Cornea (Wills Eye Institute Atlas Series)

114 4 Ectatic Conditions of the Cornea

PELLUCID MARGINAL DEGENERATION P ellucid marginal degeneration is an uncommon, bilateral condition with inferior corneal thinning, protrusion, and ir regularity. It usually presents in early to middle adulthood.

Differential Diagnosis ● Keratoconus: inferocentral corneal thinning with protrusion of cornea in the area of great est thinning. A Fleischer ring and Vogt striae may be present. Treatment ● Mild and moderate cases: RGPCL, hybrid lens, or scleral lens ● Severe and contact lens–intolerant cases ■ Deep anterior lamellar keratoplasty ■ Large inferiorly displaced penetrating keratoplasty ■ Inferior crescentic wedge resection ■ Crescentic penetrating keratoplasty (sometimes followed by a central pen etrating keratoplasty) and placement of intracorneal ring segments are occasionally performed. ■ Corneal collagen cross-linking can be considered but is generally not as success ful as when treating keratoconus. ■ Refractive surgery in patients with pellu cid marginal degeneration is unpredictable and generally not recommended. ● Acute hydrops: see section on Keratoconus earlier Prognosis ● Most patients do well with RGPCLs, al though they are harder to fit than in patients with keratoconus. Patients can also do very well with scleral lenses. The success rate with corneal transplantation in pellucid marginal degeneration is good, but not as good as kera toconus, because of more peripheral disease.

Etiology ● Sporadic Symptoms

● Gradually decreasing vision beginning in young to middle adulthood. Can develop acute decreased vision and pain due to hy drops with advanced disease Signs ● High irregular against-the-rule astigmatism (flat at 90 degrees, steep at 180 degrees) ● Recognizable “crab-claw” pattern of irregu lar astigmatism on computed corneal topog raphy and tomography ( Fig. 4-2A ), although a similar pattern can be found in eyes with keratoconus with “low-sagging cones” ● Inferior, crescent-shaped band of peripheral corneal thinning, 1 to 2 mm in width, ex tending from the 4 to the 8 o’clock positions, which is separated from the limbus by normal thickness cornea ( Fig. 4-2B , eFig. 4-2B) ● The area of greatest protrusion is located above the band of thinning ( Fig. 4-2C and D ). ● Fleischer ring and Vogt striae are absent. ● Corneal hydrops can occur on rare occa sions ( Fig. 4-2E and F , eFig 4-2E).

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