The Reliability of Successive Scheimpflug Imaging and Anterior Segment Optical Coherence Tomography Measurements Decreases With Increasing Keratoconus Severity

PURPOSE This study assesses the reliability of successive measurements of tomographic parameters in different keratoconus (KC) stages with 2 different devices. METHODS A total of 125 eyes (13 controls: 24 eyes, and 73 patients with KC: stages 1|2|3|4, n = 24|24|26|27 according to Topographical KC Classification) were repeatedly examined 5 times with the rotating Scheimpflug tomograph (Pentacam HR, Oculus, Wetzlar, Germany) and an anterior segment optical coherence tomograph (Casia 2, Tomey, Nagoya, Japan). Outcome measures included 1) mean anterior (KA) and 2) mean posterior powers (KP), 3) mean anterior (AC) and 4) posterior cylinders (PC), 5) maximal anterior power (Kmax), and 6) thinnest corneal thickness (TCT). The results were compared using the Wilcoxon matched pairs test considering P values <0.05 as statistically significant. Standard deviations (SDs) of repeated measurements with both devices were compared between and within the KC stages. RESULTS The Pentacam values for PC, Kmax, and TCT were significantly elevated compared with those of the anterior segment optical coherence tomograph (P < 0.0001). The SDs of successive measurements of KA and KP, AC and posterior cylinders, Kmax, and TCT increased from (Casia 2|Pentacam) 0.10|0.09 diopter (D), 0.01|0.02 D, 0.15|0.08 D, 0.02|0.05 D, 0.17|0.18 D, and 0.92|5.25 μm in controls to 0.47|0.66 D, 0.1|0.2 D, 0.86|0.9 D, 0.17|0.3 D, 0.89|1.65 D, and 7.68|15 μm in TKC4. Significant differences occurred between the 2 devices for eyes within the same KC stage. CONCLUSIONS The reproducibility of measurements decreases with increasing KC severity in both devices. Although both devices seem reliable, the interdevice differences regarding measured tomographic parameters lead to the conclusion that Pentacam HR and Casia 2 measurements are not interchangeable in neither healthy nor KC corneas.