Goldmann applanation tonometry (GAT) is the gold standard for intraocular pressure (IOP) measurement,1 which remains the only known modifiable risk factor for glaucoma progression.2 Still, a considerable proportion of patients will have progressive disease despite presenting IOP measurements that are compatible with a clinically-defined target.3 Therefore, the search for parameters that could further explain glaucoma progression is a topic of much relevance in the field.
Previous reports suggest that corneal biomechanical properties could possibly function as a confounder of applanation tonometry readings and could partly explain glaucoma progression unrelated to IOP.4 In a prospective observational cohort study, Susanna et al. investigated the associations between IOP measurements obtained by different tonometric methods and rates of visual field change over time.5 The tonometric measurements investigated were: GAT IOP (Haag-Streit International, Köniz, Switzerland), corneal-compensated IOP (IOPcc) yielded by the Ocular Response Analyzer (ORA, Reichert, Inc., Depew, NY), and ICare Rebound Tonometer IOP (RBT, Tiolat, Oy, Helsinki, Finland). The authors hypothesized that an IOP assessment able to account for corneal biomechanical properties would be more strongly correlated with visual field outcomes.
The three tonometric methods were applied in a randomized sequence in follow-up visits of 125 patients with primary open-angle glaucoma. All participants had at least four reliable visual field tests over a mean of 2.4 ± 0.6 years. The average baseline visual field mean deviation (MD) was -3.7 ± 5.5 dB. There was a statistically significant association between the readings from all the studied tonometers and rates of visual field change, which remained significant after adjusting for central corneal thickness, corneal hysteresis, and demographic factors. However, the ORA IOPcc had the strongest correlation with the rate of MD change (R2 = 24.5%), which was statistically higher than correlations observed with GAT (R2 = 11.1%) and RBT (R2 = 5.8%) IOP readings. Even though RBT had the weakest correlation with the rate of visual field change, it was not significantly different from GAT IOP.
The authors not only sought to validate different tonometric methods, but also applied strong methodology to investigate how their readings correlated with functional outcomes based on a well-founded hypothesis, which led to findings with important clinical implications.
Some of the limitations of the study resemble those commonly encountered in the clinical setting, such as difficulty to account for the effects of IOP reduction, pre-treatment IOP, and IOP diurnal fluctuation. Nonetheless, these limitations were largely overcome by the fact that the same eyes were tested with the three tonometers and thus shared the same sequence of visual field tests. Noteworthy is that the ORA measurements were the result of the average of three measurements, whereas the other methods were measured only once per visit, which may have contributed, at least in part, to the stronger correlation between ORA IOPcc and rates of visual field change. It would have been interesting to investigate how a single IOPcc measurement correlated with visual field progression in comparison to the single GAT IOP assessment. Notwithstanding, the techniques for measuring IOP in this study followed the recommendations from the manufacturers of each tonometer and, more importantly, resembled those employed in clinical practice, thus supporting the generalizability of their findings.
Further studies on how the different tonometers perform with regard to functional outcomes with extended follow-up, stratification by glaucoma severity, and inclusion of different types of glaucoma (e.g., normal tension) will provide valuable information on the expected trajectory of disease progression, and possibly, introduce new tools to improve clinical decision making and definition of IOP targets in clinical practice.