Both qualitative and quantitative assessments of the corneal endothelium can be made. Qualitative cellular analysis identifies abnormal endothelial structures and grades the endothelium either according to the number or size of the abnormal structures present or on the basis of an overall visual assessment of endothelial appearance. The goal is to provide a subjective evaluation of the endothelium, not to assign a precise numerical value to the specular photomicrograph. This type of analysis provides a rapid clinical evaluation of the endothelium to assess the risks of intraocular surgery, to establish a diagnosis, or to decide upon treatment. Complete qualitative analysis requires that several parameters be evaluated16 including cell conformation, cell boundaries and their intersections, configuration of the dark boundary, and the presence of acellular structures. One must be careful to eliminate optical artifacts from consideration when performing either qualitative or quantitative analysis of the corneal endothelium. Cell Conformation The specular microscope shows A pattern of contiguous cells having well defined cell boundaries. The central endothelial cells of young people with normal eyes are hexagonal and approximately the same size; the distribution of cell area is approximately Normal (Gaussian). With age, the average cell area increases, the cellular pattern becomes distinctly pleomorphic, and the cell size distribution becomes skewed toward larger cell areas. In young people with normal eyes the cell side lengths are all roughly equal. In older individuals, the side lengths lose this regularity and one sees an increasing variation.
In addition to changes in size, endothelial cells may assume a shape that is substantially different from the usual quasi-hexagonal configuration. Figure A shows endothelial cells that are both enlarged and elongated. These cells were encountered near the corneal apex in a case of keratoconus. They appear to be aligned in the same direction and to follow lines of stress as if they have been stretched by gross deformation of the cornea. Cells with scalloped rather than conventional straight sides are occasionally seen (Fig. B). So, too, are round cells (Fig. C and D), square cells (Fig. E), and triangular cells (Fig. F). As in the case with changes in endothelial cell size, alterations in shape have not been directly related to changes in the physiologic function of the affected cells. Miscellaneous Structures
A number of inter- and intra-endothelial cell structures, which may be either dark or bright in appearance, are seen in endothelial photographs.16 One type of dark structure disrupts the endothelial cell pattern and can range in size from a structure smaller than a single cell to one larger than an individual endothelial cell. Each such structure generally has dark sides and a central bright spot (Fig. A). Such structures represent a smooth excrescence of Descemet's membrane (i.e., cornea guttatae) and often are surrounded by a ring of abnormally shaped cells. Cornea guttate can be seen at a much earlier stage with the specular microscope than with the conventional slit-lamp biomicroscope.17 When these excrescences are abundant, they begin to touch one another and to coalesce. One sees this pattern illustrated in Fig. B; although the endothelial cell pattern is not visible, the bright reflection from the apex of each excrescence is clearly seen. (See the discussion of Fuchs' dystrophy later in this chapter). Two additional types of dark bodies, both intracellular in location, have been observed. The first type is small, generally located in the central or paracentral portion of the cell, and has sharp, well-defined edges showing that it is located on the posterior surface of the cell (Fig. F). This structure has been seen in clinically normal corneas, and when present, it occurs in many but not all cells. It presumably represents the base of an endothelial cilia, although histological verification of this has not yet been obtained. The second type of dark body is considerably larger and has indistinct edges, suggesting that it is located within the cell (Fig. G). It may represent an intracellular vacuole or bleb. Intercellular dark structures, lying predominantly at endothelial cell intersections, have been observed (Fig. H).16 They tend to be uniform in size, and within a given frame they are randomly positioned across the endothelial cell pattern. These structures occur in patients with anterior uveitis, and it is believed they represent invading inflammatory cells. Intracellular bright structures, some of which may be only the cell nucleus, have been seen in endothelial photomicrographs.16 They are variable in size and typically are contained completely within a single endothelial cell (Fig. C and D). Occasional exceptions do occur, however, where the bright structure seems to cross a cell boundary. These intracellular bright structures appear to be associated with stressed cells, explaining why they commonly are seen within enlarged cells such as those encountered in successful corneal transplants. Their size or number (multiple bright structures may occur within a single greatly enlarged cell) seems to be proportional to the size of the cell. That is, the larger the endothelial cell, the larger the intracellular bright structure. A second type of bright structure spans several endothelial cells (Fig. E) and is positioned at random on the endothelial cell pattern of the specular photomicrographs. When viewed directly, these structures appear to sparkle; some are orange, while others are white. Slit-lamp biomicroscopy of these corneas reveals numerous pigment deposits on the endothelium. The bright structures seen with the clinical specular photomicroscope appear to correspond to the pigmented endothelial deposits seen with the slit-lamp biomicroscope, and presumably they are the same abnormality. Morphologic variations in endothelial cell configuration, cell surface properties, and intercellular boundaries, as well as the presence of numerous intracellular structures, can be identified by the clinical specular microscope. Although the nature and significance of many of these abnormalities are not presently known, their recognition represents an initial step in the elucidation of their pathophysiological significance. |
References: 16. Koester C. Comparison of optical sectioning methods. The scanning slit confocal microscope. In: Pawley J, ed. The Handbook of Biological Confocal Microscopy. Madison: IMR Press; 1989:189-194. 17. Kino G, Chore C, Xiao Q. Imaging theory for the scanning optical microscope. Proc-SPIE. 1988;1028:104. |
