Pellucid marginal degeneration

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Pellucid marginal degeneration (PMD) is a rare, bilateral, non-inflammatory corneal thinning disorder characterized by a peripheral band of thinning, typically located in the inferior cornea. This condition leads to irregular astigmatism and can significantly affect visual acuity.

Presentation

Patients with PMD often present with progressive visual impairment due to the irregular astigmatism induced by the corneal thinning. Unlike keratoconus, another corneal ectatic disorder, PMD does not typically present with corneal scarring or Vogt's striae. The hallmark of PMD is a clear, crescent-shaped band of thinning located 1-2 mm from the inferior limbus, with the area of maximum thinning being peripheral to the visual axis.

Diagnosis

The diagnosis of PMD is primarily clinical, supported by imaging techniques such as corneal topography and corneal tomography. These imaging modalities reveal a characteristic "kissing doves" or "crab claw" pattern on the topographic map. Pachymetry can be used to measure the corneal thickness and confirm the peripheral thinning.

Differential Diagnosis

PMD must be differentiated from other corneal ectatic disorders, including keratoconus, keratoglobus, and Terrien's marginal degeneration. Unlike keratoconus, PMD does not typically present with central corneal thinning or apical protrusion. Keratoglobus involves generalized corneal thinning, and Terrien's marginal degeneration is associated with vascularization and lipid deposition.

Management

Management of PMD focuses on correcting the irregular astigmatism to improve visual acuity. Options include:

• **Spectacles and Contact Lenses**: In the early stages, spectacles or rigid gas-permeable contact lenses may be sufficient to correct the astigmatism.
• **Surgical Interventions**: In advanced cases, surgical options such as intrastromal corneal ring segments (ICRS), corneal collagen cross-linking (CXL), or keratoplasty may be considered. Penetrating keratoplasty (PK) or deep anterior lamellar keratoplasty (DALK) can be performed in cases with significant corneal thinning and ectasia.

Prognosis

The prognosis for patients with PMD varies. With appropriate management, many patients can achieve satisfactory visual acuity. However, the progressive nature of the disease may necessitate ongoing adjustments in the management plan.

See Also

• Keratoconus
• Keratoglobus
• Corneal topography
• Corneal collagen cross-linking
• Penetrating keratoplasty