Managing Posterior Polymorphous Dystrophy
BY WILLIAM L. MILLER, OD, PHD, FAAO
As we continue our series on corneal dystrophies, let's take a look at a relatively uncommon deep dystrophy called posterior polymorphous dystrophy (PPMD). Although rare, I have been fortunate enough to see several PPMD cases in the last two years including in one of my students.
PPMD predominantly affects the posterior limiting lamina (Descemet's membrane) and endothelium. It is bilateral but often asymmetric.
PPMD is an autosomal dominant dystrophy with variable expressivity that has been mapped to Chromosome 20. It typically appears in the second and third decade of life, thus it mostly affects adults; however, cases have appeared in children. It can be associated with keratoconus; in a recent study by Cremona et al (2009), nearly 14 percent of patients who had keratoconus also had PPMD. This dystrophy has also been associated with peripheral anterior synechiae (PAS) and glaucoma. PAS may occur in up to one-third of patients who have PPMD.
The biomicroscopic appearance of PPMD demonstrates grouped vesicles and bands with scalloped edges that are located at the level of the corneal endothelium. Confocal and specular microscopy reveals three different types of configurations including vesicle-like, band-like, and diffuse opacities. The vesicle-like opacities measure 100-to-1,000 microns in diameter, while the diffuse opacities are two-to-five times larger. The band-like configuration appears as horizontal lesions.
PPMD is often confused with the corneal disease iridocorneal syndrome, but you can delineate PPMD based on its bilateral and heritable pattern. However, this delineation is not foolproof and may require specular or confocal microscopy to confirm the diagnosis. In addition to helping with diagnosis, these instruments may also be beneficial in monitoring progression of the disease process.
Most patients who have PPMD are asymptomatic. In rare cases, PPMD will affect vision. Either the metabolic pumps or barrier function can be affected in the disease process with resultant corneal edema. This corneal edema can range from subtle stromal thickness changes to a more dramatic bullous keratopathy. There is no set pattern for the induction of the edema; it can occur at any age and can be rapidly progressive or completely stable. This aspect of the disease process is what we can target using the same treatment modalities indicated for managing corneal edema that occurs with other dystrophic entities. Such treatments would include hyperosmotic topical agents and, in some cases, pressure-lowering medications. The latter strategy is especially important in the small percentage of patients (14 percent) that may succumb to elevated intraocular pressure (IOP).
The prognosis for PPMD patients is generally very good. A more severe case of PPMD would decrease the prognosis and is marked by several risk factors, which include the obvious presence of iridocorneal adhesions and an elevated IOP.
In severe cases with decreased visual performance, a corneal transplantation may be indicated. On the other hand, corneal surgery may be contraindicated in patients who have a visible PAS and increased IOP. In addition, it is possible for PPMD to recur after corneal transplantation. CLS
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Dr. Miller is an associate professor and chair of the Clinical Sciences Department at the University of Houston College of Optometry. He is a member of the American Optometric Association and serves on its Journal Review Board. You can reach him at firstname.lastname@example.org.