Diagnosis
The center of the cornea shows normal thickness, with an intact central epithelium, but the inferior cornea exhibits a peripheral band of thinning, to about 1-2 mm. The portion of the cornea that is immediately adjacent to the limbus is spared, usually a strip of about 1-2 mm. In PMD we can see high against the rule astigmatism along with horizontal bow ties. The inferior peripheral thinning is seen between the 4 o'clock and 8 o'clock positions.
Unlike keratoconus, PMD lacks apical corneal scarring, Rizutti's phenomenon, Munson's sign, and the central corneal thickness is usually normal.
The gold standard diagnostic test for PMD is corneal topography. However, it may not as specific as corneal pachymetry, because corneal topography only evaluates the degree and distribution of surface irregularities on the cornea, not the thickness of the cornea. Corneal topography may show a "crab claw-like" appearance, a finding that is seen in both keratoconus and in pellucid marginal degeneration. Thus, if corneal topography is used for diagnosis, it should be in conjunction with clinical findings of peripheral, inferior corneal thinning.
Read more about this topic: Pellucid Marginal Degeneration