Overestimation of subfoveal choroidal thickness by measurement based on horizontally compressed optical coherence tomography images

Purpose: To measure the difference in subfoveal choroidal thickness between 1:1 pixel (horizontally compressed) images and 1:1 micron images in age-related macular degeneration. Methods: This study included 122 eyes from 122 patients diagnosed with age-related macular degeneration. Choroidal thickness was measured using enhanced-depth imaging optical coherence tomography. The measurement line was drawn as a perpendicular line between Bruch's membrane and the chorio-scleral interface. The thickness was compared between measurements based on a 1:1 pixel image and a 1:1 micron image. Eyes with a straight vertical measurement line and oblique measurement line were classified into vertical measurement group and oblique measurement group, respectively. Intra-group comparisons of subfoveal choroidal thickness measurements based on the 1:1 pixel images and the 1:1 micron images were performed for the two groups. Results: The mean subfoveal choroidal thicknesses measured on the 1:1 pixel images and the 1:1 micron images were 232.3 ± 106.4 μm and 228.9 ± 108.1 μm, respectively (p = 0.003). In the vertical measurement group (86 eyes), the mean subfoveal choroidal thickness was 226.3 ± 109.9 μm and 225.4 ± 112.0 μm, respectively (p = 0.423). In the oblique measurement group (36 eyes), the thickness was 246.5 ± 97.3 μm and 237.5 ± 98.9 μm, respectively (p < 0.001). Conclusions: Significant overestimation of the subfoveal choroidal thickness was noted when it was measured on a 1:1 pixel image. This finding suggests that the measurement of choroidal thickness should be performed based on a 1:1 micron image, especially if the measurement line is not vertical.