Differentiation of posterior polymorphous dystrophy from other posterior corneal opacities by specular microscopy

Deep corneal opacities due to posterior polymorphous dystrophy, especially if they are mild, are frequently confused with changes due to other corneal conditions. In a series of 64 patients with deep corneal opacities (14 with Descemet's tears, 8 with posterior polymorphous dystrophy, 12 with interstitial keratitis, and 30 with keratoconus), specular microscopy enabled the differentiation of deep corneal changes provided some degree of corneal clarity was retained. The parallel "rail track" borders of old Descemet's tears were characteristic and differed from the "snail tracks" seen in posterior polymorphous dystrophy, which also showed characteristic rounded vesicular or doughnut-like lesions in Descemet's membrane. The appearances in interstitial keratitis were varied with poor detail in most cases because of corneal opacity. However, fine opacities consistent with ghost vessels were seen in interstitial keratitis and a resolving case showed fine, intersecting straight lines different from the bundles of parallel fine vertical lines of deep corneal striae in keratoconus. Specular microscopy also enables an assessment of the corneal endothelium which is important if intraocular surgery is contemplated.