Morphological link between accommodation, presbyopia and cortical cataract

Purpose Evaluation of the gross morphology, the cortical location and the fiber cell architecture of cortical opacities in the aging human lens. Methods 39 human donor lenses were photographed in toto in frontal view using dark-field stereomicroscopy. 15 lenses were fixed, cut in axial slices and photographed the same way. Details of fiber cell architecture were investigated by fluorescent staining for membranes and by scanning electron microscopy. Results Small focal and cuneiform cortical cataracts are discrete opacities located at a specific depth below the capsule, and extending from the equatorial region in anterior and posterior direction. A sharp border is observed, between the opacities with their disorganized fiber architecture and the deeper nuclear layers, which show a regular fiber pattern. This border is at a mean depth of 700 µm below the capsular surface. Close examination of the opacities revealed fiber folds, fiber undulations, fiber-to-fiber separations, fiber breaks, water lakes and fiber displacement. Because the lens cortex and nucleus have different viscoelastic properties in young and old lenses, we hypothesize that external forces during accommodation cause shear stress predominantly in this border zone. Conclusion The location of the described changes suggests that mechanical forces may cause fiber disorganization, small cortical opacities, and, ultimately, cuneiform cataracts. Our hypothesis would be in line with recent findings by Truscott and co-workers about a two-compartmental organization of the human lens. They found biochemical arguments for a transport barrier developing at a middle age at the cortical/nuclear interface.