Corneal Biomechanical Properties of various types of glaucoma and their impact on measurement of intraocular pressure.

Introduction Corneal biomechanical properties could affect intraocular pressure (IOP) measurement. To evaluate the differences of corneal biomechanical properties of various types of glaucoma, assess their effect on IOP measurements. Methods An observational clinical study of 486 subjects including 102 normal subjects, 104 ocular hypertension (OHT), 89 normal tension glaucoma (NTG) and 191 high tension glaucoma (HTG). Corneal biomechanical parameters were measured using ocular response analyzer (ORA). The main parameters assessed were corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated pressure measurement (IOPg) and corneal compensated intraocular pressure (IOPcc). Ultrasound pachymetry was used to measure central corneal thickness (CCT). IOP was measured by Goldmann applanation tonometer (GAT) and non-contact tonometer (NCT). Visual field (VF) and refractive status were also recorded. Results were analyzed by One-way analysis of variance, univariate and multivariate linear regression analyses, and Bland-Altman plots. Results
 Multiple comparison by ANOVA showed significantly lower CH and CRF in NTG compared to HTG, OHT and normal subjects (CH:0.011, 0.015 and 0.033, CRF: 0.001, <0.001 and 0.042 respectively). CRF and CH associated with IOP measured using either GAT, NCT and IOPcc-GAT, IOPcc-NCT, yet CCT did not. GAT correlated strongly with IOPg (r＝0.79; P<0.001) and IOPcc (r＝0.77; P<0.001), but limits of agreement between the measurements were poor. CH and CRF were both negatively correlated with visual field change (P<0.01). Conclusion CH and CRF affect the measurement of IOP, and were related to types of glaucoma or severity of glaucoma. Pure CCT should not be used to correct IOP values or estimate the risk of disease.