Methods of measuring intraocular pressure independently of central corneal thickness

Introduction. Goldmann applanation tonometry (GAT) has been the gold standard for measuring intraocular pressure (IOP) for about 50 years. However, it depends on central corneal thickness (CCT) and is, therefore, prone to being incorrect. Dynamic contour tonometry (DCT) has recently been introduced to measure IOP independently of CCT; however, DCT is costly and difficult. IOP measurement using the ocular response analyzer (ORA) offers noncontact tonometry with declaration of the corneal-compensated IOP (IOPcc), which takes corneal hysteresis (CH) into account and is supposed to be independent of CCT. Patients and methods. Using the ORA instrument, IOPcc was determined in 192 glaucoma eyes and 59 nonglaucoma eyes. Subsequently, measurement by DCT and GAT was performed. IOP measurements were compared and analyzed with respect to CCT and CH. Results. Average values were as follows: IOPcc, 18.38 +/- 6.3 mmHg; GAT, 14.69 +/- 4.5 mmHg; DCT, 15.17 +/- 3.9 mmHg; CH, 9.96 +/- 2.5 mmHg; CCT, 552 +/- 57 mu m. Neither CCT nor CH differed between the two groups. There was a positive correlation between GAT and CCT that did not exist for IOPcc and DCT values. However, IOPcc and DCT differed significantly in Bland-Altman analysis (p < 0.01). Furthermore, these two IOP values differed significantly with respect to CH and the level of IOP. Conclusion. Because IOPcc is not a primarily measured variable but also takes CH into account, a direct comparison of DCT and IOPcc values is not acceptable, and a simple correction factor may not be valid.