TonoVet versus Tonopen in a high intraocular pressure monkey model

The purpose of this study is to evaluate the consistency of and deviation in intraocular pressure (IOP) measurements obtained by the TonoVet rebound tonometer and the Tonopen applanation tonometer in a primate model.

Twenty-four-hour IOPs (nine time points) were recorded in ten monkeys with normal IOP and eight monkeys with chronic high IOP (one eye was randomly selected for measurement in each animal) using a Tonopen and TonoVet device. Measurements obtained using both handheld devices were first compared in the healthy control group (90 readings). The monkeys with chronic ocular hypertension (COHT, 72 IOP readings) were divided into three subgroups according to the level of IOP. The consistency of and deviations in the measurements were analyzed using Bland-Altman plots, linear regression, and two-tailed Student t tests.

In monkeys with normal IOP, the two devices produced similar IOP readings (mean IOP deviation, 0.06 ± 2.08 mmHg, p = 0.761), with 56.67% of the deviation between -1 mmHg and 1 mmHg and 91.12% between -3 mmHg and 3 mmHg. However, in the animal model group (23-60 mmHg), the readings obtained by the TonoVet tonometer were higher than those obtained by the Tonopen tonometer (mean deviation, 13.76 ± 9.19 mmHg); furthermore, 75.68% of the TonoVet measurements deviated by ± 5 mmHg from the Tonopen measurements.

In animals with normal IOP, the TonoVet and Tonopen tonometers produced consistent measurements. However, in a monkey model of chronic high IOP, the measurements obtained by these tonometers were inconsistent, with higher IOPs associated with larger measurement errors. Therefore, it is necessary to be aware that among different tonometers, there may be systemic errors and deviations in IOP measurements. These findings should facilitate efforts to obtain more accurate individualized diagnoses and prevent the utilization of misleading IOP values.