Comparison of intraocular pressure adjusted by central corneal thickness or corneal biomechanical properties as measured in glaucomatous eyes using noncontact tonometers and the Goldmann applanation tonometer

Purpose: To investigate the correlation coefficients between intraocular pressure (IOP) before and after adjusting for central corneal thickness (CCT) and corneal biomechanical properties. Patients and methods: A total of 218 eyes of 218 patients with primary open-angle glaucoma (mean age =71.5 years; mean spherical equivalent =-0.51 D; mean deviation determined by Humphrey visual field analyzer =-3.22 dB) were included in this study. The tIOP and tIOPCCT, which were adjusted by the CCT (with tIOP meaning IOP not adjusted by CCT, as determined using the CT-1P; and tIOPCCT meaning IOP adjusted by CCT, as determined using the CT-1P), were determined using a noncontact tonometer. The IOPg and IOPCCT, which were adjusted by CCT, and IOPcc adjusted by corneal biomechanical properties were determined using a Reichert 7CR (with IOPg meaning IOP not adjusted by CCT or corneal biomechanical properties, as determined using the Reichert 7CR; IOPCCT meaning IOP adjusted by CCT, as determined using the Reichert 7CR; and IOPcc meaning IOP adjusted by corneal biomechanical properties, as determined using the Reichert 7CR). The GT and GTCCT adjusted by CCT were determined using a Goldmann applanation tonometer (with GT meaning IOP not adjusted by CCT, as determined using the Goldmann applanation tonometer; and with GTCCT meaning IOP adjusted by CCT, as determined using the GAT). Pearson's correlation coefficients among the IOPs were calculated and compared. P-values <0.05 were considered as statistically significant. Results: The tIOP, tIOPCCT, IOPg, IOPCCT, IOPcc, GT, and GTCCT were 14.8 +/- 2.5, 15.0 +/- 2.4, 13.1 +/- 3.2, 13.3 +/- 3.1, 13.7 +/- 2.9, 13.2 +/- 2.4, and 13.4 +/- 2.3 mmHg (mean +/- standard deviation), respectively. The correlation coefficient between tIOPCCT and tIOP (r=0.979) was significantly higher than that between tIOPCCT and the other IOPs (r=0.668-0.852; P<0.001, respectively). The correlation coefficient between IOPCCT and IOPg (r=0.994) or IOPcc and IOPg (r=0.892) was significantly higher than that between IOPCCT or IOPcc and the other IOPs (r=0.669-0.740; P<0.001, respectively). The correlation coefficient between GTCCT and GT (r=0.989) was significantly higher than that between GTCCT and the other IOPs (r=0.669-0.740; P<0.001, respectively). Conclusion: The IOP adjusted by CCT or corneal biomechanical properties depends on the measurement instrument itself, rather than the adjustment methods, for eyes of patients with primary open-angle glaucoma.