Pellucid Marginal Degeneration

Because PMD and keratoconus are both ectatic corneal disorders, they are sometimes confused with each other. The most important distinction is where the thinning occurs. In keratoconus, the cornea thins in the central or slightly below-center area, producing a cone-shaped protrusion at or near the visual axis. In PMD, the thinning is located in the peripheral inferior cornea, forming a crescent-shaped band well below the center. The protrusion in PMD occurs just above this band rather than at the thinnest point itself.

Keratoconus often produces visible corneal findings such as Vogt striae, a Fleischer ring, and in advanced cases, corneal scarring. In PMD, the cornea typically remains clear without lipid deposits or scarring. The absence of these classic keratoconus signs is an important diagnostic clue when distinguishing the two conditions.

Keratoconus usually begins during adolescence or early adulthood and tends to stabilize by the third or fourth decade of life. PMD generally presents later, with symptoms often emerging between the ages of 20 and 40. Notably, PMD may continue to progress throughout a patient's lifetime rather than plateauing, which makes long-term monitoring essential. Approximately 10 percent of PMD cases have features that overlap with keratoconus.

Rigid gas-permeable lenses are often the first contact lens option because they create a smooth optical surface over the irregular cornea. For patients who find rigid lenses uncomfortable or who have more advanced thinning, scleral lenses are typically the most effective choice. Scleral lenses vault entirely over the cornea and rest on the white of the eye, providing excellent comfort and stable vision. Because PMD patients rely heavily on specialty lenses, maintaining proper lens hygiene is especially important to reduce the risk of infections such as Acanthamoeba keratitis.

Corneal collagen cross-linking uses ultraviolet light and riboflavin drops to strengthen the structural bonds within the cornea, and research indicates it can help stabilize PMD and slow or halt its progression. While cross-linking does not reverse thinning that has already occurred, it may reduce the need for more invasive surgical procedures in the future. Cornea specialists will assess whether cross-linking is appropriate based on your corneal thickness and the degree of progression.

When contact lenses and cross-linking are not sufficient, several surgical approaches may be considered. Intracorneal ring segments can be implanted to help reshape the cornea and reduce irregular astigmatism. For advanced cases, a corneal transplant may be recommended, and the specific technique depends on the extent and location of thinning. Options include deep anterior lamellar keratoplasty, which replaces the outer corneal layers while preserving the inner endothelium, or a full-thickness penetrating keratoplasty when the thinning is too severe for a partial transplant.

PMD tends to progress gradually, and unlike keratoconus, it may continue to worsen throughout adulthood rather than stabilizing in middle age. The rate of progression varies widely from person to person. Some patients maintain functional vision with contact lenses for many years, while others experience more rapid thinning that eventually requires surgical intervention. Regular monitoring with corneal imaging allows cornea specialists to detect changes early and adjust treatment accordingly.

Scleral lenses are considered one of the most effective nonsurgical treatments for PMD because they bypass the irregular corneal surface entirely. By creating a tear-filled vault over the cornea, scleral lenses provide a smooth, uniform refracting surface that corrects even severe irregular astigmatism. Most patients with PMD experience a meaningful improvement in visual clarity and comfort with properly fitted scleral lenses.

Yes, PMD typically affects both eyes, although the severity is often asymmetric, meaning one eye may be more advanced than the other. Because of this bilateral tendency, patients who are diagnosed in one eye should have the other eye carefully evaluated and monitored as well. Early detection in the less-affected eye provides more time to plan treatment and potentially intervene with cross-linking before significant vision changes develop.