Does air gas aesthesiometry generate a true mechanical stimulus for corneal sensitivity measurement?

Background: Belmonte Ocular Pain Meter (OPM) air jet aesthesiometry overcomes some of the limitations of the Cochet-Bonnet aesthesiometer. However, for true mechanical corneal sensitivity measurement, the airflow stimulus temperature of the aesthesiometer must equal ocular surface temperature (OST), to avoid additional response from temperature-sensitive nerves. The aim of this study was to determine: (A) the stimulus temperature inducing no or least change in OST; and (B) to evaluate if OST remains unchanged with different stimulus durations and airflow rates. Methods: A total of 14 subjects (mean age 25.14 2.18 years; seven women) participated in this clinical cohort study: (A) OST was recorded using an infrared camera (FLIR A310) during the presentation of airflow stimuli, at five temperatures, ambient temperature (AT) +5C, +10C, +15C, +20C and +30C, using the OPM aesthesiometer (duration three seconds; over a four millimetre distance; airflow rate 60 ml/min); and (B) OST measurements were repeated with two stimulus temperatures (AT +10C and +15C) while varying stimulus durations (three seconds and five seconds) and airflow rates (30, 60, 80 and 100 ml/min). Inclusion criteria were age <40 years, no contact lens wear, absence of ocular disease including dry eye, and no use of artificial tears. Repeated measures (analysis of variance) and appropriate post-hoc t-tests were applied. Results: (A) Stimulus temperatures of AT +10C and +15C induced the least changes in OST (−0.20 0.13C and 0.08 0.05C). (B) OST changes were statistically significant with both stimulus temperatures and increased with increasing airflow rates (p < 0.001), and were more marked with stimulus temperature AT +10C. Conclusion: A true mechanical threshold for corneal sensitivity cannot be established with the air stimulus of the Belmonte OPM because its air jet stimulus with mechanical setting is likely to have a thermal component. Appropriate stimulus selection for an air jet aesthesiometer must incorporate stimulus temperature control that can vary with stimulus duration and airflow rate

Agreement and repeatability of four different devices to measure non-invasive tear breakup time (NIBUT)

Purpose: Since tear film stability can be affected by fluorescein, the Dry Eye Workshop (DEWSII) recommended non-invasive measurement of tear breakup time (NIBUT). The aim of this study was to investigate the agreement and repeatability of four different instruments in the measurement of NIBUT. Methods: 72 participants (mean 24.2 ± 3.6 years) were recruited for this multi-centre, cross-sectional study. NIBUT was measured three times from one eye using each of the instruments in randomized order on two separate sessions during a day, separated by at least 2 h. NIBUT was performed at three sites (Switzerland, Germany and UK) using three subjective instruments, Tearscope Plus (Keeler, Windsor, UK) (TS), Polaris (bon Optic, Lübeck, Germany) (POL), EasyTear Viewplus (Easytear, Rovereto, Italy) (ET) and the objective Keratograph 5 M (Oculus Optikgeräte GmbH, Wetzlar, Germany) (KER). As the latter instrument only analyses for 24 s, all data was capped at this value. Results: NIBUT measurements (average of both sessions) between the four instruments were not statistically significantly different: TS (median 10.4, range 2.0–24.0 s), POL (10.1, 1.0–24.0 s), ET (10.6, 1.0–24.0 s) and KER (11.1, 2.6–24.0 s) (p = 0.949). The objective KER measures were on average (1.2 s ± 9.6 s, 95 % confidence interval) greater than the subjective evaluations of NIBUT with the other instruments (mean difference 0.4 s ± 7.7 s, 95 % confidence interval), resulting in a higher limits of agreement. The slope was -0.08 to 0.11 indicating no bias in the difference between instruments with the magnitude of the NIBUT. Repeated measurements from the two sessions were not significantly different for TS (p = 0.584), POL (p = 0.549), ET (p = 0.701) or KER (p = 0.261). Conclusions: The four instruments evaluated for their measurement of tear stability were reasonably repeatable and give similar average results