ConditionEyes & vision
Pellucid Marginal Degeneration (PMD)
Pellucid marginal degeneration (PMD) is a rare, slowly progressive eye condition in which a crescent-shaped band of the lower cornea, the clear front window of the eye, becomes thin. The cornea above the thin band bulges forward, causing high irregular astigmatism and blurred, distorted vision. It usually affects both eyes and appears between ages 20 and 50. It is related to keratoconus. Glasses help early, specialty contact lenses give the best vision for most people, and corneal cross-linking can slow progression.
Also called: PMD, pellucid marginal corneal degeneration, pellucid degeneration
At a glance
- What it is
- Thinning of the lower peripheral cornea causing the cornea to bulge and warp.
- Who gets it
- Adults usually 20 to 50. Rare. Often both eyes.
- Symptoms
- Gradually blurred and distorted vision, frequently changing glasses prescriptions, glare.
- Diagnosis
- Corneal topography or tomography showing a characteristic pattern.
- Treatment
- Glasses, scleral or rigid contact lenses, corneal cross-linking, and surgery in advanced cases.
When to get help
See a doctor within a day or two
- Sudden painful, watery eye with a white or cloudy cornea and a big drop in vision (possible corneal hydrops)
- A red, painful eye with light sensitivity while wearing contact lenses (possible corneal infection)
- Eye pain or vision loss after rubbing or minor trauma to an eye with known PMD
Book a routine appointment
- Blurred or distorted vision that keeps changing despite new glasses
- Increasing glare, halos or ghost images
- Difficulty tolerating contact lenses
What happens in PMD
The cornea focuses light onto the retina. In PMD, a band 1 to 2 mm wide near the lower edge of the cornea becomes thin, typically between the 4 and 8 o'clock positions. Unlike keratoconus, the thinnest area is below the point of maximum bulging, and the cornea is clear (pellucid means clear) without scarring or iron lines in most cases.
The bulging distorts the cornea's shape, producing large amounts of against-the-rule astigmatism, where the cornea is steeper horizontally. Corneal maps often show a characteristic "crab claw" or "kissing doves" pattern.
PMD vs keratoconus
| Pellucid marginal degeneration | Keratoconus | |
|---|---|---|
| Location of thinning | Lower periphery, a crescent band | Central or slightly below center |
| Typical age at diagnosis | 20 to 50 | Teens to 20s |
| Astigmatism | Against-the-rule | With-the-rule or oblique |
| Corneal clarity | Usually clear | Scarring and iron lines common in advanced cases |
| Contact lens fitting | More challenging | Usually easier |
The two conditions may overlap and sometimes coexist. See keratoconus.
Symptoms
- gradually blurred vision, worse at distance
- distortion, ghosting or doubled images in one eye
- frequent changes in glasses prescription
- glare and halos around lights at night
- eye strain and headaches
Symptoms often progress slowly over years, and many people are first told they simply have high astigmatism.
Causes and risk factors
The cause is not fully understood. Contributing factors may include:
- inherited weakness of corneal collagen
- eye rubbing
- associations with some connective tissue and allergic conditions
It is not caused by contact lens wear.
Diagnosis
An eye specialist (ophthalmologist or optometrist) will:
- measure vision and refraction
- examine the cornea with a slit lamp
- perform corneal topography or tomography, the key test, to map the shape and thickness of the cornea
- measure corneal thickness (pachymetry)
- repeat maps over time to check for progression
Treatment
Vision correction
- Glasses or soft toric contact lenses work in mild cases.
- Scleral lenses: large rigid lenses that vault over the cornea and rest on the white of the eye. They often give the best vision and comfort in PMD.
- Rigid gas permeable (RGP) lenses or hybrid lenses in some people.
Slowing progression
- Corneal cross-linking: riboflavin eye drops activated by ultraviolet light strengthen corneal collagen. It is approved for progressive keratoconus and used for PMD, though the thin band near the edge makes treatment more technically challenging.
- Avoid eye rubbing, and treat eye allergies and dry eye.
Surgery
For advanced disease or contact lens intolerance:
- corneal transplant (often a larger, off-center graft, or a lamellar graft that replaces only part of the cornea's thickness)
- crescentic wedge resection or other tectonic procedures
- intracorneal ring segments in selected cases
- toric intraocular lens implants combined with cataract surgery in stable cases
Laser vision correction such as LASIK is not suitable, because it further weakens the cornea.
Complications
- Corneal hydrops: a sudden break in the inner layer of the cornea lets fluid in, causing painful clouding. It usually settles over weeks to months, sometimes with corneal scarring.
- Perforation: rare, in severe thinning.
- Infection with contact lens wear. See contact lens complications.
Outlook
PMD usually progresses slowly, and many people maintain good vision with specialty lenses for decades. Cross-linking can stabilize progressive cases. Regular monitoring with corneal maps helps time treatment.
Frequently asked questions
What is pellucid marginal degeneration?
It is a rare eye condition in which a band near the lower edge of the cornea thins, causing the cornea to bulge and warp. This produces high, irregular astigmatism and blurred, distorted vision, usually in both eyes.
How is pellucid marginal degeneration different from keratoconus?
Both cause the cornea to thin and bulge. In keratoconus the thinning is central and starts in teens or 20s. In PMD the thinning is in a crescent near the lower edge, starts later, and causes a different astigmatism pattern.
Can pellucid marginal degeneration cause blindness?
It rarely causes blindness. Vision can become very blurred, but most people achieve good vision with scleral or rigid contact lenses. Advanced cases can be treated with surgery, including corneal transplant.
Is there a cure for pellucid marginal degeneration?
There is no cure, but treatments control it well. Corneal cross-linking can slow or stop progression, specialty contact lenses correct vision, and surgery helps advanced cases.
Can I have LASIK with pellucid marginal degeneration?
No. LASIK and similar laser procedures remove corneal tissue and can make the thinning and bulging worse. People with PMD should avoid laser vision correction and ask about lenses or cross-linking instead.
Sources
This guide was written from current patient and clinical guidance, including:
- American Academy of Ophthalmology: Pellucid marginal degeneration (EyeWiki)
- National Keratoconus Foundation: Pellucid marginal degeneration
- American Academy of Ophthalmology: Corneal ectasia preferred practice pattern
- Cornea Society: Patient information on corneal cross-linking
- MedlinePlus (US National Library of Medicine): Corneal disorders