•Bilateral, non-inflammatory, non-hereditary peripheral corneal thinning usually inferiorly (4 to 8 ‘o’ clock).
•2nd to 5th decade of life.
•Thought to be secondary to collagen abnormalities, similar to keratoconus.
•The thin, weakened cornea is said to protrude as a result of intraocular pressure.
•Progressive deterioration of visual acuity due to irregular astigmatism.
•Rare – pain due to acute corneal hydrops.
•Peripheral band of corneal thinning in the inferior cornea (4 to 8 ‘o’ clock) 1-2mm from the limbus.
•The steepest corneal protrusion occurs above the area of thinning in PMD.
•Vogt striae, hydrops, vascularization relatively rare in PMD.
•Corneal topography/tomography: Classic butterfly/ crab claw appearance with PMD, showing low power along the central vertical axis, increasing power in the inferior cornea and high power along the infero-oblique meridians
•Pachymetry: Used to measure for inferior corneal thinning, which is a reversal of the typical pattern in which the cornea thickens from center to periphery.
•Refraction and keratometry showing against-the-rule astigmatism.
-Crab claw/ Butterfly patterns on the sagittal topographic map of anterior corneal curvature reveal steepening of the inferior corneal periphery and flattening of the cornea along the vertical meridian.
•Contact lens correction when ectasia is mild. (But CL fit is usually more difficult than in keratoconus).
•Peripheral lamellar crescentic keratoplasty.
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