In Horner's syndrome, on one side of the face, the eyelid droops, the pupil is small (constricted),  and sweating is decreased.  The cause is disruption of the nerve fibers that connect the eye and the brain. 

• Horner's syndrome may occur on its own or result from a disorder that disrupts nerve fibers connecting the eyes and brain. 
  
• 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's syndrome can develop in people of any age. 

Causes 

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's  syndrome results. Horner's 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 thoracic aortic aneurysm,  and injuries. Horner's syndrome may be present at birth (congenital). 

Symptoms

Horner's 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. 

Diagnosis and Treatment 

The disorder is suspected based on symptoms. To confirm the diagnosis, doctors may apply eye drops that contain small amounts  of cocaine to the affected eye. If the pupil does not widen (dilate) after 30 minutes, Horner's  syndrome is diagnosed. Doctors  may apply other drugs to the eye later. How the pupil reacts to them indicates the general location of the damage. 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's syndrome. Often, no treatment  is necessary because, typically,  the eyelid only droops very slightly.