Posterior polymorphous dystrophy is an inherited condition of the posterior layer of the cornea. The cornea has three basic layers. The outer or skin layer is called the epithelium. The middle layer is thick, tough tissue, contains very few cells, and is called the stroma. The inner layer lining the undersurface of the cornea is called the endothelium.
The endothelium is a different, specialized cell that is required to keep the cornea clear by pumping fluid out of the cornea and into the front of the eye. Posterior polymorphous dystrophy means that some of the endothelial cells do not work properly and form a small mark on the endothelium.
Usually, this condition has no effect on the patient, but if extensive, it can permit corneal clouding because these abnormal endothelial cells can no longer pump fluid out of the cornea, at least to the extent needed.