Comparing Different Self-Tonometry Devices against Goldmann Applanation Tonometry - Special Focus on Biomechanical Properties of the Cornea.

Background: Interest in self-tonometry as an additional component to glaucoma monitoring has been increasing for several years. Two self-tonometry devices, Ocuton S and iCare HOME, have seen clinical use, but reports are lacking as to how individual biomechanical corneal properties might influence readings. We compared the two devices against Goldmann applanation tonometry (GAT) as a reference method.

Patients and methods: Intraocular pressure (IOP) was measured by GAT and the two self-tonometers, Ocuton S*TT-MV and iCare HOME, on 108 patients with glaucoma, suspected glaucoma, or ocular hypertension at different times in alternating order during diurnal profiling for intraocular pressure. Corneal thickness (CCT), corneal hysteresis (CH), and corneal resistance factor (CRF) were recorded using an Ocular Response Analyzer^® (ORA) to determine biomechanical corneal properties.

Results: We first determined measurement deviations using GAT as the established reference method. The mean difference was - 1.73 ± 2.84 (- 3.95 - 7.41 mmHg) between iCare HOME and GAT, and + 1.49 ± 2.9 mmHg (- 7.29 - 4.31 mmHg) between Ocuton S*TT-MV and GAT. Differences between iCare HOME readings within measurement sequences were statistically significant (p < 0.001). Ocuton S*TT-MV did not show any significant difference from GAT using the measurement sequence (p = 0.262; p = 0.063). In contrast, previous tonometry measurements exerted a significant influence on iCare HOME readings. We found a significant correlation in both self-tonometers with corneal thickness on investigating the possible impact of biomechanical corneal properties on self-tonometry results. The impact of corneal thickness on readings was moderate to strong in iCare HOME. IOP values measured using both self-tonometers correlated significantly with CRF values (p < 0.001). Ocuton S*TT-MV values also correlated significantly with CH values (p = 0.024), unlike iCare HOME (p = 0.853).

Conclusion: We consider differences in readings using the two self-tonometers compared against GAT as the established reference method to be within acceptable tolerances. Monitoring changes in readings using either device over an extended period - such as gradual increases in intraocular pressure values - is far more important in clinical practice. The measurement sequence requires special consideration due to interplay between measurement methods when comparing readings from different tonometers. We recommend taking iCare HOME readings before applanation tonometry. Biomechanical corneal properties especially need to be considered in iCare HOME and Ocuton S*TT-MV self-tonometers. CRF values affect readings from both self-tonometers. CH values should also be considered when interpreting readings from Ocuton S*TT-MV.