Pellucid marginal degeneration

Lore & Background

Pellucid marginal degeneration is an idiopathic, non-inflammatory condition in which the cornea thins, sometimes approaching 20% of normal thickness. The thinning occurs in a crescent or arcuate shape in the inferior periphery, sparing a 1–2 mm strip adjacent to the limbus. The cornea above the thinning may protrude anteriorly, creating a 'beer belly' appearance and irregular astigmatism. Unlike keratoconus, PMD lacks apical scarring, Rizutti's phenomenon, and Munson's sign.

Reader's Guide

Pellucid marginal degeneration is significant as a rare corneal ectatic disorder often confused with keratoconus, yet distinct in its peripheral, inferior thinning pattern. Diagnosis relies on corneal topography, which may show a 'crab claw-like' appearance, but should be combined with clinical findings of peripheral thinning. Treatment begins with eyeglasses or contact lenses, including scleral rigid gas permeable lenses, which often provide exceptional vision. Surgical options such as Intacs implants, corneal collagen cross-linking, and corneal transplant are reserved for those who cannot tolerate contact lenses. The condition's rarity and unknown prevalence contribute to diagnostic challenges, and its association with other diseases (e.g., chronic open angle glaucoma, retinitis pigmentosa) remains non-causal. Prognosis involves progressive visual decline due to astigmatism, but no standard surgical approach has been established.

Did You Know?

• The word 'pellucid' means clear, indicating that the corneas retain clarity in this condition.
• PMD typically presents with painless vision loss affecting both eyes, but rare perforation can cause sudden vision loss and excruciating pain.
• Corneal topography may show a characteristic 'crab claw-like' appearance, a pattern not typically seen in keratoconus.
• People with PMD are poor candidates for LASIK and photorefractive keratectomy due to corneal thinning.