Horner’s Syndrome

Horner’s syndrome is a collection of clinical signs that are seen together and often have a limited number of causes. These particular clinical signs seen with Horner’s occur due to damage to the sympathetic nerve supply to the eye:

These symptoms are not painful though they may interfere with vision due to the third eyelid elevation.

Sympathetic Nerve Pathway to the Eye

The sympathetic nervous system is part of the autonomic nervous system that “automatically” controls a variety of bodily functions, including pupil size, without conscious effort. It also controls the fight or flight response. The sympathetic nerve supply to the eye originates at the base of the brain and has 3 sections. The first section of the nerve then travels down the neck within the spinal cord. The nerve exits the spinal cord in the area near the shoulders and then travels through the chest cavity. The second section of the nerve travels up the neck to the base of the skull. The third section of the nerve then takes an elaborate route through the skull where it is closely associated with the bone of the middle ear before finally reaching the orbit. Inside the orbit, it branches to innervate certain structures of the eye. Damage to this nerve anywhere along its path results in loss of sympathetic innervation leading to the characteristic signs listed above.

Determining the Location of the Damage

Damage can occur anywhere along the nerve pathway:

To try to determine the location of nerve damage, your veterinary ophthalmologist may apply eye drops that stimulate different sections of the nerve and monitor the response. This usually takes about 15 to 30 minutes. Determining the location of the damage is important because it may help to uncover the cause of the damage or other problems in that area. For example, patients with First order Horner’s syndrome may have a history of brain or spinal cord trauma. Studies also have demonstrated a correlation between endocrine diseases, like hypothyroidism or Cushing’s disease, in patients with Horner’s syndrome. Patients with Second order Horner’s syndrome might have thoracic disease, including tumors within the chest cavity or neck trauma. Third order Horner’s syndrome is the most commonly seen form in dogs and is most often associated with inner or middle ear disease. You may be referred back to your veterinarian for evaluation of systemic disease or a close examination of the ear canal. If the Horner’s is suspected to be First or Second order, the help of a veterinary neurologist may be recommended. Many times the location of damage cannot be determined without advanced imaging.


We can temporarily treat the symptoms of Horner’s syndrome with the eye drops used for diagnostic purposes. If your pet is having difficulty with vision due to Horner’s syndrome, your veterinary ophthalmologist may prescribe eye drops called sympathomimetics that mimic the sympathetic system supply. Also, an anti-inflammatory may be recommended to symptomatically treat conjunctivitis (inflammation of the pink tissue around the eye), if present.

While most patients with Horner’s syndrome do not warrant medical therapy for the eye, determining and treating the underlying cause of this syndrome can be important as some diseases are very serious and potentially life-threatening. Luckily, most patients have few complications from Horner’s syndrome and the signs clear up on their own in a few months.

If you have any questions regarding Horner’s Syndrome, or any other ocular condition, please do not hesitate to call us at Eye Care for Animals.

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