Rarely, it may involve superior part of cornea. The central cornea is usually of normal thickness, and the epithelium overlying the area of thinning is intact.
This association in the same patient suggests that pellucid marginal degeneration, keratoconus and keratoglobus (generalised thinning and globular protrusion of the cornea) may be varying spectrum of the same pathophysiology rather than separate disease processes. Pellucid marginal degeneration and keratoconus may exist in the same eye and in different eyes of the same patient. It differs from keratoconus in that it has no iron ring, its thinning is in inferior part without a cone and the corneal protrusion is located above (rather than) in the area of thinning. The condition results in high irregular astigmatism and rarely may develop acute hydrops (stromal oedema due to leakage of aqueous humour through breaks in Descemet’s membrane) or even perforation. The condition affects both men and women between 20 to 40 years of age.
Band of thinning measures about 1 to 2mm in width and extends from 4 to 8 o’clock position (about 6 to 8mm), leaving a normal band of 1 to 2mm from the limbus. There is no vascularisation, infiltration or scarring in the area of involved cornea. Pellucid marginal degeneration (PMD) is a bilateral (may be asymmetric), progressive, inferior crescentic peripheral thinning of the cornea.
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