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Horner Syndrome

(Horner's Syndrome)

by Phillip Low, MD

Horner syndrome affects one side of the face, causing the eyelid to droop, the pupil to become small (constricted), and sweating to decrease. The cause is disruption of the nerve fibers that connect the brain to the eye.

  • Horner syndrome may occur on its own or result from a disorder that disrupts nerve fibers connecting the brain with the eye.

  • The upper eyelid droops, the pupil remains small, and the affected side of the face may sweat less.

  • Doctors test the pupil to see whether it can widen, and may do imaging tests to look for a cause.

  • The cause, if identified, is treated.

Horner syndrome can develop in people of any age.


Some of the nerve fibers that connect the eyes and brain take a circuitous route. From the brain, they go down the spinal cord. They exit the spinal cord in the chest, then go back up the neck beside the carotid artery, through the skull, and into the eye. If these nerve fibers are disrupted anywhere along their pathway, Horner syndrome results.

Horner syndrome may occur on its own or be caused by another disorder. For example, it can be caused by disorders of the head, brain, neck, or spinal cord, such as lung cancer, other tumors, swollen lymph glands in the neck (cervical adenopathy), dissection of the aorta or carotid artery (a tear in the lining of the artery's wall), a thoracic aortic aneurysm (a bulge in the aorta's wall), and injuries. Horner syndrome may be present at birth (congenital).


Horner syndrome affects the eye on the same side as the disrupted nerve fibers. Symptoms include a drooping upper eyelid (ptosis) and a constricted pupil (miosis). The affected side of the face may sweat less than normal or not at all, and rarely, it appears flushed. In the congenital form, the iris of the affected eye remains blue-gray as it is at birth.


The disorder is suspected based on symptoms. To confirm the diagnosis and determine where the problem is, doctors do a two-part test. First, they apply eye drops that contain small amounts of cocaine or another drug (apraclonidine) to both eyes. How the pupils react indicates whether Horner syndrome is likely. If it is likely, doctors do another test 48 hours later. They apply hydroxyamphetamine drops to both eyes. How the pupils react to this drug indicates the general location of the problem.

Magnetic resonance imaging (MRI) or computed tomography (CT) of the brain, spinal cord, chest, or neck is often needed to look for tumors and other serious disorders.


The cause, if identified, is treated. However, there is no specific treatment for Horner syndrome. Often, no treatment is necessary because typically, the eyelid only droops very slightly.

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