Corneal hysteresis and its relationship with intraocular pressure in primary open angle glaucoma, low tension glaucoma and ocular hypertension

Purpose: To compare intraocular pressure (IOP) measurement obtained from the ocular response analyzer (ORA) and Goldmann applanation tonometer (GAT), corneal hysteresis (CH) and corneal resistance factor (CRF) in primary open angle glaucoma (POAG), normal tension glaucoma (NTG), ocular hypertension (OHT) and normal subjects.Patients and Methods: 127 eyes of 66 cases with POAG, 48 eyes of 24 cases with NTG, 110 eyes of 60 cases with OHT and 172 eyes of 89 control subjects were enrolled in the study. First, corneal-compensated IOP (IOPcc), Goldmann-correlated IOP (IOPg), CH, and CRF were measured with ORA, then IOP was measured with GAT (GIBGAT) and central corneal thickness (CCT) was measured with ultrasonic pachymetry (IOPac, Heidelberg). Four readings were obtained with ORA, and three measurements were taken with the GAT and ultrasonic pachymetry. The mean of the all measurements was used for the analysis.Results: The CH was 9.31±1.79 mmHg, 9.23±1.65 mmHg, 9.94±1.84 mmHg, 10.58±1.71 mmHg; CRF was 10.36±1.73 mmHg, 9.02±1.67 mmHg, 12.08±1.77 mmHg, 10.53±1.74 mmHg; CCT was 551.10±37.59 µm, 517.27±38.50 µm, 578.53±41.30 µm, 548.52±30.94 µm, in POAG, NTG, OHT, and control subjects respectively. In all study group, IOPGAT, IOPcc and IOPg were positively correlated with CCT (r= 0.17, 0.37, 0.42, respectively), but correlation was less with IOPcc when compared to IOPGAT and IOPg. KH was lower in POAG group when compared to OHT and control group (p < 0.05).Conclusion: IOPcc is affected less by CCT when compared to GAT and thus IOPcc may be more advantageous than GAT for measuring IOP. CH was lower in glaucoma subjects and it seems to be an useful parameter for early diagnosis of glaucoma.Key Words: central corneal thickness, corneal hysteresis, corneal resistance factor.