![]() Thus, if corneal topography is used for diagnosis, it should be in conjunction with clinical findings of peripheral, inferior corneal thinning. Corneal topography may show a "crab claw-like" appearance, a finding that is seen in both keratoconus and in pellucid marginal degeneration. 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. ![]() The gold standard diagnostic test for PMD is corneal topography. PMD lacks apical corneal scarring, Rizutti's phenomenon, Munson's sign, and the central corneal thickness is usually normal. The inferior peripheral thinning is seen between the 4 o'clock and 8 o'clock positions. In PMD we can see high against the rule astigmatism along with horizontal bow ties. The portion of the cornea that is immediately adjacent to the limbus is spared, usually a strip of about 1–2 mm. 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 Bowman's layer of the cornea may be absent, irregular, or have ruptured areas. There may be an increase in the number of mucopolysaccharides in the corneal stroma. The thinning of the corneas may approach 20% of normal thickness. PMD is an idiopathic, non-inflammatory condition. ![]() Normally, PMD does not present with vascularization of the cornea, scarring, or any deposits of lipid. This is described as a "beer belly" appearance since the greatest protrusion occurs below the horizontal midline (unlike keratoconus). The cornea just above the region of thinning is of normal thickness, and may protrude anteriorly, which creates an irregular astigmatism. The distribution of the degeneration is crescent or arcuate shaped. PMD is characterized by bilateral thinning (ectasia) in the inferior and peripheral region of the cornea. While PMD usually affects both eyes, some unilateral cases have been reported. However, in rare cases, PMD may present with sudden onset vision loss and excruciating eye pain, which occurs if the thinning of the cornea leads to perforation. Pain is not typically present in pellucid marginal degeneration, and aside from vision loss, no symptoms accompany the condition.
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