Protrusion of the cornea occurs above the band of thinning (Fig 7-32), which occurs 1–2 mm from the limbus and extends up to 4 clock-hours. In keratoconus, the cornea protrudes at the point of maximal thinning, which is typically just below the center. In contrast, PMD is associated with peripheral thinning (Fig 7-33) and exhibits protrusion central to the area of maximum thinning. At times, a clear distinction between PMD and keratoconus is not possible. In PMD, no vascularization or lipid deposition occurs, but posterior stromal scarring has been noted within the thinned area. PMD is diagnosed in most patients between 20 and 40 years of age, and men and women are affected equally. Decreased vision results from high irregular astigmatism. Acute hydrops has been reported, and, though rare, spontaneous corneal perforation has also occurred. While the “crab claw” pattern is the typical map appearance in PMD (Fig 7-34), inferior keratoconus has a similar appearance. Clinical correlation and review of elevation and pachymetric maps are important for more accurate differentiation.
Figure 7-32 Typically in pellucid marginal degeneration there is inferior protrusion of the cornea above the band of thinning, which occurs 1–2 mm from the limbus and extends up to 4 clock-hours.
(Courtesy of Vincent P. deLuise, MD.)
Figure 7-33 Slit view demonstrates protrusion of the cornea above a band of stromal thinning in pellucid marginal degeneration.
(Reproduced with permission from Feder RS, Neems LC. Noninflammatory ectatic disorders. In: Mannis MJ, Holland EJ, eds. Cornea. Vol 1. 4th ed. Philadelphia: Elsevier; 2017:836.)
Treatment consists of contact lens fitting early in the disease, although the fit is more difficult to achieve in PMD than in keratoconus. Hybrid or scleral lenses may be options. Eventually, PK may be required to restore vision. Because of the location of the thinning, the grafts tend to be large and close to the limbus, making surgery technically more difficult and the graft more prone to rejection. Wedge resection and lamellar tectonic grafts have been advocated as alternative or adjunctive procedures. Corneal crosslinking may also be considered for some of these patients.
Belin MW, Asota IM, Ambrosio R Jr, Khachikian SS. What’s in a name: keratoconus, pellucid marginal degeneration, and related thinning disorders. Am J Ophthalmol. 2011;152(2): 157–162.
Rasheed K, Rabinowitz YS. Surgical treatment of advanced pellucid marginal degeneration. Ophthalmology. 2000;107(10):1836–1840.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.