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Ptosis is from the Greek, to fall, and describes a drooping of one or both upper eyelids.
It may be congenital or acquired, and in either, may be hereditary. In the normal eye position, the upper lid is situated about 2 mm below the limbus and 2 mm above the pupillary margin. The degree of ptosis is assessed by the amount of excursion of the lid margin from full downgaze to full upgaze - less than 4mm is poor, more than 8mm, good.