Objective assessment of the corneal endothelium in Fuchs' endothelial dystrophy

Abstract

Purpose: To develop a standardized method of endothelial cell density (ECD) assessment in Fuchs' endothelial dystrophy that maximizes the sample area and uses the clearest endothelial cells in confocal images.

Methods: The corneal endothelium of 51 eyes from 30 patients, with varying degrees of Fuchs' endothelial dystrophy, was examined using confocal microscopy. In two or three distinct images of the central endothelium, local contiguous cell density was determined using a variable frame method. The effective ECD was the product of the local cell density and the fraction of the image that was free of guttae. Two examiners assessed the severity of disease in each eye during slit-lamp examination and assigned a severity grade of 1 to 6. In a second group of 55 eyes with Fuchs' dystrophy from 30 patients, the clinical grade was predicted from the effective ECD and the regression coefficients of the first group and compared to the subjective clinical grade assigned by one examiner.

Results: The effective ECD decreased linearly with subjective grade (r = -0.93, P < 0.001). The grade predicted from the effective ECD differed from the subjective clinical grade by -0.1 ± 0.8 (mean difference ± standard deviation).

Conclusions: The effective ECD in confocal images provides an objective means of assessing the corneal endothelium in Fuchs' dystrophy and might be a useful tool in clinical studies.

Keywords: Fuchs' endothelial dystrophy; corneal endothelium; endothelial cell density; guttae.

Figure 1

Methods of assessing endothelial cell density from confocal images in Fuchs' endothelial dystrophy. Local endothelial cell density (left) was the number of cells in contiguous patches of clear cells divided by the total area of the cells. The effective cell density was calculated from the local cell density and the fraction of the image that was not covered by guttae, as determined by image processing (center). With the fixed-frame method, all visible cells were counted in a fixed rectangular area superposed on the endothelial image (right). By convention, cells that overlapped the lower or left boundary (solid line) were counted while cells that overlapped the upper or right boundary (dashed line) were not counted. The fixed frame was always the same size and centered on the image. In this example, if the frame had been repositioned in the lower right portion of the image, the method would have yielded a much higher cell density than if it had been positioned in the upper left corner.

Figure 2

Mean local endothelial cell density decreased as severity of Fuchs' dystrophy increased. Each point was the mean cell density in two or three frames. Frames that were filled with guttae and had no visible cells had a cell density of zero. The regression line decreased to approximately 332 cells/mm² at a clinical grade of 6.

Figure 3

Mean endothelial cell density determined by the fixed-frame method decreased as severity of the disease increased to a clinical grade of 3. At higher grades, mean densities were typically less than 500 cells/mm² and changed minimally. The fitted line is a second-order polynomial.

Figure 4

The mean fraction of the image covered by guttae increased as disease severity increased.

Figure 5

The mean effective endothelial cell density decreased as severity of the disease increased. The fitted line decreased to zero at grade 5.5.

Figure 6

Grade of severity in the test group of patients was predicted by the effective endothelial cell density and regression coefficients from the trial group. The regression through these data (solid line, Predicted Grade = Clinical Grade × 0.87 + 0.37) was close to the identity line (dashed line). Corneas with zero effective ECD had predicted grades of 4, 5, and 6, and multiple samples with the same value were displaced slightly so they could be distinguished from each other.

Figure 7

Difference between predicted and subjective clinical grade of disease severity with mean of predicted and subjective grade, according to the method of Bland and Altman. The mean difference was −0.1 and is illustrated as the wide broken line. No difference is indicated by the solid line. The limits of agreement (−1.7 to 1.6), which include approximately 95% of the differences, are indicated by the narrow broken lines. Multiple samples of corneas with the same subjective and predicted grades were displaced so they could be distinguished from each other.

Figure 8

Mean image brightness in the anterior cornea increased as effective cell density decreased in the test group. Corneas with the brightest images and greatest backscatter were those covered with guttae and with no visible endothelial cells.