Bilateral progressive essential iris atrophy and keratoconus with coincident features of posterior polymorphous dystrophy: a case report and proposed pathogenesis.

We report the first case known to us of an apparent bilateral association of essential iris atrophy (EIA) and keratoconus (KC), with coincident features of posterior polymorphous dystrophy (PPD). Based on this case and the published natural history and findings of both the irido corneal endothelial (ICE) syndrome and PPD, we propose a new hypothesis for the pathogenesis of the ICE syndrome with associated KC and/or PPD. We suggest that, similar to the genetics of retinoblastoma, the predisposition for either the ICE syndrome or for PPD is inherited as an inactive allele, the so-called "first hit." Inactivation of the second allele, or "second hit," which could occur at any time, might be the product of the background mutation rate or of an environmental trigger. Dedifferentiation or an abnormality in normal development could occur after the first or second hit, resulting in varying clinical patterns. We also concur with other investigators that PPD could be part of the spectrum of the ICE syndrome, owing to similarities in their clinical presentations, histopathology, specular and electron microscopy, and natural history.