Overestimation of Corneal Endothelial Cell Density in Smaller Frame Sizes in In Vivo Confocal Microscopy.

PURPOSE: To evaluate the effect of frame size on the calculated corneal endothelial cell density (CECD) in images of laser scanning in vivo confocal microscopy (IVCM).

METHODS: Forty-nine corneal endothelial images acquired by laser scanning IVCM (Heidelberg Retina Tomograph 3 with Rostock Corneal Module) with different endothelial cell densities were analyzed. In each image (160,000 μm), the CECD was calculated using the fixed-frame method by counting cells in the following frame sizes: 80,000 μm, 40,000 μm, 20,000 μm, 10,000 μm, 5000 μm, and 2500 μm. The calculated CECD was then compared with that of the variable-frame method as the reference value.

RESULTS: There was no significant difference in the calculated CECD between the variable-frame method (2004 ± 832 cells/mm), and the fixed-frame method using a 40,000-μm frame (2023 ± 810 cells/mm). On the other hand, the calculated CECD showed significant overestimations in frame sizes of 20,000 μm (2066 ± 820 cells/mm), 10,000 μm (2156 ± 785 cells/mm), 5000 μm (2352 ± 783 cells/mm), and 2500 μm (2715 ± 754 cells/mm), with P < 0.001 in all. This resulted in overestimations of 4.8 ± 9.8%, 11.9 ± 16.2%, 24.9 ± 23.1%, and 49.1 ± 38.8% for these frame sizes, respectively. Images with lower CECD demonstrated higher overestimations of cell density in smaller frame sizes.

CONCLUSIONS: In laser scanning IVCM images, there is significant overestimation of CECD if the cells are counted in frames smaller than 25% of the image. Similar frame sizes should be used when monitoring CECD over time.