Pellucid marginal degeneration (PMD) is a rare eye condition that affects the cornea, the clear, dome-shaped front surface of the eye. It is characterized by a non-inflammatory, progressive thinning disorder of the cornea. This thinning leads to changes in the cornea’s shape, which can cause significant visual disturbances.
Understanding Pellucid Marginal Degeneration
Pellucid marginal degeneration involves a distinct thinning of the inferior peripheral cornea, typically in a crescent or arcuate shape. This thinning usually occurs between the 4 o’clock and 8 o’clock positions, about 1 to 2 millimeters from the limbus, which is the border between the cornea and the white part of the eye. Above this thinned band, the cornea often bulges outward, while the central cornea usually maintains normal thickness.
This unique pattern of thinning and protrusion gives the cornea a characteristic “crab-claw” or “kissing doves” appearance when viewed with specialized imaging. From a side view, the cornea can resemble a “beer belly” due to the greatest protrusion occurring below the horizontal midline. The exact cause of PMD is often unknown, and it is considered a non-inflammatory condition.
Recognizing Symptoms and Diagnosis
Individuals with pellucid marginal degeneration often experience a gradual decline in vision that cannot be fully corrected with standard eyeglasses. Common symptoms include blurred or distorted vision and increasing irregular astigmatism. Some people may also notice glare or difficulty with night vision. Rarely, in advanced cases, sudden blurred vision and sensitivity to light, along with intense pain, can occur due to acute corneal hydrops.
Diagnosis of PMD is performed by eye care professionals through a comprehensive eye examination. A slit-lamp examination allows the professional to observe the characteristic clear, crescent-shaped thinning in the inferior cornea. Corneal topography and tomography are also used, providing detailed maps of the corneal surface and thickness. These imaging techniques help identify inferior corneal steepening and can measure the thinning.
Distinguishing from Other Corneal Conditions
Differentiating pellucid marginal degeneration from other corneal ectatic disorders, particularly keratoconus, is important for accurate diagnosis and management. While both conditions involve corneal thinning and protrusion, their locations and topographic patterns differ. In PMD, the thinning is in the inferior peripheral cornea, forming a crescent shape, with the greatest protrusion occurring above the thinned area.
Keratoconus, on the other hand, involves thinning and steepening in the central or paracentral region of the cornea, creating a more cone-like protrusion. Topographic maps in keratoconus may show an asymmetric bowtie pattern, whereas PMD presents with the “crab-claw” or “kissing doves” pattern. While some advanced cases of keratoconus can present with inferior steepening that might resemble PMD, pachymetry maps (which measure corneal thickness) can help distinguish them by revealing the absence of inferior crescentic thinning in keratoconus. Other rare conditions like keratoglobus, which involves more widespread corneal thinning from limbus to limbus, can also be considered in a differential diagnosis.
Approaches to Management and Treatment
Managing pellucid marginal degeneration involves various strategies aimed at improving vision and stabilizing the cornea. For mild cases, eyeglasses may offer some vision correction, but they often become insufficient as astigmatism increases. Contact lenses are a common non-surgical option, with rigid gas permeable (RGP) lenses and scleral lenses being particularly effective. RGP lenses provide a smooth optical surface over the irregular cornea, while larger scleral lenses vault over the entire cornea, resting on the white part of the eye, to correct for corneal irregularities.
For cases where the condition is progressing or contact lenses are not well-tolerated, surgical interventions may be considered. Corneal cross-linking (CXL) is a procedure that uses riboflavin eye drops and ultraviolet-A light to strengthen the corneal tissue and can help halt the progression of thinning. In advanced cases or when other treatments fail, corneal transplantation may be necessary. Deep anterior lamellar keratoplasty (DALK) can reduce severe astigmatism and improve visual outcomes while avoiding some risks associated with full-thickness transplants. Penetrating keratoplasty (PK) is a last resort for very advanced cases.