Pellucid Marginal Degeneration
Thinning of the lower cornea that distorts vision
Quick Facts
- Type: Corneal thinning (ectatic) disorder
- Affected area: Lower (inferior) cornea
- Main effect: Irregular astigmatism, blurred vision
- Related to: Keratoconus
Overview
Pellucid marginal degeneration (PMD) is a rare, slowly progressive eye condition in which a band of the cornea near its lower edge becomes thin. As the tissue thins and weakens, the cornea above it bulges forward into an irregular shape. Because the cornea must keep a smooth, regular curve for clear focus, this distortion blurs and warps vision.
PMD belongs to a group of conditions called corneal ectasias, which also includes the more common keratoconus. Unlike keratoconus, where the cornea thins near its center, PMD thins along the bottom, producing a characteristic pattern of irregular astigmatism. The condition usually appears in early to middle adulthood and tends to progress gradually. Vision can often be improved with specialized lenses, and surgery is reserved for more advanced cases.
The name pellucid refers to the fact that the thinned cornea usually stays clear rather than becoming cloudy, which helps distinguish it from some other corneal conditions. Because the changes are gradual and the cornea remains transparent, PMD is sometimes overlooked early on and may be mistaken for ordinary astigmatism until corneal mapping reveals its characteristic pattern.
Symptoms
Symptoms develop gradually and stem from the distorted corneal shape.
- Blurred or distorted vision
- Increasing astigmatism that needs frequent changes in glasses prescription
- Difficulty achieving clear vision with regular glasses
- Glare and halos around lights
- Reduced vision quality that is worse than glasses alone can correct
The astigmatism in PMD is often irregular, meaning it cannot be fully corrected with standard glasses, which is a common reason people seek evaluation.
Causes
The exact cause of pellucid marginal degeneration is not known.
- Corneal weakening: The lower cornea loses structural strength and thins, allowing the area above it to bulge.
- Possible genetic and biological factors: As with related corneal thinning disorders, changes in the cornea's collagen and supporting structure are thought to play a role, though no single cause is established.
Unlike some corneal problems, PMD is not caused by infection or injury, and it is not contagious. It typically affects both eyes, though one may be worse than the other.
Risk Factors
- A personal or family history of corneal thinning disorders such as keratoconus
- Onset typically in early to middle adulthood
- Eye rubbing has been associated with corneal thinning disorders in general
Diagnosis
An eye doctor diagnoses PMD by examining the shape and thickness of the cornea.
- Corneal topography: A map of the corneal surface that shows the characteristic pattern of PMD, often described as a butterfly or crab-claw shape.
- Slit-lamp examination: To see the band of thinning along the lower cornea.
- Corneal thickness measurement (pachymetry): To document the area and degree of thinning.
- Vision and refraction testing: To measure the irregular astigmatism.
These tests help distinguish PMD from keratoconus and other corneal conditions.
Treatment
Treatment focuses on improving vision and, when needed, strengthening or reshaping the cornea.
- Glasses: May help in early stages but often cannot fully correct the irregular astigmatism.
- Specialized contact lenses: Rigid gas-permeable or scleral lenses create a smooth front surface and usually give the best vision.
- Corneal cross-linking: A procedure that can strengthen the cornea and help slow progression in suitable cases.
- Surgical options: For advanced disease, procedures including certain corneal transplant techniques may be considered, though they can be technically challenging because of where the thinning is.
Because PMD can progress, regular monitoring helps determine when to adjust treatment.
Prevention
- There is no proven way to prevent PMD, but early detection helps preserve vision
- Avoid vigorous eye rubbing, which is discouraged in corneal thinning disorders
- Have regular eye examinations, especially if you have a family history of corneal conditions
- Report frequent changes in vision or prescription to your eye doctor
- Follow up regularly so progression can be caught and managed early
When to See a Doctor
See an eye doctor if you have progressively blurred or distorted vision, frequently changing glasses prescriptions, or astigmatism that glasses do not fully correct. An eye specialist can map the cornea to make the diagnosis and recommend lenses or other treatment. Seek prompt care for sudden, severe eye pain, sudden vision loss, or a suddenly cloudy cornea, which are uncommon in PMD and may indicate another problem needing urgent attention.
Frequently Asked Questions
What is pellucid marginal degeneration?
It is a rare condition in which a band of the lower cornea thins and the area above it bulges forward, distorting vision. It is one of the corneal thinning disorders, related to keratoconus, and usually causes irregular astigmatism.
How is it different from keratoconus?
Both involve corneal thinning and bulging, but in keratoconus the thinning is near the center of the cornea, while in pellucid marginal degeneration it occurs along the lower edge. This produces a distinct pattern on corneal mapping and a characteristic irregular astigmatism.
Can glasses correct pellucid marginal degeneration?
Glasses may help in early stages, but because the astigmatism is irregular, they often cannot fully sharpen vision. Specialized rigid or scleral contact lenses usually provide much better vision by creating a smooth front surface.
Does it get worse over time?
It tends to progress slowly over years. Regular monitoring allows treatment to be adjusted as needed, and corneal cross-linking may help slow progression in suitable cases.
How is it treated in advanced cases?
When lenses no longer provide adequate vision, surgical options including certain corneal transplant techniques may be considered. These can be technically demanding because of the low position of the thinning, so they are reserved for advanced disease.
References
- American Academy of Ophthalmology. Corneal ectatic disorders.
- National Eye Institute (NEI). Corneal Conditions.
- MedlinePlus, U.S. National Library of Medicine. Astigmatism.
- National Keratoconus Foundation. Pellucid marginal degeneration.