Pellucid marginal degeneration: causes, symptoms and treatment

The cornea is the clear, curved front surface of the eye. In pellucid marginal degeneration a crescent-shaped band of the lower cornea becomes thin. EyeWiki describes this as a band of inferior corneal thinning approaching 20 percent of normal thickness, 1 to 2 mm in height and 6 to 8 mm across, lying 1 to 2 mm from the limbus, with no inflammation and a central cornea of normal thickness. The name comes from the clear (pellucid) look of the thinned area.

The weakened cornea bulges under the eye's normal pressure, in a region above the thinned band, which produces high irregular astigmatism. A 2025 review in the open-access literature describes PMD as a rare ectatic corneal disorder characterised by inferior peripheral thinning and significant irregular astigmatism, with clinical similarities to keratoconus but its own diagnostic and therapeutic challenges.

EyeWiki says the cause remains unknown. It is thought to be secondary to collagen abnormalities, similar to keratoconus. The condition does not appear to be hereditary, although moderate to high astigmatism has been noted in families with affected patients. EyeWiki also says keratoconus, keratoglobus and PMD are postulated to be related because they coexist in families, with 10 percent of PMD cases associated with keratoconus.

It typically presents in the second to fifth decade and affects both sexes and all races. There is no known prevention, but because eye rubbing is associated with keratoconus, EyeWiki says it could be considered to advise people not to rub their eyes.

EyeWiki says patients typically present with progressive deterioration of vision despite spectacle correction because of irregular astigmatism, and usually report a gradual decrease in vision uncorrected by spectacles. Rarely, the cornea suddenly swells with pain and a sharp drop in vision, called acute hydrops.

Sudden pain, redness and loss of vision in an eye with a known corneal condition needs urgent assessment. The NHS advises an urgent appointment with a GP or optician or NHS 111 for a very painful red eye, and A&E or 999 for changes to sight such as loss of vision. An optician may be the first to notice that glasses no longer give a sharp image and that corneal scanning is needed.

EyeWiki says PMD is a clinical diagnosis based on slit lamp examination and history and supported by corneal topography or tomography. On slit lamp examination there is a band of inferior thinning from about the 4 to 8 o'clock positions, which is epithelialised, clear and avascular. Topography typically shows a pattern described as a butterfly, and pachymetry measures the thinning. There are no laboratory tests for PMD.

The conditions to be distinguished are keratoconus, keratoglobus, Terrien marginal degeneration, furrow degeneration and peripheral corneal melting disorders such as Mooren ulcer. The 2025 review describes a crab-claw pattern on Scheimpflug-based tomography. Accurate diagnosis matters because treatment choices and the safety of refractive surgery differ from keratoconus.

EyeWiki says management is typically with spectacles, contact lenses or surgery and is difficult because of the abnormal corneal shape. Spectacles often fail early as astigmatism increases. Contact lens correction is the standard medical therapy; fitting can be harder than in keratoconus because lenses tend to decentre downwards. Hybrid and scleral lenses can also be considered. EyeWiki stresses these lenses improve vision but there is no evidence that they prevent progression.

If lenses are unsatisfactory, surgery may be required. Penetrating keratoplasty has been the classic operation, but large inferiorly decentred grafts carry a higher risk of rejection and are technically demanding. Other procedures have been described, including lamellar and crescentic resection techniques and intrastromal corneal ring segments. The 2025 review lists corneal cross-linking, ring segments and toric lens implants as options reserved for advanced cases, with varying success, and describes penetrating and deep anterior lamellar keratoplasty as definitive treatments for severe disease, with risks including graft rejection and astigmatism.

The NHS describes a cornea transplant as an operation to replace all or part of a damaged cornea, usually considered when glasses or contact lenses are no longer enough, and says recovery of vision can take up to 6 months to 2 years depending on the type.

EyeWiki says PMD is slowly progressive and should be followed at regular, extended intervals. Most people are managed without surgery. Practical points include attending regular corneal reviews, replacing contact lenses as advised, avoiding eye rubbing, telling any laser eye surgery provider about the diagnosis and seeking urgent help for sudden pain or visual loss.

Living with irregular astigmatism can affect driving at night, reading and screen work; an optometrist experienced in specialist lens fitting is often central to day-to-day management.

PMD is usually suspected by an optometrist, confirmed in a hospital eye or cornea clinic, and managed there or through specialist contact lens services. Corneal transplant is performed in NHS eye units. Private ophthalmology and specialist contact lens practices are also available in the UK. No UK patient leaflet on PMD was found while preparing this page; the main sources are the American Academy of Ophthalmology's EyeWiki and a 2025 journal review, which are not UK-specific.