Jawad Ahmad

Learning Points

• Pellucid marginal degeneration (PMD) is a non-inflammatory corneal thinning disorder that causes high, often irregular, astigmatism.
• It primarily affects the inferior peripheral cornea in an arcuate band, with relative central corneal sparing.
• Diagnosis relies on corneal topography/tomography and pachymetry; a characteristic “crab-claw” pattern is typical but not pathognomonic.
• Management includes rigid corneal, corneoscleral or scleral contact lenses, with corneal cross-linking, intracorneal ring segments and keratoplasty reserved for progressive or advanced disease.
• Differentiating PMD from keratoconus and other ectasias is critical, as this influences monitoring, treatment choice and surgical planning.

Summary

Pellucid marginal degeneration (PMD) is a rare, bilateral, non-inflammatory corneal ectasia characterised by a peripheral band of inferior thinning with relative central sparing. Patients usually present in early to mid-adulthood with progressive visual distortion from high against-the-rule and irregular astigmatism. Clinical examination typically reveals a clear, quiet eye with a crescentic band of thinning located 1–2 mm from the inferior limbus. Corneal topography and tomography are essential for diagnosis and commonly show a “crab-claw” pattern of inferior steepening with corresponding thinning on pachymetry. PMD may be misdiagnosed as keratoconus, particularly when only anterior curvature maps are reviewed.

Non-surgical management focuses on optical rehabilitation with spectacles in early disease and rigid, corneoscleral or scleral contact lenses in more advanced cases. Corneal collagen cross-linking (CXL) has emerged as a strategy to stabilise progression, although evidence is largely limited to small series and requires modification of protocols for thin peripheral corneas. Intracorneal ring segments (ICRS) can regularise corneal shape in selected patients, while lamellar or penetrating keratoplasty is reserved for advanced disease, scarring or contact lens intolerance. Overall prognosis is favourable with early recognition, judicious use of contact lenses and targeted surgical intervention, although keratoplasty in PMD can be more challenging than in keratoconus with a higher risk of residual astigmatism.

Introduction

First described by Schlaeppi in 1957, pellucid marginal degeneration is an uncommon form of corneal ectasia that predominantly affects the inferior peripheral cornea. It is characterised by a band of thinning located parallel to the limbus, with overlying ectasia just central to the thinned zone. Unlike keratoconus, where ectasia is more central or paracentral, the biomechanical weakness in PMD is displaced inferiorly. Histopathological and biomechanical studies suggest abnormalities in stromal collagen and corneal rigidity, but the exact aetiology remains uncertain. PMD is important to recognise because it can cause profound irregular astigmatism and is frequently mislabelled as keratoconus, which can lead to suboptimal management.

Epidemiology

PMD is rare and its true prevalence is difficult to ascertain, partly because of under-recognition and misclassification as atypical or inferior keratoconus. It typically presents in the second to fifth decades of life and affects both sexes, with a slight male predominance in some series. The condition is usually bilateral but often asymmetric, and many patients report slowly progressive blur or distortion rather than acute symptoms.

Pathophysiology and Morphology

The hallmark of PMD is a peripheral band of thinning, most commonly between the 4 and 8 o’clock positions, located 1–2 mm from the inferior limbus and separated from it by a narrow zone of relatively normal cornea. Above this band, the cornea protrudes, producing the characteristic “beer-belly” configuration on slit-lamp examination and corneal mapping. Unlike inflammatory marginal thinning disorders, the epithelium is intact, vascularisation is absent and lipid deposition is not a feature. Scarring is uncommon unless there has been prior hydrops or significant epithelial compromise.

From a biomechanical perspective, the inferior band of thinning leads to focal weakening, triggering progressive deformation under intraocular pressure. The exact trigger for this weakening is unknown; hypotheses include intrinsic collagen abnormalities, altered stromal remodelling and genetic predisposition.

Clinical Presentation

Patients typically present with gradually worsening distance vision and visual distortion due to high against-the-rule and irregular astigmatism. Common symptoms include ghosting, halos, glare and particularly poor night vision. The condition is painless, and the eye is usually white and quiet unless complicated by acute hydrops or epithelial breakdown.

On slit-lamp examination, the cornea is clear with a non-ulcerated, crescentic band of inferior thinning separated from the limbus by a clear zone. The overlying epithelium is intact, and there is little or no vascularisation. Acute hydrops is rare but can occur and may result in localised scarring. PMD is classically bilateral but may be markedly asymmetric, with one eye significantly more ectatic than the other.

Differentiating PMD from keratoconus, keratoglobus and Terrien marginal degeneration is essential. In inferior keratoconus, thinning and protrusion tend to be more central without a clear peripheral band. Terrien marginal degeneration typically shows inflammatory signs, vascularisation and lipid deposition, which are absent in PMD.

