Pellucid marginal degeneration: Detection, discrimination from other corneal ectatic disorders and progression.

Pellucid marginal degeneration (PMD) is a non-inflammatory ectatic corneal disease characterized by a narrow band of corneal thinning separated from the limbus by a relatively uninvolved area 1-2 mm in width. It is a rare corneal disorder that shares many clinical characteristics with other corneal ectasias, such as keratoconus, keratoglobus or Terrien marginal degeneration. PMD usually starts later in life than keratoconus and progresses slower than keratoconus. Slit-lamp examination is very useful to distinguish PMD from other corneal ectatic disorders with inflammatory nature. Corneal topographic indices and the classical crab-claw topographic pattern cannot be used as the main tool to distinguish between PMD and keratoconus. New Scheimpflug imaging-based devices have shown the importance and usefulness of the pachymetric map for an appropriate diagnosis of PMD. In addition, biomechanical and densitometric properties have been studied as complementary techniques to help in the diagnosis of PMD.