So, more vertical coma is produced, reflecting this topographic asymmetry, he adds.Ī small percentage of patients may actually have both conditions. In keratoconus, while the ectasia is more centrally located, its usually still inferior to the apex. This trefoil is the optical effect of the inferior area of elevation on the topography, says Jay S. 1 Because PMD is located inferiorly, significant trefoil is the typical finding on wavefront aberrometry. Measure the distance from this point to the corneal apex, Dr. To determine the peak elevation index, place the cursor over the highest elevation point on the topographers elevation map. It demonstrates that keratoconus is a mid-peripheral thinning disorder, while PMD is a peripheral thinning disorder. This is the main differentiating feature between keratoconus and PMD, Dr. Two additional ways to determine which ectatic disease the patient has:Ī peak elevation index (PEI).The average PEI for keratoconic eyes is about 1.95mm from the corneal apex, while the average PEI for eyes with PMD is an average of 3.5mm from the corneal apex. Note, however, the increased invagination present with PMD. The axial map shows a topographic similarity between keratoconus and PMD with both manifesting superior flattening and inferior steepening. Typically, a prolate shape factor in excess of 0.6 (numerical value) on a corneal topographer is almost diagnostic of keratoconus. The higher the shape factor, the greater the difference between the center and the periphery, as opposed to an oblate shape in which the peripheral cornea is steeper than the central cornea, Dr. A prolate shape indicates that the center of the cornea has a steeper curvature than the periphery. If this is the case, consider these differences: PMD corneas are typically low prolate to oblate in shape, while keratoconus corneas are almost always markedly prolate in shape. However, the topography pattern can appear similar to that of PMD, Dr. Patients who have very advanced keratoconus typically show inferior steepening. PMD gives a characteristic topography pattern of birds kissing, a gull-winged shape or a moustache pattern, with significant invagination along the flat axis of the cornea, says optometrist Kenneth A. How can I distinguish between keratoconus and pellucid marginal degeneration (PMD)?Ī: PMD typically shows marked against-the-rule astigmatism on the axial map. Q: I have several patients with inferior steepening on topography and non-specific corneal findings, suggestive of some form of non-inflammatory thinning disorder.
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