Pellucid marginal degeneration

Pellucid marginal degeneration (PMD) is a rare, non-inflammatory corneal ectatic disorder that affects the eyes. It is characterized by a crescent-shaped band of corneal thinning, typically located in the inferior peripheral cornea between the 4 o'clock and 8 o'clock positions, approximately 1-2 mm from the limbus (the border between the cornea and the white of the eye). Unlike keratoconus, another corneal thinning disorder, the area of maximum corneal protrusion in PMD occurs above the zone of thinning rather than at the thinnest point itself. This distinctive pattern produces a characteristic "against-the-rule" or irregular astigmatism that progressively impairs vision. Patients with pellucid marginal degeneration typically experience gradually worsening blurred or distorted vision due to the irregular corneal shape. The condition is usually bilateral, affecting both eyes, though it may be asymmetric in severity. The cornea in the thinned area remains clear (pellucid means transparent), without vascularization, lipid deposition, or significant scarring in early stages. Acute corneal hydrops (sudden swelling due to a break in Descemet's membrane) can occur in advanced cases, causing sudden pain and vision loss. Management of PMD depends on disease severity. In early stages, spectacles or soft contact lenses may adequately correct vision. As the condition progresses, rigid gas-permeable contact lenses, scleral lenses, or hybrid lenses are often required to provide a regular refracting surface. Corneal collagen cross-linking (CXL) has been explored as a means to stabilize corneal thinning and halt progression, though evidence remains limited. In advanced cases where contact lens fitting becomes impossible or vision is severely compromised, surgical options include crescentic lamellar keratoplasty, wedge resection, or penetrating keratoplasty (corneal transplant). The etiology of PMD remains poorly understood, and no definitive genetic basis has been established.

Data sourced from FDA regulatory filings and ClinicalTrials.gov. Updated periodically.

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