Pellucid marginal degeneration (PMD) thins a crescent along the lower edge of the cornea, letting the surface above it sag into a distinctive shape. Like keratoconus it distorts vision beyond what glasses can fix; unlike keratoconus it hides at the edge, where only good imaging looks.
Diagnosis leans on tomography and thickness mapping, which catch the peripheral thinning a routine exam can miss. Management mirrors keratoconus: monitor for progression, consider cross-linking referral when it moves, and fit scleral lenses (which vault the entire irregular zone) for the vision glasses can’t deliver.
Because the thinning sits low, lens design matters even more; free-form scleral designs built from profilometry handle the asymmetry well.
The thinning sits at the lower margin rather than centrally, producing a distinctive map and heavy against-the-rule astigmatism. Treatment paths are nearly identical.
PMD hides at the corneal edge; it takes tomography — not just a glasses check — to see it.
Early on, partially. As the irregularity grows, scleral lenses become the reliable route to sharp vision.
Pellucid marginal degeneration (PMD) is a rare condition in which a band of the lower cornea near its edge, roughly from the 4 o'clock to the 8 o'clock position, slowly thins. The cornea just above the thinnest point bulges, causing astigmatism (uneven focusing) and blurrier vision, while the center of the cornea keeps its normal thickness. PMD affects both eyes, causes no inflammation and progresses slowly. The thinned area stays clear, which fits the name: "pellucid" means transparent.
PMD is rare, and it affects both eyes; it is a lesser-known relative of keratoconus that thins the lower edge of the cornea instead of its center. Because the thinning sits low at the edge, PMD is easy to miss: it shows up as astigmatism, and only tomography that maps the corneal edge reveals the thinning, which a routine glasses check does not. If your astigmatism keeps getting harder to correct with glasses, a corneal map is the way to check.
