Comparison of corneal thickness measurements with the Sirius and Ultrasound Pachymetry

Objective To assess the repeatability of corneal thickness measurements in healthy eyes obtained with a new Scheimpflug/Placid imaging system (Sirius,CSO Inc.) and the agreement of the measurements with those obtained by Ultrasound Pachymetry (UP).Methods In this prospective study,corneal thickness was measured in 36 subjects (36 eyes) by a single operator.Three consecutive scans were acquired for each eye.The central comeal thickness (CCT) and thinnest comeal thickness (TCT) of the Sirius were compared to UP,respectively.Data were compared using paired t test,the Pearson correlation test and Bland-Altman plot.Results The mean values of CCT,TCT obtained from Sirius and UP were 533.06±34.21,526.25±34.51,and525.94±34.51μtm,respectively.BothCCTandTCT showed high intra-observer repeatability,with all intra-class correlation coefficients and Cronbach's alphas were more than 0.99,and yielded aCoefficient of variation (COV)ofless than 0.6％.Significant difference was found between CCT and UP (P ＜0.05);No significant difference was found between TCT and UP (P ＞0.05).And regression analysis showed a high correlation between the measurements obtained with both devices(r =0.984,P ＜0.05; r =0.987,P ＜0.05).The Bland-Altman Plots displayed relatively good agreement (95％ limits of agreement were-5.2 to +19.4μm and-10.9 to +11.5μm) between CCT and TCT to UP.Conclusions In healthy eyes,the repeatability of Sirius is good.The agreement of Sirius and UP is high,and Sirius is better than the UP in observing the whole corneal thickness change.Because of high agreement between these devices,the Sirius is a non-contact method that may be an alternative to UP for measurement of corneal thickness. Key words: Corneal thickness;  Ultrasound Pachymetry;  Sirius;  Repeatability;  Agreement