What is Horner syndrome?

Horner syndrome typically involves one side of the face which causes the drooping of the eyelids where one pupil is small in size and sweating is reduced. The usual cause is the disturbance of the nerve fibers which links the brain to the eye. The syndrome can develop in individuals of all ages.

What are the possible causes of Horner syndrome?

Some of the nerve fibers that link the eyes to the brain take on a convoluted route. Starting at the brain, it goes down to the spinal cord and exit the spinal cord in the chest and return up to the neck alongside the carotid artery via the skull and into the eye. In case these nerve fibers are disrupted anywhere in its path, it results to Horner syndrome.

Horner syndrome can develop on its own or triggered by another condition such as lung cancer, tumors, injuries, swollen lymph glands in the neck, thoracic aortic aneurysm and dissection of the aorta or carotid artery. In addition, it might also be present at birth.

What are the symptoms?

Horner syndrome affects the eye on the same side that has disrupted nerve fibers. The symptoms include the following:

  • Drooping upper eyelid (ptosis)
  • Constricted pupil

In some individuals, the constricted pupil disrupts with ability to see in the dark. Nevertheless, most do not experience any difference in vision.

Horner syndrome

Horner syndrome affects the eye on the same side that has disrupted nerve fibers.

Remember that the affected side of the face might sweat less than usual or not at all and rarely appears flushed. In its congenital form, the iris of the involved eye remains bluish-gray as it is during birth.

How is it diagnosed

The condition is diagnosed based on the symptoms experienced by the individual. When confirming a diagnosis and determining where the issue is, the doctor will perform a 2-part test.

Initially, eye drops are applied that contain small amounts of cocaine or a different drug (apraclonidine) on both eyes. The reaction of the pupils indicates whether Horner syndrome is a possibility. If likely, the doctor will perform another test 48 hours later. Hydroxyamphetamine drops are applied on both eyes. The reaction of the pupils will indicate the general location of the issue.

MRI or CT scan of the spinal cord, brain, chest or the neck are often requested to check for the presence of tumors or other serious conditions that can disrupt the nerve fibers connecting the brain and the eye.


In case the cause is identified, it should be treated. Nevertheless, there is no exact treatment for Horner syndrome. Oftentimes, treatment is not required since the eyelid only appears to droop minimally.


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