Correlation of Serial Scleral and Corneal Pneumatonometry

Purpose To evaluate the usefulness of scleral pneumatonometry as an alternative for corneal measurements of intraocular pressure (IOP) over a broad range of IOPs. Design Prospective, observational cohort study. Participants The study was conducted in the University of California, San Francisco, Retina Clinic between August and November 2013 in 33 adult patients (age range, 34–94 years; mean ± standard deviation, 74.1±13.4 years) receiving anti–vascular endothelial growth factor intravitreal injections, which transiently increase IOP. Methods Corneal pachymetry and serial corneal and temporal scleral pneumatonometry (baseline, immediately after, and 10, 20, and 30 minutes after injection) were collected. One-time baseline corneal and scleral pneumatonometry readings were obtained in the noninjected eye. Main Outcome Measures Correlation analysis and a Bland-Altman plot were used to evaluate reliability and agreement between scleral and corneal measurements of IOP. A linear mixed model was used to determine the relationship between measurements and to perform covariate analyses. Results Scleral and corneal pneumatonometry showed nearly 1:1 linear correlation, although scleral pneumatonometry was biased toward higher values ( r = 0.94; P < 0.001). Scleral pneumatonometry averaged 9.0 mmHg higher than corneal pneumatonometry (95% limits of agreement, −1.5 to 19.5 mmHg). A linear mixed model resulted in the following equation: corneal IOP = 1.04 × scleral IOP − 10.37. Age, central corneal thickness, laterality, and glaucoma and lens status did not impact this relationship. The difference between corneal and scleral pneumotonometry was correlated between the two eyes of individual patients ( r = 0.75; P < 0.001). Conclusions Differences between serial scleral measurements reflect differences between serial corneal measurements. Scleral pneumatonometry should be considered as an alternative to corneal pneumatonometry for following patients in whom corneal measurements are unreliable or unobtainable.