The repeatability and comparison between swept-source optical coherence tomography and Scheimpflug imaging in healthy eyes and various grades of keratoconus

To evaluate the repeatability of a swept-source anterior segment optical coherence tomography (SS-OCT) and a high-resolution rotating Scheimpflug system and compare them in patients with various grades of keratoconus. In this prospective, cross-sectional study, 117 eyes (86 keratoconus and 31 healthy controls) with various grades of keratoconus were enrolled during their regular follow-up visit. Three consecutive measurements were taken with SS-OCT (CASIA SS-1000) and Scheimpflug device (Pentacam HR) by the same operator. Anterior and posterior keratometry, elevation, pachymetry, pupil diameter, and Q-value were recorded. Raw data from both devices were used to determine cone location and size. Repeatability was assessed via relative standard deviation (SD), and compared them using Pearson correlation and Bland-Altman analysis. In healthy eyes, repeatability was comparable for most parameters, though SS-OCT showed significantly better repeatability for pachymetry, pupil diameter, and posterior keratometry (p < 0.001). In keratoconus eyes, SS-OCT demonstrated better repeatability in posterior keratometry and pachymetry. Despite strong correlations between devices (r ≥ 0.90 for most parameters), systematic differences were present in almost all measurements, including Kmax (mean difference 4.57 ± 1.68 D), with wide limits of agreement. Cone location also differed significantly between devices (p < 0.001). We found significantly better overall repeatability of keratometry and pachymetry with the SS-OCT in keratoconus eyes, these parameters are essential for progression monitoring and treatment planning. Furthermore, the devices cannot be used interchangeably. Awareness of these differences is essential for interpreting measurements.