![]() 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. This is described as a "beer belly" appearance since the greatest protrusion occurs below the horizontal midline (unlike keratoconus). The cornea may protrude anteriorly just above the region of thinning cornea. 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. Normally, PMD does not present with vascularization of the cornea, scarring, or any deposits of lipid. 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. Unlike keratoconus, pain is not typically present in pellucid marginal degeneration, and aside from vision loss, no symptoms accompany the condition. The term "Pellucid Marginal Degeneration" was first coined in 1957 by the ophthalmologist Schalaeppi.The word "pellucid" means clear, indicating that the corneas retain clarity in Pellucid Marginal Degeneration. It is typically characterized by a bilateral thinning ( ectasia ) in the inferior and peripheral region of the cornea, although some cases are unilateral. Tomographic and densitometric evaluations may facilitate the differential diagnosis.Pellucid Marginal Degeneration (PMD keratotorus), is a degenerative corneal condition, often confused with keratoconus. Therefore, analyzing only the anterior corneal surface is not sufficient in differential diagnosis. The densitometry values of PMD were significantly higher than those of the controls in all zones and layers (P<0.01) and significantly higher than the densitometry values of inferior keratoconus in the 6-10 and 10-12 mm zones (P<0.05).ConclusionThere is a higher probability of a patient with crab claw pattern on the topography of having inferior keratoconus than having PMD. ![]() In the PMD, all deviation indices were higher than the controls (P<0.01), whereas the deviation indices, except Dt (P=0.960), were lower than the inferior keratoconus (P<0.01). The thinnest corneal point and maximum anterior and posterior elevation points were located lower in the PMD than in the inferior keratoconus (P<0.01). The control group consisted of 40 patients (33.1±4.6 years). The topographic, tomographic, and densitometric measurements were analyzed.ResultsPMD was detected in 11 eyes of eight patients (mean age 50.2☑1.1 years), and inferior keratoconus was detected in 36 eyes of 24 patients (mean age 34.7☑0.1 years). They were divided into two groups, inferior keratoconus and PMD, based on clinical findings. Forty-seven eyes of 32 patients with crab claw patterns were identified from 2751 patients with corneal ectasia. PurposeTo evaluate the topographic, tomographic, and densitometric properties of patients with pellucid marginal degeneration (PMD) and inferior keratoconus.Patients and methodsRetrospective, comparative case series. ![]()
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