Corneal endothelial assessment with special references to keratoconus.

This review examines problems associated with in vivo assessment of corneal endothelium with special reference to keratoconus (KC) corneas. The main difficulty appears to be that a single assessment of less than 1/400 of the total number of endothelial cells in any eye may not detect any abnormal levels of heterogeneity. A diagnosis of polymegethism, pleomorphism, corneal guttae, or incipient Fuchs dystrophy may be missed if the sample observed is not representative. Evaluation of the endothelium may be more important in KC when the endothelium may be more susceptible to ultraviolet radiation due to stromal thinning and/or be more susceptible to damage from wearing thicker contact lenses that place hypoxic demands on endothelial deturgescing function. Also, assessment appears to be a greater problem for KC corneas for which central observations are likely to miss any changes in the region of an eccentric cone or in noncentral areas that may be affected by ruptures of the posterior limiting lamina. Apart from genetic influences and ethnic and/or racial considerations, factors such as exposure to ultraviolet and other geographic or vocational variables, atopy, or specific ocular allergies, which may be related to chronic habits of abnormal rubbing and any associated damage due to elevated intraocular pressure, may all contribute to variable presentations of the endothelium. Because of the phenotypic diversity of KC, the status of the endothelium may vary according to many other criteria including age of onset, rate of progression, degree of asymmetry, the type and number of biomicroscopic signs, degree of thinning, and the location, type, and area of the cone, for example. For in vivo assessments, observation of multiple sites is required, but for KC, it should include the cone area to increase the chance of detecting any significant differences from normal.