Image courtesy of Mitch Ibach, OD, FAAO, Vance Thompson Vision.ĭiagnosis of PPMD is mainly clinical, requiring slit-lamp microscopy and in some cases, specular microscopy to determine the changes in Descemet’s membrane. Iridocorneal endothelial syndrome (ICE)įigure 1: Vesicular changes on slit-lamp biomicroscopy consistent with PPMD.Additionally, the resulting thickened cornea may result in a false reading of applanation tonometry. The thickened Descemet’s membrane and possible resulting stromal edema may result in blurred vision and corneal clouding. These abnormal epithelialized cells are keratinized, migrate randomly, secrete defective basement membrane cells, cause peripheral synechiae, and ineffectively remove fluid from the corneal stroma, increasing the risk of edema and glaucoma. The epithelium-like endothelial cells in a PPMD lesion are usually pleomorphic, multi-layered, exhibit positive staining for cytokeratin and have multiple microvilli on their surfaces. Pathophysiologyĭue to abnormal developmental differentiation, the endothelial layer transforms into cells similar to stratified, squamous epithelium, thus causing an abnormal basement membrane and thickening of Descemet’s membrane. The collagen type VIII alpha 2 gene (COL8A2) is found at the PPCD2 locus (1p34.3–p32.3) and has been proposed as a causative agent in the development of PPCD2 however, insufficient evidence in the literature has supported this notion. GHRL2 has also been shown to directly repress transcription of ZEB1 and thus play a role in EMT. Interestingly, recent developments have identified the transcription factor GHRL2 expressed at the PPCD4 locus (8q22.3–q24.12). Together these genes control epithelial-to-mesenchymal transition (EMT) and its converse process, mesenchymal-to-epithelial transition (MET) both are important for embryological development of the cornea. ZEB1, another zinc-finger transcription factor, is expressed at the PPCD3 loci (10p11.2). The PPCD1 locus (20p11.2–q11.2) encodes OVOL2, a zinc-finger transcription factor that directly represses expression of ZEB1. PPMD is inherited in an autosomal dominant manner, and it has been mapped to four different loci on chromosomes 1, 8, 10, and 20, thus showing locus heterogeneity. Disease Entity Molecular mechanisms and genetics Other infantile corneal dystrophies include congenital hereditary endothelial dystrophies, congenital hereditary stromal dystrophies, and posterior amorphous corneal dystrophies. PPMDs have also been associated with extraocular diseases, such as abdominal hernias and Alport Syndrome. Ocular diseases correlated with PPMD include secondary glaucoma and keratoconus. PPMD usually presents in early childhood or adolescence as clouding of the stroma that may cause blurred vision. estimate that 60% of congenital corneal dystrophies have endothelial involvement. Although the exact prevalence of PPMD is not known, studies of the prevalence of corneal dystrophies in the U.S. Whereas, CHED Type 2 is an autosomal recessive disease and presents immediately after birth. Congenital hereditary endothelial dystrophies have recently been classified into two types PPMD, formerly CHED Type 1, is the autosomal dominant disorder that presents after the first year of life and is on the same locus on chromosome 20 as PPMD. PPMD is a subtype of congenital hereditary corneal dystrophies, which often manifest as bilateral, non-inflammatory corneal opacities that may result in corneal edema of the stroma and degradation directly affecting vision. Posterior Polymorphous Corneal Dystrophy (PPMD, PPCD), also known as Schlichting dystrophy, is an autosomal dominant disorder of the corneal endothelium and Descemet’s membrane, producing a wide variability in clinical presentation.
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