Pellucid marginal degeneration (PMD) is a degenerative condition that causes vision to deteriorate as the cornea slowly changes shape. While it sometimes can affect only one eye, PMD most often causes thinning, or ectasia, in both eyes.
PMD is very similar to keratoconus, but is more rare. Since both conditions cause bilateral thinning of the cornea, the two are often confused—particularly if PMD is not detected in its early stages.
Researchers are still trying to determine with certainty, whether PMD and keratoconus are entirely different diseases or are simply two variations of the same disease. In either case, the striking similarities have inspired some to contend that, since PMD is often misdiagnosed as keratoconus, it may be more prevalent than we think.
What Causes Pellucid Marginal Degeneration?
PMD occurs in men and women of every race, usually in the second to fifth decade of life. Unfortunately, however, the etiology of PMD remains unknown. Although it does not appear to be hereditary, moderate to high astigmatism has been noted in families with affected patients.
How Is Pellucid Marginal Degeneration Diagnosed?
The good news is that PMD progresses very gradually, so if you are diagnosed early on, your doctor can be diligent in the effort to minimize vision loss. On the other hand, diagnosing PMD can be challenging—even for the most experienced clinician.
Usually, pain is not associated with PMD, except in rare cases in which the thinning has gotten so severe that it causes the cornea to perforate. In the absence of any noticeable symptoms, recognizing PMD in a timely manner is generally left in the hands of your eye doctor. This is one of the many reasons why regular eye exams are so important, even when it seems like your eyes are healthy and your vision is fine.
Other than by noting vision changes, your doctor can detect PMD using non-invasive diagnostic imaging technology that generates a topographical map of your eye. Because PMD has a unique appearance, these maps can be very informative. If you have PMD, the areas of corneal thinning and steepening will be most noticeable in specific locations. For example, PMD involves more peripheral thinning of the cornea, while keratoconus involves more central thinning.
Although it is important to recognize PMD in every case, it is especially critical if you are considering a refractive procedure, such as LASIK, since the disease can cause you to have a poor surgical outcome.
Scleral Lenses for Pellucid Marginal Degeneration
Most people who have PMD are treated with contact lenses. However, conventional soft lenses are not generally a suitable long-term solution. Due to the abnormal shape of a PMD cornea, more customized lenses are a better option. The most successful lens options take the corneal irregularity out of the fitting process and utilize a scleral contact lens design to better align with the regular curvature of the sclera.1
A scleral lens is a large-diameter contact lens designed to vault the entire corneal surface and rest on the white part of the eye, known as the sclera. Liquid fills the space between your eye and the back surface of the scleral lens. This liquid buffer can help protect your corneal tissue. It also contributes to more comfortable contact lens wear while simultaneously creating a smooth and uniform optical surface that may improve vision.
Surgical Treatments for Pellucid Marginal Degeneration
If PMD progresses to a point where spectacles and contact lenses can no longer provide adequate vision correction, surgery may be a wise next step. Fortunately, the advent of modern scleral lenses is now reducing the need for surgical intervention in many cases.
Refractive procedures such as LASIK cannot be used to correct vision lost to PMD because this would require additional thinning of corneal tissue. However, other surgeries, which often involve a corneal graft, may be helpful. These procedures are complicated due to the size and location of damaged tissue in PMD cases, but a skilled corneal surgeon can advise you on risks and benefits. There is also a possibility that you may be a candidate for newer surgical interventions using corneal collagen crosslinking or intracorneal ring segments such as Intacs.
If you have pellucid marginal degeneration, visit your eye doctor to determine the most appropriate management strategy for you.
 Messer B. Part II: Troubleshooting Astigmatism. Review of Cornea & Contact Lenses; March 2012;149(2):6.