![]() Several surgical procedures have been used in an attempt to improve visual acuity when spectacles and contact lenses do not provide adequate vision correction. The vast majority of PMD patients are managed using spectacles and contact lenses. In rare cases, patients may present with a sudden loss of vision and excruciating ocular pain due to corneal hydrops or spontaneous perforation. Visual signs and symptoms include longstanding reduced visual acuity or increasing against-the-rule irregular astigmatism leading to a slow reduction in visual acuity. Unless corneal topography is evaluated, early forms of PMD may often be undetected however, in the later stages PMD can often be misdiagnosed as keratoconus. Ocular signs and symptoms of patients with PMD differ depending on the severity of the condition. The prevalence and aetiology of this disorder remain unknown. The condition is most commonly found in males and usually appears between the 2nd and 5th decades of life affecting all ethnicities. ARVO abstract #2893, 2004.Pellucid marginal corneal degeneration (PMD) is a rare ectatic disorder which typically affects the inferior peripheral cornea in a crescentic fashion. Wavefront aberrations in patients with keratoconus and pellucid marginal degeneration. Within that area, you may see two distinct zones of thinning, along with the clinical signs of both disorders.ġ. In these cases, you will see a much broader area of thinning that starts almost from the center of the cor-nea or just inferior to the center, but it expands almost like a band right down to the inferior aspect of the cornea, Dr. 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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