Ptosis is the medical term for drooping of the upper eyelid, a condition that may affect one or both eyes. Droopy eyelids in general occur when the edge of the upper eyelid (the eyelid margin) falls from its normal position. When the edge of the eyelid falls too low and covers part of the pupil, it can block the upper part of your vision. In most cases, a drooping upper eyelid results from aging of previously normal structures. Typically, the tendon of the muscle responsible for lifting the eyelid stretches and the eyelid falls. Surgical correction of a drooping upper eyelid involves repairing the stretched tendon.
It is not uncommon for a person to develop a droopy upper eyelid following cataract or other eye surgeries. This is because manipulation of the eyelid during a procedure can cause weakening of the muscle that holds the eye open.