Posterior Polymorphous Corneal Dystrophy

Corneal dystrophies (CD) are a group of genetically determined diseases that are restricted to one or more layers of the cornea. These can be characterized as bilateral, symmetric, slowly progressive and have no systemic manifestations. Most often these are inherited as a dominant trait. Recessive inheritance occurs seldomly in a limited number of pedigrees. Ocular symptoms of CD can include blurred vision, foreign body sensation, eye pain, photophobia, and lacrimation . Posterior polymorphous corneal dystrophy (PPCD) was described first by Koeppe in 1916. PPCD is a rare, hereditary condition passed on in an autosomal dominant manner. Manifestations of this condition are highly variable. Affected family members may have distinctly different ocular presentation. The Descemet basement membrane laid down by endothelial cells is abnormally thickened in PPCD.  Corneal edema associated with PPCM may be evident at a very young age. However, the majority of patients are asymptomatic and diagnosis occurs years later at the time of a routine ocular health examination .   Bilateral presentation is typical for corneal dystrophies but an asymmetric appearance is not uncommon with PPCD. Corneal findings are either nonprogressive or very slowly progressive . This condition is most easily identified with careful inspection of the posterior cornea, specifically the Descemet membrane and the adjacent corneal endothelium. In PPCD, the Descemet basement membrane laid down by endothelial cells is abnormally thickened. Groupings of blister-like vesicles and gray-white plaques lead to opacification of the posterior cornea. The appearance of the opacities at the Descemet membrane may be characterized by one of three different distribution patterns; vesicular, band-like or diffuse. These polymorphous lesions may be visualized at the slit lamp biomicroscope with moderate to high magnification using direct or, preferentially, retro-illumination techniques. Vesicular lesions are frequently clustered together. They have a cystic appearance, variably sized. The well-circumscribed lesions have a translucent center that is banded by a darker perimeter. The band-like lesions are most often seen as two parallel bands with serrated edges. These bands can be seen in any orientation but most often they are oriented horizontally. The band length can vary from 2 to 10mm. In contrast to the Descemet's findings associated with trauma or congenital glaucoma (Haab striae), in PPCD the bands do not taper at the ends . Diffuse polymorphous opacities are the least common finding but tend to be the most visually disruptive. Geographic aggregations of small opacities are dispersed across a portion of the Descemet membrane creating a hazy appearance of the posterior stroma. A peau d'orange appearance is appreciated with retro-illumination . PPCD may also involve the iris and the anatomical angle. Rather than confining themselves to the posterior surface of the cornea, the aberrant endothelial cells can migrate erratically across the chamber angle and onto the peripheral iris surface. Abnormal basement membrane is secreted, creating iridocorneal adhesions and a glass-like appearance that can be appreciated gonioscopically. On the iris surface, this membrane can lead to corectopia and pupillary ectropion. Areas of peripheral anterior synechiae (PAS) with segmental zones of angle-closure can occur, increasing the risk for glaucoma . Additionally, peripheral corneal edema may be seen overlaying any broad regions of iridocorneal adhesion .