Pellucid Marginal Degeneration (PMD) Clinical Presentation

Updated: Oct 30, 2017
  • Author: Jean Deschênes, MD, FRCSC; Chief Editor: Hampton Roy, Sr, MD  more...
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Presentation

History

Pellucid marginal degeneration (PMD) is usually asymptomatic, except for the progressive deterioration in uncorrected and spectacle corrected visual acuity caused by the irregular astigmatism induced by the corneal ectasia.

The increased corneal higher-order aberrations (HOAs), especially the trefoil and spherical aberrations, are the leading causes of optical errors, along with the induced against-the-rule astigmatism and visual acuity decline. [7]

Episodes of corneal hydrops with resultant pain, as seen in keratoconus, have been reported, but they occur only rarely. [8]

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Physical

Uncorrected visual acuity is often severely reduced. Visual acuity, as measured by using a pinhole, is close to normal. Refraction and keratometry show against-the-rule astigmatism. Visual acuity typically cannot be restored by using a spherocylindrical combination of lenses. Rigid gas permeable contact lenses are often required to treat pellucid marginal degeneration, but patients often experience glare and decreased contrast sensitivity despite achieving good Snellen acuity. It is unclear if this condition is due to the corneal disease or the contact lens wear.

Results of slit lamp examination are characterized by a peripheral band of thinning of the inferior cornea from the 4-o'clock position to the 8-o'clock position; this is shown in the image below.

Slit lamp image of the inferior cornea in a patien Slit lamp image of the inferior cornea in a patient with advanced pellucid marginal degeneration. Image illustrates inferior corneal thinning, a hallmark of this disease.

This thinning is accompanied by 1-2 mm of normal cornea between the limbus and the area of thinning. Corneal ectasia is most marked just central to the band of thinning. The central cornea is usually of normal thickness, and the epithelium overlying the area of thinning is intact.

Both eyes are usually affected, but the degree of involvement may be asymmetric. The area of thinning typically is epithelialized, clear, avascular, and without lipid deposits. Sporadic reports have described unilateral pellucid marginal degeneration, [9, 10] but a 2016 study suggested that this rate might be higher than previously recognized, precisely 25% in a cohort of 162 eyes with pellucid marginal degeneration. [6]

On careful slit lamp evaluation, prominent lymphatics often are detected at the inferior limbus parallel to the area of thinning. Vertical striations at the level of the Descemet membrane (similar to the Vogt striae) may be seen in rare instances.

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Causes

The cause of pellucid marginal degeneration is unknown.

A study of 162 eyes with pellucid marginal degeneration suggested that patients with unilateral involvement might have a different disease background than those with bilateral disease. One-fifth of the patients with unilateral involvement had allergic disorders, such as atopic dermatitis, allergic rhinitis, and conjunctivitis. [3]

Relationship between pellucid marginal degeneration and keratoconus

Some cases of  PMD may overlap with keratoconus. In a 2016 study, 40% (7 of 17) of patients with unilateral PMD had either keratoconus or were suspected of having keratoconus in the contralateral eye. [6]

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