Influence of ocular stiffness on intraocular pressure estimation using Goldmann Applanation Tonometry

Accurate intraocular pressure (IOP) measurement is important for management of glaucoma. Ocular stiffness is considered to be an important biomechanical factor influencing corneal deformation during IOP measurements by a Goldmann applanation tonometer (GAT). The purpose of this study was to investigate how ocular stiffness is related to clinical parameters. Ocular stiffness was defined as the ratio between the change in the force applied to the cornea and the resultant displacement of the corneal apex. Fifty-nine Japanese subjects had their ocular stiffness measured. A GAT was used to apply pressure to the cornea and a high-resolution camera was used to measure corneal apex displacement by photographing the cornea in profile. Multiple regression analyses were used to assess how ocular stiffness is associated with seven factors: IOP measured by the GAT (IOPGAT), radius of curvature of the cornea (R), ocular axial length, anterior chamber depth, central corneal thickness, age, and gender. Ocular stiffness was found to be proportional to the product of (IOPGAT)0.66 and (R)0.92; the other factors were not significantly associated with ocular stiffness. For an individual eye, because the value of the radius of corneal curvature was unchanged, ocular stiffness was proportional to (IOPGAT)0.66. This means that the rate at which ocular stiffness diminishes is greater in the lower, compared to the higher, IOPGAT range. Hence, the rate at which the true IOP diminishes is greater in the lower IOPGAT range because the true IOP is considered to be the primary factor influencing ocular stiffness. This suggests that, in patients with lower IOPGAT, the change of true IOP should become progressively greater than the value indicated by IOPGAT. This result allows improved evaluation of treatment effectiveness in normal-tension glaucoma patients.