Novel corneal biomechanical parameters in myopes vs emmetropes

Abstract

PURPOSE: While population-based studies have shown that myopia is a risk factor for glaucoma, the underlying basis of this correlation is unknown. We aim to identify novel corneal biomechanical parameters that differentiate myopic from normal eyes using a novel technology. METHODS: This prospective, cross-sectional study of 80 subjects with varying degrees of myopia and 62 emmetropies was conducted at Queen Mary Hospital in Hong Kong. The Corvis ST device (Oculus, Wetzlar, Germany), which couples a pneumotonometer with a high speed Scheimpflug camera, was used to measure IOP, CCT, and new biomechanical parameters including corneal deformation amplitude, inward and outward applanation velocity, and highest concavity peak-to-peak distance. The right eye of all subjects and controls underwent Corvis analysis. Myopes were subsequently categorized as being high (vision correction at least -9.0 D), moderate (-6.0 D to -9.0 D), or mild (-3.0 D to -6.0 D) myopes. Exclusion criteria included known corneal disease, intraocular surgery within three months preceding the study, or prior history of refractive surgery. RESULTS: Significant findings included differences in outward applanation velocity (myope: -0.38±0.08 m/s; control: -0.33±0.05 m/s; P=3.2E-5) and peak-to-peak distance (myope: 2.43±0.24 mm; control: 2.32±0.20 mm; P=3.6E-3). Interestingly, high (n=19), but not moderate (n=25) or low (n=36), myopes exhibited statistically significant differences in these two corneal biomechanical parameters as compared to controls. There was moderate correlation between IOP, CCT and visual acuity with outward applanation velocity (R=0.43; 0.34; 0.50, respectively), and with peak-to-peak distance (R=0.40; 0.15; 0.32, respectively). No correlation was found between age and outward applanation velocity (R=0.060) or peak-to-peak distance (R=0.039). Average IOP in myopes and emmetropes were 15.2±2.2 mm Hg and 14.9±2.1 mm Hg, respectively (P=4.0 E-1); average CCT in myopes and emmetropes was 554±34 μm and 554±39 μm, respectively (P=9.5 E-1). CONCLUSIONS: Myopes had a greater mean outward applanation velocity and greater peak-to-peak distance at highest concavity than emmetropic controls. In particular, high myopes demonstrated a corneal biomechanical profile distinct from that of emmetropes. Increased corneal deformability in high myopes, also found in glaucoma patients from another study, may indicate a relationship between high myopia and glaucoma.