Measuring Corneal Haze by Using Scheimpflug Photography and Confocal Microscopy

Abstract

Purpose: We compared corneal backscatter estimated from a Scheimpflug camera with backscatter estimated from a clinical confocal microscope across a wide range of corneal haze.

Methods: A total of 59 corneas from 35 patients with a range of severity of Fuchs' endothelial corneal dystrophy and 15 corneas from 9 normal participants were examined using a Scheimpflug camera (Pentacam) and a confocal microscope (ConfoScan 4). The mean image brightness from the anterior 120 μm, midcornea, and posterior 60 μm of the cornea across the central 2 mm recorded by the Scheimpflug camera and analogous regions from the confocal microscope were measured and standardized. Differences between instruments and correlations between backscatter and disease severity were determined by using generalized estimating equation models.

Results: Backscatter measured by the two instruments in the anterior and midcornea were correlated (r = 0.67 and 0.43, respectively, P < 0.001), although in the posterior cornea they were not correlated (r = 0.13, P = 0.66). Measured with the Scheimpflug camera, mean backscatter from the anterior and midcornea were greater, whereas backscatter from the posterior cornea was lower (P < 0.001) than that measured by the confocal microscope. Backscatter from the anterior cornea was correlated with disease severity for both instruments (Scheimpflug, r = 0.55, P < 0.001; confocal, r = 0.49, P = 0.003).

Conclusions: The Scheimpflug camera and confocal microscope should not be used interchangeably to measure corneal haze. The ability to detect changes in backscatter with disease severity is superior with the Scheimpflug camera. However, the confocal microscope provides higher resolution of corneal structure.

Figures

Figure 1

Corneal volumes selected by the Pentacam Scheimpflug camera for determining backscatter in this study. Mean image brightness was determined in each of three cylindrical volumes, with 2-mm diameter centered on the apex of the cornea. The anterior cylindrical volume included the anterior 120 μm of cornea, the posterior included the posterior 60 μm, and the midcornea included the region between these boundaries.

Figure 2

Brightness of Amco Clear in Scheimpflug images. Image brightness of Amco Clear in a contact lens (S) increased as the concentration of Amco Clear (A) increased. Data were fitted to a third-order polynomial, which was used to determine the concentration of Amco Clear that gave the same scatter as the region of interest as indicated by the right-directed and down-directed arrows. Backscatter in SU refers to the concentration of Amco Clear that produced the same image brightness as the cornea.

Figure 3

Sample images from a patient with Fuchs' endothelial dystrophy. The Scheimpflug image (left) shows one slit image. Backscatter was determined in the region identified by the rectangle. The measurement region of the confocal microscope was scanned axially approximately along the broken line. The image brightness profile through the cornea from the confocal microscope (lower right) shows brightness peaks at the epithelial surface (A), the stromal surface (B), and the endothelial surface with guttae (D), and a region of relatively uniform brightness at the midcornea (C). Representative images from these regions show characteristic structures (upper right). The mean image brightness was calculated from all video frames in the regions represented by the anterior and posterior shaded areas (Anterior and Post.) and the midcornea as described in the text. These regions corresponded to the anterior 120-μm, midcornea, and posterior 60-μm regions automatically selected by the Scheimpflug camera.

Figure 4

Relationships between backscatter (in SU) from the Scheimpflug camera (S) and confocal microscope (C) in all participants. In the anterior (left) and midcornea (center), measurements from the two instruments were correlated, whereas they were not correlated in the posterior cornea (right). The regression lines in the anterior and midcornea were approximately parallel to each other, but were offset and had slopes equal to 0.40 and 0.29, considerably less than 1. The diagonal line in each graph represents the identity line.

Figure 5

Mean corneal backscatter (in SU) from the Scheimpflug camera and confocal microscope in three regions of the cornea. Mean backscatter measured by confocal microscopy was less than that measured by Scheimpflug photography in the anterior and midcornea. However, the mean backscatter was higher in the posterior cornea by confocal microscopy compared to Scheimpflug photography. The higher backscatter in confocal microscopy in this region is likely associated with specular reflection from the endothelium.

Figure 6

Corneal backscatter as a function of severity of Fuchs' dystrophy. Backscatter (in SU) in the anterior cornea measured by both instruments increased with severity of the disease. In the midcornea, the relationship with disease severity was slightly weaker than in the anterior cornea by Scheimpflug photography and not significant by confocal microscopy. In the posterior cornea, when measured by Scheimpflug photography, backscatter increased with disease severity, but when measured by confocal microscopy backscatter trended weakly downward. In regression equations, S = backscatter from Scheimpflug camera, C = backscatter from confocal microscope, Gr = grade of severity of Fuchs' endothelial dystrophy (grades 1–6) and normal (grade 0).

Figure 7

Optical design of the Scheimpflug camera and the confocal microscope. At specular surfaces, such as the endothelium, the specular reflection of the illumination from the Scheimpflug camera is directed back at the light source and image brightness is dominated by backscatter. In the confocal microscope, the specular reflection from the light source is directed into the detection path, because the optical axis of the illuminator and detector are at equal angles to the axis perpendicular to the surface, and image brightness is dominated by specular reflection. This reflection appears as a large peak at the endothelial surface with the confocal microscope and is not seen in the Scheimpflug camera. Instrument lenses and the cornea are not drawn to scale.