Pellucid marginal degeneration (PMD)
Pellucid marginal degeneration, or PMD, is often confused with keratoconus, and indeed it is a type of irregularity affecting the cornea which results in visual impairment. PMD is caused by a thinning of a narrow band of the cornea between 4 and 8 o’clock, approximately 2mm in from the edge of the cornea and the white of the eye.