New perspectives on keratoconus as revealed by corneal confocal microscopy

Confocal microscopy (CM) of keratoconus is reviewed. In the Manchester Keratoconus Study (MKS), slit scanning CM was used to evaluate 29 keratoconic patients and light microscopy (LM) was performed on two of the keratoconic corneas post-keratoplasty. The findings of the MKS are compared with other CM studies. Consideration of the differences between studies of cell counts is confounded by the use of different experimental controls. A consensus exists among studies with respect to qualitative observations. The epithelium appears more abnormal with increasing severity of keratoconus. In severe disease, the superficial epithelial cells are elongated and spindle shaped, epithelial wing cell nuclei are larger and more irregularly spaced and basal epithelial cells are flattened. Bowman's layer is disrupted and split in the region of the cone and intermixed with epithelial cells and stromal keratocytes. Stromal haze and hyper-reflectivity observed with CM correspond with apical scarring seen with the slitlamp biomicroscope (SLB). Hyper-reflective keratocyte nuclei are thought to indicate the presence of fibroblastic cells. Increased haze detected with CM is found with LM to be due to fibroblastic accumulation and irregular collagen fibres. Dark stromal bands observed with CM correlate with the appearance of Vogt's striae with SLB. Desçemet's membrane appears normal with both CM and LM. Some evidence of endothelial cell elongation is observed with CM. The application of CM to ophthalmic practice has facilitated a greater understanding of medical and surgical approaches that are used to treat keratoconus. This review offers new perspectives on keratoconus and provides a framework, against which tissue changes in this visually debilitating condition can be studied in a clinical context in vivo using CM.