Investigations

Corneal topography and tomography

Corneal topography is central to the diagnosis of PMD. A classic “crab-claw” or “butterfly” pattern of inferior peripheral steepening with relative central flattening is commonly observed, especially on tangential or axial curvature maps. However, similar patterns can be seen in other inferior ectasias, so topography alone is not definitive.

Scheimpflug tomography and elevation maps provide additional information, demonstrating the location of maximal curvature just above the thinning band and highlighting posterior corneal changes. These modalities are particularly useful for discriminating PMD from keratoconus and for monitoring progression.

Pachymetry and anterior segment OCT

Pachymetric mapping shows an arcuate band of thinnest cornea inferiorly, again typically between 4 and 8 o’clock, with a relatively preserved central thickness. Anterior segment OCT can delineate stromal thinning and curvature changes and is helpful in ruling out other peripheral thinning disorders.

Differential diagnosis

Differentiation from keratoconus, keratoglobus and Terrien marginal degeneration is crucial, as these conditions differ in their natural history and surgical management. Terrien marginal degeneration, for example, shows peripheral thinning with lipid deposition and superficial vascularisation, whereas PMD is non-inflammatory with a clear cornea.

Management

Non-surgical management

In early disease, spectacles may suffice to correct regular astigmatism, but they rapidly become inadequate as irregular astigmatism increases. Rigid gas-permeable (RGP) lenses, corneoscleral lenses and scleral lenses are the mainstay of visual rehabilitation in moderate to advanced PMD. The larger diameter and vaulting characteristics of scleral lenses allow them to mask irregularities and provide excellent optical quality while resting on the relatively regular scleral surface. Careful fitting is essential to avoid excessive pressure over the thinned inferior cornea.

Corneal collagen cross-linking

Corneal collagen cross-linking (CXL) aims to increase stromal rigidity and halt ectatic progression. While most evidence arises from keratoconus, several small series have reported encouraging short-term visual and topographic stabilisation in PMD following standard or modified CXL protocols. Because thinning is located peripherally and often falls below the conventional safety threshold of 400 µm, treatment protocols may need to be decentered, customised or combined with stromal swelling techniques. At present, CXL should be considered in carefully selected, documented progressive PMD, with the understanding that the evidence base remains limited compared with keratoconus.

Intracorneal ring segments

Intracorneal ring segments can be used in selected PMD cases to regularise corneal shape and reduce astigmatism, often in combination with CXL. Segments are typically implanted above the area of thinning, within the ectatic but structurally sufficient stroma. This can improve best-corrected visual acuity and reduce dependence on rigid lenses, although long-term data are limited and some patients will still require contact lenses for optimal vision.

Keratoplasty and other surgical options

Keratoplasty is reserved for advanced disease with severe thinning, central involvement, scarring or contact lens intolerance. Options include crescentic lamellar grafts, lamellar wedge resections, deep anterior lamellar keratoplasty (DALK) and penetrating keratoplasty (PK). Penetrating keratoplasty has been shown to improve visual acuity in PMD, but grafts can be more challenging than in keratoconus because of peripheral thinning, the need for large or decentered grafts and a higher risk of residual astigmatism. Lamellar techniques can preserve host endothelium and reduce rejection risk while improving tectonic support. Careful pre-operative planning, often guided by tomography, is critical to optimise outcomes.

Prognosis

PMD usually progresses slowly over years rather than months. With early diagnosis, appropriate refractive correction and close monitoring, many patients can maintain good functional vision for long periods. Contact lens technologies – particularly scleral lenses – have transformed visual rehabilitation and can substantially delay or avoid the need for keratoplasty.

When surgery is required, both ICRS and keratoplasty can offer significant visual improvement, but outcomes are somewhat less predictable than in keratoconus, and residual astigmatism is common. Acute hydrops and perforation are uncommon but can lead to scarring and more complex surgery. Overall, the prognosis is favourable when PMD is correctly identified, patients are counselled about the chronic nature of the disease and management is tailored to the stage and pattern of ectasia.

Conclusion

Pellucid marginal degeneration is an uncommon but important cause of irregular astigmatism and visual impairment. Its characteristic pattern of inferior peripheral thinning with central sparing distinguishes it from other ectatic disorders, yet misdiagnosis as keratoconus remains common. Accurate diagnosis requires careful slit-lamp examination, corneal topography or tomography and pachymetric mapping.

Management is stepwise: spectacles in early disease, progression to rigid or scleral lenses in more advanced cases, and consideration of cross-linking, intracorneal ring segments or keratoplasty when progression or visual disability persists. Although the evidence base for interventions such as CXL and ICRS in PMD is smaller than for keratoconus, emerging data are encouraging. With early recognition, modern contact lens options and judicious use of surgery, most patients with PMD can achieve and maintain good visual function.

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