Abstract
Abstract: :
Purpose: Goldmann applanation tonometry (GAT) is influenced by corneal viscoelastic properties, but investigation of these effects has been thus far limited to the measurement of central corneal thickness (CCT). The Reichert Ocular Response Analyser (ORA) has been developed to measure these properties, and presents two new parameters, corneal hysteresis (CH) and excess ocular pressure (EOP). CH is a measure of corneal viscoelastic properties and is strongly correlated with CCT. EOP incorporates both CH & IOP and is said to remove the corneal effects from the pressure measurement. The aim of this study was to test the independence of EOP from CCT. Methods: 204 subjects (55 normals, 44 OHTs and 105 glaucoma suspects, all untreated) had GAT, ORA IOP, CH & EOP and ultrasound pachymetry CCT measurements. Relationships investigated were: association between EOP & CCT, GAT & CCT and CH & CCT, association between EOP & GAT IOP, and agreement between GAT & ORA IOP. Analyses were performed using linear regression and Bland–Altman analyses in SPSS version 10, with significance defined at the p<0.05 level. Results: A weak, statistically and clinically non–significant relationship was found between EOP and CCT (r2 = 0.02, p = 0.07, slope –1.1mmHg per 100µ CCT). There was a positive relationship between GAT and CCT (r2 = 0.10, p<0.0001, slope 3.6 mmHg per 100µ CCT). CH was positively correlated with CCT (r2 = 0.39, p = 0.0001, slope 3.8 mmHg per 100µ CCT), indicating a strong relation between corneal viscoelastic properties and CCT, with a similar slope to the GAT/CCT relationship. EOP was positively related to GAT IOP (r2 = 0.12, p<0.0001, slope 0.28 mmHg EOP per 1.0 mmHg GAT IOP), indicating a higher EOP with higher GAT IOP. Bland Altman plots indicate good agreement between GAT and ORA IOP measures (mean difference 0.4 mmHg, standard deviation of differences 3.5 mmHg). Conclusion: The results indicate that ORA IOP is in good agreement with GAT IOP and that EOP, which takes into account corneal viscoelastic properties, is effectively independent of CCT. EOP may, therefore, be a more accurate measure of true IOP than GAT.
Keywords: intraocular pressure • clinical (human) or epidemiologic studies: systems/equipment/techniques • cornea: clinical science