A secondary muscle that raises the eyelid is Muller's muscle. This muscle is supplied by different nerves called sympathetic nerves. If these nerves are damaged, a milder sagging of the eyelid is present. Because these sympathetic nerves travel at the top of the chest cavity before going up the neck to the head, abnormalities in the upper areas of the lungs can lead to ptosis. A Pancoast tumor is an example of a type of lung cancer that can lead to this condition.
Mechanical ptosis occurs when the weight of the eyelid is too great for the muscles to lift.
Tumors or excess skin and fat can weigh down the eyelid. Involutional ptosis develops when the attachments of the muscle to the eyelid degenerate. This occurs with aging or with stretching of the eyelid tissues, due to swelling. In this condition, the nerve and muscle work properly. Since the muscle is no longer fully attached to the solid structures inside the eyelid, the muscle contracts but the eyelid does not come up.