Corneal Cross-Linking for Pellucid Marginal Degeneration

Pellucid marginal degeneration (PMD) is a rare, bilateral, asymmetric noninflammatory disorder of the inferior cornea characterized by an arcuate band of thinning, occurring in the 4- to 8-o’clock position, 1 to 2 mm from the limbus. The resulting progressive ectasia, just above the thinned region of the cornea, causes a reduction of visual acuity due to astigmatism and, rarely, acute hydrops and perforation. Shlaeppi first described the disease in 1957, using the word “pellucid” (ie, “clear”), referring to the absence of corneal scarring or neovascularization despite protrusion and ectasia.