Reliability analysis of successive Corneal Visualization Scheimpflug Technology measurements in different keratoconus stages

Background This study assesses the reliability of successive corneal biomechanical response measurements by the Corneal Visualization Scheimpflug Technology (CST, Corvis ST(R), Oculus Optikgerate, Wetzlar, Germany) in different keratoconus (KC) stages. Methods A total of 173 eyes (15 controls: 15 eyes, and 112 KC patients: stages 1|1-2|2|2-3|3|3-4|4, n = 26|16|36|18|31|26|5 according to Topographical KC Classification, TKC) were repeatedly examined five times with the CST, each after repositioning the patient's head and re-adjusting the device. Tomographical analysis (Pentacam HR(R); Oculus, Wetzlar, Germany) was performed once before and once after CST measurements. Outcome measures included (1) A1 velocity, (2) deformation amplitude (DA) ratio 2 mm, (3) integrated radius, (4) stiffness parameter A1 and (5) Ambrosio relational thickness to the horizontal profile (ARTh). The Corvis Biomechanical Index (CBI) is reported to be extracted out of these parameters. Mean values of the five measurements and Cronbach's alpha were calculated as a measure for reliability. Results Ambrosio relational thickness to the horizontal profile and SPA1 were significantly higher in controls (534|123) compared to TKC1 (384|88), TKC2 (232|66), TKC3 (152|55) and TKC4 (71|27; p < 0.0001). The other parameters were similar in controls and TKC1 (A1 velocity: 0.148|0.151 m/s; integrated radius: 8.2|8.6 mm(-1)), but significantly higher in TKC stages 2 to 4 (DA ratio 2 mm: 5.5|6.3|8.0; A1 velocity: 0.173|0.174|0.186 m/second; integrated radius: 10.9|12.8|19.0 mm(-1); p < 0.0001). All parameters proved to be highly reliable (Cronbach's alpha >= 0.834) and the corneal tomography remained unaffected. Conclusions The individual parameters included in the CBI (consisting of ARTh, SPA1, DA ratio 2 mm, A1 velocity and integrated radius) are highly reliable but differ KC stage-dependently.