to your dog’s health. Chest x-rays will be used to rule out the possibility of your pet having a chest tumor. Treatment is then based on these test results.