Akorn Animal Health

Horner's Syndrome Horner's Syndrome

Horner's Syndrome A syndrome is a collection of symptoms that often has a limited number of causes and can thus help doctors diagnose the underlying problem. The following clinical signs collectively indicate damage to the sympathetic nerve supply to the eye, and is referred to as Horner’s Syndrome. The cardinal signs of horners syndrome include:

  • A constricted pupil that does not dilate (miosis)
  • Drooping of the upper eyelid (ptosis)
  • A sunken appearance of the eye (enophthalmos)
  • Elevation of the third eyelid
  • Red eye (conjunctival hyperemia)

These symptoms are not painful, however may interfere with vision due to the third eyelid elevation and drooping of the upper eyelid.


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. The sympathetic nerve supply to the eye originates at the base of the brain. 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. There, it branches to innervate the 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.


Damage can occur anywhere along the nerve pathway:

  • Damage within the brain and spinal cord, is called first order Horner’s syndrome.
  • Damage between the chest cavity and the base of the skull, is called second order Horner's syndrome
  • Damage between the base of the skull and eye, is called third order Horner's syndrome.

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. Determining the location of the damage is important because it may highlight 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 of endocrine diseases like hypothyroidism in patients with Horner’s syndrome. Patients with second order Horner’s syndrome might have thoracic disease, including tumors or infection within the chest cavity or neck trauma. Patients with third order Horner’s syndrome have a higher probability of middle ear diseases or unknown cause (idiopathic).

Depending on the suspected location of the nerve damage further diagnostic testing may be advised such as advanced imaging (CT/MRI), thoracic radiographs, and blood work.


We can temporarily treat the symptoms of Horner’s syndrome for diagnostic purposes. These medications are not prescribed to treat horners syndrome as can cause dangerous side effects such as high blood pressure. An anti-inflammatory medication 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 is important. If detected early, the disease causing the syndrome is more likely to be able to be treated. If left undetected for too long, treatment options may be limited.

If you have any question about this information please contact your veterinarian.