![]() , However, it has been found that this characteristic, claw-like pattern of peripheral steepening on corneal topographic analysis is not always associated with the sure diagnosis of PMD. Indeed, the other typical morphological patterns which are called the “crab claw” pattern or “butterfly” pattern are considered the most characteristic patterns in PMD due to the presence of the corneal protrusion below the area of thinning, with normal thickness of the central cornea. įigure 1: Flattening of the vertical meridian with characteristic keratometric pattern of a marked “against-the-rule” astigmatism obtained by a rotating Scheimpflug device Pentacam ® corneal tomographer (OCULUS Optikgeräte GmbH, Germany) The classic topographic presentation of PMD is flattening of the vertical meridian above a cresentric band of thinning, with characteristic keratometric pattern of a marked “against-the-rule” astigmatism. It is essential to measure the degree and location of the thinning, which is the main clinical key to distinguish the different entities of corneal ectasia. Topographic Patterns of Pellucid Marginal DegenerationĬorneal topographic evaluation is the main method to confirm the presence of an underlying ectatic condition. The major limitation for differentiating between KCN and PMD by slit lamp is in the detection of early and subclinical stages of the diseases, because these are not associated with any biomicroscopic signs or these signs are unremarkable. īy slit-lamp biomicroscopy, a 1–2-mm margin of normal cornea was found between the thinning and limbus with peripheral corneal thinning from the 4 to 8-o'clock positions. This disease is usually asymptomatic however, a progressive deterioration in uncorrected and best-corrected visual acuity can occur due to irregular astigmatism induced by the corneal ectasia in the most advanced cases. Īge of presentation of PMD is usually diagnosed between the second and the fifth decades, in comparison to KCN, which is diagnosed mostly between the puberty and the third decade of life. The clinical diagnosis of PMD depends on the suspicion of the disease and careful evaluation by various available diagnostic tools including retinoscopy, slit-lamp biomicroscopy, keratometry, keratoscopy, pachymetry, and corneal topography. ![]() Several studies, have proposed that PMD is a form of KCN of peripheral type due to the close similarities between both conditions.Ĭlinical Diagnosis of Pellucid Marginal Degeneration Īccording to multiple studies about the incidence of PMD, this disorder is considered as a rare condition, less common than other ectatic corneal diseases, such as keratoconus (KCN), but more common than others, such as keratoglobus or posterior KCN. Patients with PMD mostly presented in their third to fourth decade of life with decreased visual acuity due to increase in against-the-rule astigmatism. This ectatic disease commonly involves the inferior cornea, with an area of thinning extending from the 4-o´ clock to the 8-o´ clock positions. The term “pellucid” means clear and first time was used by Schlaeppi to describe the corneal clarity and the absence of lipid deposition, scarring or vascularization, despite the presence of ectasia. Pellucid marginal degeneration (PMD) is an idiopathic, progressive, noninflammatory, ectatic corneal disease characterized by a peripheral crescent of inferior corneal thinning. Updates on management of pellucid marginal degeneration: Topographic patterns, differential diagnosis, and surgical options. Keywords: Differential diagnosis, pellucid marginal degeneration, surgical options, topographic patterns Different surgical techniques are available for PMD management however, none of them were found to be effective, so further studies will be needed in the future. Spectacles, soft and rigid gas permeable contact lens are the main visual correcting method in early stage of the disease. Corneal topography is the main diagnostic tool of PMD with characteristic diagnostic patterns “crab-claw” or “butterfly.” PMD could be mistaken as KCN, keratoglobus, and other peripheral thinning conditions such as Terrien marginal degeneration and Mooren's ulcer. It is difficult to differentiate between keratoconus (KCN) and PMD by slit lamp, especially in the detection of early and subclinical stages of the diseases. Pellucid marginal degeneration (PMD) is a rare ectatic corneal disease involving the inferior part of the cornea.
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