Assessing the True Intraocular Pressure in the Non-human Primate

Abstract

Significance: For glaucoma patients, high intraocular pressure (IOP) is a risk factor for progressive neuropathy. Similarly, animal models used to study the disease are based on an experimental elevation of IOP. Thus, accurate IOP measurements are important in characterizing experimental models and resulting effects.

Purpose: The purpose of the present study was to investigate IOP measurements in a non-human primate model of experimental glaucoma by comparing clinical tonometry (Tono-Pen and TonoVet) to the true IOP from intracameral manometry.

Methods: A total of 17 rhesus macaque eyes from 12 animals were used for this study. Eleven eyes had no previous experimental intervention, whereas six eyes were at varying stages of laser-induced experimental glaucoma. IOPs were adjusted by inserting a needle in the anterior chamber that was attached to a pressure transducer and syringe pump system. The anterior chamber IOP was adjusted to values between 10 and 50 mmHg and corresponding measures with Tono-Pen and TonoVet were taken.

Results: The IOPs by TonoVet and Tono-Pen were linearly related over the range of pressures tested (slope = 0.68 normal/healthy and 0.72 experimental glaucoma). For the most, TonoVet measures overestimated IOP at all anterior chamber pressure settings (mean difference of 3.17 mmHg, 95% CI 12.53 to -4.74 normal and 3.90 mmHg, 95% CI 12.90 to -6.53 experimental glaucoma). In contrast, Tono-Pen measures overestimated IOP at lower IOPs and underestimated at higher IOP (slope = -0.26 normal and -0.21 experimental glaucoma).

Conclusions: The TonoVet and Tono-Pen tonometers that are often used to assess IOP in both clinical and experimental settings generally reflect the status of IOP, but the results from this study suggest that the instruments need calibration with true anterior chamber pressure for accurate measures in experimental models of glaucoma.

Figure 1

(A) Correspondence between pressure setting and rise of water column converted to mmHg and comparison to the 1:1 line. (B) Bland-Altman plot for the agreement between pressure setting and water column rise, illustrating the bias and 95% limits of agreement.

Figure 2

(A & B) The relationship between TonoVet IOP and intracameral pressure in normal healthy and experimental glaucoma eyes. (C & D) Difference in tonometer IOP measures from that of the pressure transducer. Healthy eyes are represented by circles and experimental glaucoma with triangles. Each animal is represented by a unique color.

Figure 3

(A & B) The relationship between Tono-Pen IOP and intracameral pressure in normal healthy and experimental glaucoma eyes. (C & D) Difference in tonometer IOP measures from that of the pressure transducer. Healthy eyes are represented by circles and experimental glaucoma with triangles. Each animal is represented by a unique color.

Figure 4

Scatter plots illustrating the comparison of Tono-Pen and TonoVet IOP measures in normal (A) and experimental glaucoma (B) eyes for all anterior chamber pressure settings. Bland-Altman plot for the agreement between the two tonometers in normal (C) and experimental glaucoma (D) eyes, illustrating the bias and 95% limits of agreement. Each animal is represented by a unique color.