Inter-corneal Biomechanical Symmetry in Keratoconus Patients

Objective: To evaluate inter-corneal symmetry of biomechanical parameters in keratoconus patients as measured by an ultra-high frequency corneal visualization Scheimpflug technology (CorVis-ST). Methods: In this case series study, 40 bilateral keratoconus patients diagnosed with the Amsler-Krumeich keratoconus classification were recruited from December 2013 to July 2015 at the Optometry Outpatient of the Eye Hospital, Wenzhou Medical University. The same operator, using the same CorVis-ST instrument, measured each eye three times. Twelve inter-corneal parameters measured by the instrument were recorded, including 10 biomechanical parameters, intraocular pressure, and central corneal thickness. Student's t-test and Mann-Whitney U-test were used to compare inter-corneal differences, while inter-corneal agreement was evaluated by the Bland-Altman plot. Results: The 95% limits of agreement and agreement ratios of the 12 parameters were as follows: applanation 1 time (AP1 T), -0.56-0.61 ms (8.8%, Z=0.441, P=0.229); applanation 1 length, -0.75-0.77 mm (45.0%, t=-0.178, P=0.907); applanation 1 velocity, -0.063-0.053 m/s (37.0%, Z=-1.528, P=0.171); applanation 2 time (AP2 T), -1.01-0.84 ms (4.6%, t=-0.848, P=0.269); applanation 2 length, -0.96-1.02 mm (65.8%, t=-0.342, P=0.715); applanation 2 velocity, -0.22-0.22 m/s(50.0%, t=-0.087, P=0.812); maximum deformation time (T), -1.38-1.11 ms (8.5%, Z=-1.170, P=0.162); maximum deformation peak distance, -1.70-3.93 mm (78.8%, Z=-3.321, P=0.001); maximum deformation curvature, -2.30-2.74 mm (46.5%, t=1.014, P=0.287); maximum deformation amplitude, -0.34-0.28 mm (29.1%, t=-1.057, P=0.221); intraocular pressure, -3.61-3.91 mmHg (32.6%, t=-1.152, P=0.267); and central corneal thickness, -135.70-152.10 μm (32.3%, t=-0.698, P=0.323). Of all measured parameters, only AP1 T, AP2 T, and T had low agreement ratios, indicating high inter-corneal symmetry. Conclusions: In our study population of binocular keratoconus patients, the parameters AP1 T, AP2 T, and T had high inter-corneal symmetry. Research protocols using the CorVis-ST to study keratoconus should take these results into consideration during data analysis. Key words: keratoconus; biomechanical; symmetry; Scheimpflug