What is Horner's syndrome?
A person with Horner's syndrome has irritation of the nerves that supply the eye and face. The nerves do not function correctly, which results in a drooping eyelid, small pupil and the lack of perspiration on one side of the face. Horner's syndrome can be caused by a tumor in the neck or upper chest that is irritating the nearby nerves.

What are the symptoms of Horner's syndrome?
Symptoms of Horner's syndrome include eyelid weakness, drooping eyelid, constricted pupil, and absence of sweating: all on the same side of the face. Additional symptoms of Horner's syndrome depend on the underlying cause.

How does the doctor treat Horner's syndrome?
The treatment for Horner's syndrome depends on the underlying cause.