Background
Pellucid marginal degeneration (PMD) is a bilateral, noninflammatory, peripheral corneal thinning disorder characterized by a peripheral band of thinning of the inferior cornea. The cornea in and adjacent to the thinned area is ectatic. Schalaeppi first coined the term pellucid marginal degeneration in 1957.
Pathophysiology
The etiology of pellucid marginal degeneration has not been clearly established, but collagen abnormalities, as seen in keratoconus, have been reported. The thinned and presumably weakened cornea may protrude as a result of the positive intraocular pressure. The corneal area between the thinned band‑like region and the limbus is characterized by the absence of lipid deposition, scarring, or vascularization and is typically epithelialized. The degree of thinning of the cornea can be severe resulting in up to 80% stromal tissue loss [1, 2, 3]
Epidemiology
Frequency
United States
Pellucid marginal degeneration is rare, and the exact incidence and prevalence are unknown. However, the incidence may be considerably underestimated, as this condition often is misdiagnosed as keratoconus.
India
Das et al published a multicenter study with an 8 year follow up and they found that the overall prevalence of PMD was 0.02% of all eye diseases diagnosed between 2012 and 2020. They compared the incidence in the diagnosis of Keratocous in the same period, and found that it was 0.62%, which is 31 times more common than PMD. [4]
Mortality/Morbidity
Deterioration of visual function results from the irregular astigmatism induced by asymmetric distortion of the cornea. The deterioration in visual function commonly is severe.
Refractive surgery, such as photorefractive keratectomy (PRK), can cause severe corneal haze, and results with laser-assisted in situ keratomileusis (LASIK) are unpredictable.
Race
No racial preponderance has been identified for pellucid marginal degeneration.
Sex
There are controversies concerning the sexual predilection of pellucid marginal degeneration. Some studies have reported no difference, [1, 2] while others have reported a male predilection. [3, 5, 6] In a 2016 Japanese study of 162 eyes with pellucid marginal degeneration, 75% of the patients were men. [7]
Age
Patients are usually aged 20-40 years at the time of clinical presentation.
Prognosis
The affected cornea maintains its clarity. However, patients experience progressive deterioration of vision due to irregular astigmatism. [7] The prognosis of pellucid marginal degeneration is clinical.
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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.
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Corneal topography of early (right eye) and moderate (left eye) pellucid marginal degeneration.
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Optical coherence tomography (OCT) of cornea with moderate pellucid marginal degeneration.
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Image shows simultaneous central penetrating keratoplasty and inferior peripheral lamellar keratoplasty performed to treat pellucid marginal degeneration.
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Representative photographs of corneal topography. Courtesy of the Japanese Journal of Ophthalmology, published by the Japanese Ophthalmological Society.
