Ptosis occurs when the muscles that raise the eyelid are not strong enough to do so properly. It can affect one eye or both eyes. People can be born with ptosis. Congenital ptosis is hereditary. Ptosis may also be caused by damage/trauma to the muscle which raises the eyelid, or damage to the nerve which controls this muscle. Such damage could be a sign or symptom of an underlying disease which may cause weakness in muscles or nerve damage. Depending upon the severity, ptosis can need to be treated with surgery.