Pellucid marginal degeneration

A rare disorder of the anterior segment of the eye characterized by slowly progressive, bilateral, non-ulcerative, non-inflammatory, clear thinning of the inferior portion of the peripheral cornea (extending from the 4 o'clock to the 8 o'clock position), with an area of corneal protrusion above the point of maximal thinning, resulting in against-the-rule astigmatism with decreased visual acuity. The central cornea is of normal thickness.