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Pellucid Marginal Degeneration (PMD)
Cornea
1. Clinical Analogy - The Crescent Moon Thinning
Exam pearl
- Clinical Analogy: Imagine a tyre that develops wear not at the centre but at its lower rim - the rubber thins inferiorly, and as the rim weakens, the tyre wall above it bulges outward. This is PMD. The cornea thins along an inferior crescent band (the 'tyre rim'), and the cornea just above this thin zone ectasias forward - giving the characteristic crab-claw or butterfly pattern on topography. Unlike keratoconus where the cone is at the apex, PMD's ectasia sits ABOVE/superior to the thin zone - a critical anatomical distinction.
2. Definition, Epidemiology & Aetiology
- Pellucid Marginal Degeneration (PMD) is a bilateral, non-inflammatory, peripheral corneal ectatic disorder characterised by a band of inferior stromal thinning located 1–2 mm from the limbus, with protrusion of the cornea above the thin zone rather than at the site of thinning itself.
- The overlying epithelium is intact, and the thinning zone is clear, avascular, and without significant inflammation, distinguishing PMD from ulcerative or inflammatory peripheral corneal conditions.
| Parameter | Details |
|---|---|
| Prevalence | • Rare • Less common than keratoconus • Exact prevalence uncertain |
| Age of Onset | • Typically presents in the second to fifth decades • Reported cohorts: 8–66 years (mean 34.0 ± 14.8 years) • Median onset reported at 37 years |
| Sex | • Male predominance reported • Do not assume a universal 3:1 ratio |
| Laterality | • Usually bilateral • May be asymmetric • Unilateral involvement has been reported |
| Associated conditions | • Keratoconus may coexist • Keratoglobus is a differential diagnosis • No strong evidence links pellucid marginal corneal degeneration with Ehlers–Danlos syndrome, Down syndrome, or atopy |
| Genetic basis | • No established genetic inheritance • Familial high astigmatism has been reported |
| Aetiology | • Unknown • Abnormal stromal collagen architecture and biomechanical fragility are proposed mechanisms |
| Parameter | Details |
|---|---|
| Prevalence | • Rare • Less common than keratoconus • Exact prevalence uncertain |
| Age of Onset | • Typically presents in the second to fifth decades • Reported cohorts: 8–66 years (mean 34.0 ± 14.8 years) • Median onset reported at 37 years |
| Sex | • Male predominance reported • Do not assume a universal 3:1 ratio |
| Laterality | • Usually bilateral • May be asymmetric • Unilateral involvement has been reported |
| Associated conditions | • Keratoconus may coexist • Keratoglobus is a differential diagnosis • No strong evidence links pellucid marginal corneal degeneration with Ehlers–Danlos syndrome, Down syndrome, or atopy |
| Genetic basis | • No established genetic inheritance • Familial high astigmatism has been reported |
| Aetiology | • Unknown • Abnormal stromal collagen architecture and biomechanical fragility are proposed mechanisms |
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Keep reading Pellucid Marginal Degeneration (PMD)
9 more sections are included with the Clinical plan.
- Pathophysiology & Biomechanics
- Clinical Features
- Corneal Topography & Imaging (High-Yield)
- Management - Comprehensive Algorithm
- Progression Monitoring
- Evolving Concepts & Newer Therapies