Comparison of central corneal thickness using anterior segment optical coherence tomography vs ultrasound pachymetry

Abstract

Purpose: To determine if there is a systematic difference in central corneal thickness (CCT) measured using anterior segment optical coherence tomography (AS-OCT) as compared with ultrasound pachymetry.

Design: Prospective observational study.

Methods: Consecutive subjects with clinically normal corneas underwent CCT measurement by both ultrasound and AS-OCT while participating in a population-based study in Ghana, West Africa. One eye of each subject was randomly selected for analysis. Two measurements were taken and averaged. Agreement and interobserver variability were also analyzed.

Results: One hundred and fifty-five subjects of African ethnicity and average age 57 years (standard deviation [SD] 12; range, 40 to 98 years) were included. Measurements by AS-OCT and US were taken a mean of 15 days (maximum, six weeks) apart. The mean (SD) [range] US CCT was 525.3 microm (33.5) [422, 653] and 499.0 microm (32.0) [428, 613] with AS-OCT. Measurements by the two modalities were strongly correlated (r(2) = 0.82; P < .001), and a significant difference was observed between mean US and AS-OCT CCT (SD) [range] of 26.3 microm (14.2) [-63, 12] (P < .001). The width of the limits of agreement was 28 microm, about 6% of the average pachymetry reading. In 50 eyes randomly remeasured with OCT by a second observer, the intraclass correlation coefficient was 0.91. There was a small but significant systematic difference between observers (mean 6.9 microm, SD 10.9 microm), or 1.4% (P < .001), increasing the difference noted above.

Conclusion: There is a reproducible systematic difference between CCT measurements taken with ultrasound and OCT. It is important to note in clinical practice, that measurements acquired by these two modalities are not directly interchangeable.

Figure 1

Cross-sectional image of a subject’s anterior chamber acquired by AS-OCT, demonstrating the method used to measure CCT. The corneal apex was identified after correcting ocular rotation, from the peak of the reflectivity profile on the horizontal axis (below the image). The callipers were then aligned on the peak reflections of the anterior and posterior tissue boundaries of the cornea in the axis of the corneal apex (to the right of the image). The two measurements were averaged for each eye.

Figure 2

Scatterplot of CCT measurements by AS-OCT versus ultrasonography for each subject. Almost all measurements lie below the equivalence line demonstrating that CCT measurements are slightly higher with ultrasonography than AS-OCT in almost all subjects.

Figure 3

Bland-Altman plot of AS-OCT – ultrasonography difference versus average of ultrasonic and AS-OCT CCT measurements. This demonstrates the level of the mean difference between the two instruments (bold horizontal line), the limits of agreement (horizontal lines above and below the bold line), and that the systematic difference observed is unrelated to mean CCT.

Figure 4

Scatterplot of AS-OCT pachymetry measurements made by two observers (N=50), demonstrating high correlation (r²=0.91) but with observer 1 measuring slightly higher CCT readings (mean difference 6.9 µm, P < 0.001) than observer 2 in most subjects.