•Bilateral, non-inflammatory, non-hereditary peripheral corneal thinning usually inferiorly (4 to 8 ‘o’ clock).
ONSET:
•2nd to 5th decade of life.
PATHOPHYSIOLOGY:
•Thought to be secondary to collagen abnormalities, similar to keratoconus.
•The thin, weakened cornea is said to protrude as a result of intraocular pressure.
SYMPTOMS:
•Progressive deterioration of visual acuity due to irregular astigmatism.
•Rare – pain due to acute corneal hydrops.
SIGNS:
•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.
DIAGNOSIS:
•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.
MANAGEMENT:
•Contact lens correction when ectasia is mild. (But CL fit is usually more difficult than in keratoconus).
•Peripheral lamellar crescentic keratoplasty.
•Penetrating keratoplasty.
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