Pellucid Marginal Degeneration (PMD) is a rare, non-inflammatory eye condition that affects the cornea, the clear, dome-shaped front surface of the eye. This progressive disorder leads to a distinctive thinning of the lower portion of the cornea, which causes significant vision distortion. The exact cause of PMD remains unclear, but it is not typically associated with pain.
Understanding Pellucid Marginal Degeneration
PMD involves a specific band of thinning in the inferior peripheral cornea. This thinning is usually found 1 to 2 mm from the limbus, the border between the cornea and the sclera. The central cornea generally maintains its normal thickness, yet the peripheral thinning leads to significant irregular astigmatism.
Common symptoms of PMD include blurred or distorted vision, often uncorrectable with standard eyeglasses, and difficulty with night driving. Patients may also experience glare and increased sensitivity to light. The condition typically manifests in young adulthood, between 20 and 40 years of age, and progresses slowly over time.
Diagnosing PMD with Corneal Topography
Corneal topography is a non-invasive imaging technique that creates a detailed map of the cornea’s curvature, akin to a topographical map of land. This is the primary diagnostic tool for identifying PMD. Topography reveals characteristic patterns of corneal steepening and flattening that are unique to the condition.
The most common topographical pattern seen in PMD is the “crab claw” appearance, or sometimes a “kissing doves” or “butterfly” pattern. These patterns indicate inferior steepening with central flattening and horizontal bow ties, reflecting the crescent-shaped band of thinning in the lower cornea. This detailed mapping aids accurate diagnosis and monitoring progression.
Differentiating PMD from Keratoconus
PMD is often confused with keratoconus due to some overlapping symptoms and topographical similarities. Both conditions cause corneal thinning and irregular astigmatism, but their locations of thinning and characteristic topographical patterns differ significantly. Keratoconus typically involves thinning and protrusion in the central or paracentral cornea, leading to a more localized cone-like bulge.
In contrast, PMD features a band-shaped thinning in the inferior periphery, distinct from the more central steepening seen in keratoconus. While both can show inferior steepening, PMD corneas tend to be low prolate to oblate in shape, whereas keratoconus corneas are almost always markedly prolate. Additionally, the peak elevation in PMD is typically about 3.5 mm from the corneal apex, compared to approximately 1.95 mm in keratoconus, providing a measurable distinction.
Treatment Options for PMD
Management of PMD begins with conservative approaches. Corrective eyeglasses or soft contact lenses can sometimes improve vision in the early stages. However, as the irregular astigmatism progresses, these options often become insufficient.
Rigid gas permeable (RGP) contact lenses, along with specialized lenses like scleral or hybrid lenses, play a primary role in improving vision in more advanced PMD. These lenses create a new, smooth refractive surface over the irregular cornea, enhancing visual acuity. For severe or progressive cases, surgical interventions may be considered, though they are less common than contact lens management. These include corneal cross-linking to strengthen the cornea and potentially halt disease progression. Various forms of corneal transplantation, such as lamellar keratoplasty or penetrating keratoplasty, are typically reserved for individuals who do not tolerate contact lenses or have significant vision impairment.