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ptosis pto·sis (tō'sĭs)
n. pl. pto·ses (-sēz)
Abnormal lowering or drooping of an organ or a part, especially a drooping of the upper eyelid caused by muscle weakness or paralysis.
drooping of the upper eyelid. The condition may be congenital or acquired and can cause significant obscuration of vision. In congenital ptosis the muscle that elevates the lid, called the levator palpebrae superioris, is usually absent or imperfectly developed. If severe and not corrected in a timely manner, congenital ptosis can lead to amblyopia and permanent vision loss. Congenital palsy of the third (oculomotor) cranial nerve (which normally stimulates elevation of the upper lid) is a more rare cause of congenital ptosis.