The corneal endothelium plays a critical role in the maintenance of overall corneal transparency essential for normal visual function. Transparency is regulated, in part, by modulation of stromal water content, counterbalancing a steady inward “leak” of water and nutrients from the anterior chamber, which is driven both by intraocular pressure exogenously and by an endogenous stromal swelling pressure. The apical junctions between endothelial cells are “leaky,” and water is “pumped” out of the stroma by a number of ion-specific independent pumps (H⁺, Na⁺/K⁺, HCO₃⁻) located around the circumference of each cell in the endothelial monolayer. Unfortunately, corneal endothelial cells in the adult human become growth downregulated and do not regenerate if damaged by eye surgery, external chemical or blunt trauma, or internal inflammatory diseases.

The noncontact specular microscope is a widely available clinical instrument that is ubiquitously used to evaluate the corneal endothelium of both the normal and diseased or injured cornea in vivo, or in ex vivo assessment of tissue viability for corneal transplantation in eye banking (Fig. 10.2-1). As is discussed in more detail in Chapter 10.1, the optical principles of specular microscopy represent a special case of general confocal microscopy, which may ultimately replace the specular microscope in general clinical use.

David Maurice (1) developed the first specular microscope in 1974 to visualize the corneal endothelium at high magnification and x, y plane resolution. This was successful because of two “tricks”: (a) When the angle of the light passing into the cornea was placed exactly at the same angle as the observer’s view (angle of incidence equals angle of reflection), all other backscattered light is out of phase and hence “not seen,” producing an excellent view of the endothelial monolayer; and (b) endothelial cells are thin (3 to 6 μm), and thus focus on the apex of the cells provides an image of a flat x, y sheet of cells. If there are protuberances that prolapse individual cells or groups of cells forward (i.e., toward the aqueous), these are seen as black areas that represent no backscattering of light from the normal cell sheet focal plane. Such protuberances occur in Fuchs’ endothelial corneal dystrophy as “warts” or guttata on the underlying basal limiting membrane (Descemet’s), pushing forward the apex of endothelial cells, which are then out of focus and appear black on specular micrographs. Scattered areas of endothelial cell damage and cell swelling that may occur in the excision of scleral rim-corneal tissue in eye banking also are imaged as out-of-focus or black areas.

Unfortunately, there are important limitations of specular microscopy in providing useful clinical information. To obtain clear images, the overlying cornea must be transparent (i.e., no corneal edema, Descemet’s folds, and the like) and the endothelial apical surface must be smooth and uniform. The second major disadvantage is that specular microscopy does not permit three-dimensional optical sectioning of the cornea; viewing is restricted to a thin, single-cell plane. Thus, views of the corneal epithelial surface obtained with specular microscopy show light and dark cells with overlapping borders, making quantitative assessment of surface cell areas or number infeasible, even when a contact lens is used to flatten the corneal surface. By contrast, as discussed in Chapter 10.1, confocal microscopy easily overcomes both of these limitations.

Despite these problems, noncontact specular microscopy is a low-cost technology that can provide a useful assessment of the corneal endothelium in vivo in many cases, and remains in widespread use in most clinics and eye banks (2).