As mentioned above, accurate CCT measurements are essential for refractive surgery and the diagnosis and management of patients with glaucoma and endothelial disorders. Although the E-pach is a new, portable pachymeter, no study has yet to investigate the precision and agreement of this instrument. Our results demonstrated that the E-pach had great repeatability and reproducibility in CCT measurements. The agreement between the E-pach and other instruments is, however, not good. The E-pach yielded CCT values that were significantly lower than those from the A-scan and Pentacam but significantly higher than those from the Corvis ST.
Our study showed great repeatability by the E-pach: the TRT value was 3.7007 μm, and the ICC value was 0.9981 (far beyond 0.9). Repeatability of CCT values measured by the A-scan, Corvis ST and Pentacam devices were almost equally great.
The repeatability of CCT values measured by the A-scan, Corvis ST and Pentacam devices in the present study are consistent with previous studies. Our study showed satisfying repeatability using the A-scan: the TRT value was 14.0345 μm, and the ICC value was 0.9797. Bao et al. assessed the repeatability of CCT with the A-scan. They found that the TRT value was 7.65 μm, and the ICC value was 0.994. Moreover, Gokcinar et al. also demonstrated that the ICC value of the A-scan is 0.994. Our study also showed satisfying repeatability by Corvis ST: the TRT value was 15.0946 μm, and the ICC value was 0.9679. Yu et al. and Chen et al. both assessed the repeatability of CCT with the Corvis ST, they found that the TRT values were 13.0 μm and 12.56 μm, and the ICC values were 0.971 and 0.99. Meanwhile, our study also showed satisfying repeatability using the Pentacam: the TRT value was 11.3098 μm, and the ICC value was 0.9852. Huang et al., Wiswanathan et al., and Crawford et al. evaluated the repeatability of CCT with Pentacam and indicated that the ICC values were 0.980, 0.984 and 0.979, respectively.
In addition to repeatability, interobserver and intersession reproducibility of CCT measurements acquired with the abovementioned devices were also estimated. For interobserver reproducibility of the E-pach, the TRT value was 5.0082 μm, and the ICC value was 0.9971; for intersession reproducibility of the E-pach, the TRT value was 12.5110 μm, and the ICC value was 0.9825. As expected, the intersession reproducibility was slightly worse than the interobserver reproducibility; however, the general reproducibility was far beyond good (ICC=0.90). Reproducibility of CCT values measured by the A-scan, Corvis ST and Pentacam devices were almost equally great.
Reproducibility of CCT values measured by the A-scan, Corvis ST and Pentacam devices in the present study were similar to previous studies. Nam et al. evaluated the interobserver reproducibility of CCT measurements using A-scan. They found a TRT value of 5.2 μm and an ICC value of 0.995. Ali et al. evaluated the intersession reproducibility of CCT measurement using Corvis ST. They found a TRT value of 11 μm and an ICC value of 0.980. Bourges et al. assessed the interobserver and intersession reproducibility using Pentacam at ICC values of 0.993 and 0.980, respectively.
When the CCT readings were compared among the four devices, a significant difference in average CCT values was observed. Specifically, in the results of the T test, average CCT values of the A-scan were significantly higher than those of the Pentacam; however, these Pentacam values were significantly higher than those of the E-pach, which were significantly higher than those of the Corvis CT. The results of the Bland-Altman plots were consistent with the T test.
Several previous studies have reported that CCT readings of the A-scan are higher than those of the Pentacam and Corvis ST devices. Smedowski et al. assessed the agreement of CCT readings using the Pentacam, Corvis ST and A-scan devices. They indicated that the mean CCT value was the highest for the A-scan and the lowest for the Corvis ST. Another two studies [13, 24] also reported that a small underestimation of CCT values with the Pentacam compared to the A-scan. There are two possible explanations for this underestimation: (1) the CCT readings by the A-scan were affected after giving topical 0.5% proparacaine hydrochloride, which increased corneal thickness (8.6 μm increase in 80 seconds) ; (2) the accuracy of the A-scan is influenced by whether the probe is placed as perpendicular as possible to the centre of the cornea.[38-40] The results, however, are still controversial. For example, Tai et al. showed that the CCT value of the Pentacam is overestimated by 10 μm compared with that of the A-scan.
Despite controversy, we did obtain some valid results. Because the CCT readings of the E-pach were statistically and clinically different (>10 µm) from those of the other instruments (whether by T test or by Bland-Altman), the E-pach is not interchangeable with them.
There were some limitations in our present study. We only evaluated the precision and agreement of CCT measurements in normal eyes. In further research, ocular disorders including glaucoma, myopia, keratoconus, leucoma, or post-refractive surgery could be included. In addition, other types of instruments could be included in future studies.
In conclusion, the new versions of the A-scan and E-pach are portable, relatively inexpensive and reliable (our study indicated that the E-pach displayed great repeatability and reproducibility). However, the CCT values obtained from the E-pach are not interchangeable with those from the traditional A-scan, Pentacam and Corvis ST devices.