Stromal and central corneal thicknesses influence tonometry readings. Photo: Natalie A. Townsend, OD, and John J. McSoley, OD. Click image to enlarge.

Goldmann applanation is considered the gold standard of measuring IOP, but other methods have been developed to measure IOP as well. In a recent study, researchers evaluated the influence of total, stromal and central corneal thicknesses (CCT), as well as 0mm to 2mm and 2mm to 6mm corneal density zones, in the accuracy of IOP measurements using three different tonometry techniques—Goldmann applanation, iCare and non-contact tonometry.

IOPs of 45 patients’ right eyes were measured. Corneal parameters were obtained using the Pentacam Camera System. Data was analyzed using Paired t Test, Pearson’s correlation coefficient, multiple linear regression analysis and Bland-Altman plots.

Significant correlation between the three methods and CCT was found, as well as a negative correlation between CCT and full-thickness corneal density, which was similar to previous studies. The stromal density, however, showed higher correlation with the three tonometry methods than did the total density.

“In this study, the multiple regression analysis showed that CCT and stromal density are significant influential factors of reliable IOP readings obtained using the three tonometry methods, but stromal density showed significant coefficient for the iCare readings only,” the authors explained. “It demonstrates that corneas with high density could lead to an underestimation, while corneas with low density could lead to an overestimation of IOP readings. This effect was more obvious in rebound tonometry (iCare) than non-contact or applanation tonometry methods, which provides an explanation for the differences in IOP-correction equations in the previously mentioned studies.”

Based on these findings, the authors suggested the necessity of considering more corneal biomechanical properties and exercising caution in any new attempts toward adjusting an IOP-correction equation.