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1. A sinking down or prolapse of an organ. 2. SYN: blepharoptosis. [G. p., a falling]

- p. adiposa SYN: blepharochalasis.

- aponeurogenic p. drooping of the eyelid caused by dehiscence of the tendon of the levator muscle.

- p. sympathetica SYN: Horner syndrome.

pto·sis 'tō-səs n, pl pto·ses -.sēz a sagging or prolapse of an organ or part <renal \ptosis> esp a drooping of the upper eyelid (as from paralysis of the oculomotor nerve)

drooping of the upper eyelid, for which there are several causes. It may be due to a disorder of the third cranial nerve (oculomotor nerve), in which case it is likely to be accompanied by paralysis of eye movements causing double vision and an enlarged pupil. When part of Horner's syndrome, ptosis is accompanied by a small pupil and an absence of sweating on that side of the face. It may be due to myasthenia gravis, in which the ptosis increases with fatigue and is part of a more widespread fatiguable weakness. Ptosis is associated with the severe pain and other symptoms of a cluster headache. It may also occur as an isolated congenital feature or as part of a disease of the eye muscles, when it is associated with weak or absent eye movements.