Topographic analysis in pellucid marginal corneal degeneration and keratoglobus.

Pellucid marginal corneal degeneration (PMCD) is an uncommon cause of inferior peripheral corneal ectasia, affecting patients between the ages of 20 and 40 years. Although histopathologically it is considered a variant of keratoconus, it differs in that the marked corneal steepening occurs more inferiorly, above a narrow band of corneal stromal thinning concentric to the inferior limbus. Here we present two cases. The first case is a clinically typical bilateral PMCD with a characteristic pattern of irregular against-the-rule astigmatism on corneal topography. The second case had an uncommon presentation of hydrops in a clinically keratoglobic eye which showed a marked steepening of the inferior corneal periphery on corneal topography. The other eye showed both clinically and topographically the features of PMCD. Corneal topography suggested that in the second patient, PMCD may have preceded the development of keratoglobus. Keratoconus, PMCD and keratoglobus are considered to be associated as part of the spectrum of non-inflammatory corneal thinning disorders. However, although the finding of PMCD and keratoconus in fellow eyes has been reported, to the best of our knowledge progression from PMCD to keratoglobus has not previously been shown.