Calibration of the TonoLab tonometer in mice with spontaneous or experimental glaucoma

Abstract

Purpose: To measure the accuracy of TonoLab (TioLat, Helsinki, Finland) tonometry in mice with spontaneous or induced experimental glaucoma.

Methods: Chronic intraocular pressure (IOP) elevation was induced in one eye of 32 mice by injection of polystyrene beads and viscoelastic material. Three to 6 weeks later, the eyes were cannulated and manometrically set to 10, 20, 30, 40, or 50 mm Hg. The mice were 8-week and 8-month-old C57BL/6, 8-week-old DBA/2J, and 8-week-old CD1. The TonoLab calibration was also tested on five aged DBA/2J mice with spontaneous glaucoma. The relation of the TonoLab reading to manometric IOP was evaluated in multivariate linear regression models with axial length, IOP history, and mouse strain as independent variables.

Results: The slope of the relationship between TonoLab and manometric IOP in all the mice was 0.998, with an intercept of 2.3 mm Hg (adjusted R in univariate regression = 0.86). Neither the mice with bead-induced glaucoma nor those with spontaneous glaucoma (older DBA/2J mice) differed significantly from the control animals in having an excellent correlation between TonoLab and manometer IOP. Longer and wider mouse eyes had slightly higher tonometrically measured IOP, whether glaucomatous or control (multivariate regression, adjusted R(2) = 0.90, P < 0.0001). There was no difference in tonometric accuracy among the three mouse strains: CD1, C57BL/6, and DBA/2J, nor between 8-week and 8-month-old C57BL/6 mice (multivariate regression, P = 0.32).

Conclusions: The TonoLab accurately reflects IOP in both normal mice and in eyes of mice with experimental or spontaneous glaucoma, with no detectable effect of age.

Figure 1.

Linear regression of eyes with bead-induced glaucoma and control eyes shows a high level of correlation between manometric IOP and TonoLab (TioLat, Helsinki, Finland) readings at each setting. Solid black line: perfect agreement between the TonoLab and manometric-set IOP. Error bars, 95% CI for each data point.

Figure 2.

Linear regression of DBA/2J eyes with spontaneous glaucoma shows a high level of correlation between manometric IOP and TonoLab (TioLat, Helsinki, Finland) readings at each setting. Solid black line: Perfect agreement between the TonoLab and manometric-set IOP. Error bars, 95% CI for each data point.

Figure 3.

A Bland-Altman plot to examine whether the difference between tonometric and manometric IOP (y-axis) is related to the manometric IOP level (x-axis), with a comparison of eyes divided into two groups by the median eye width of 3.37 mm. Eyes smaller than 3.37 mm (blue line, circles) had an overestimation of manometric IOP by tonometry that decreased with increasing IOP, whereas the larger eyes (>3.37 mm) had a modest increase in the tonometric overestimation at higher set IOP (P = 0.04). Even at the greatest disparity, the larger eye group was within 10% of set IOP.

Figure 4.

Linear regression of tonometer IOP compared with manometer-set IOP in older compared with younger C57BL/6 mice (combined control and bead-induced glaucoma eyes) shows that age does not have a statistically significant effect on the accuracy of the TonoLab (TioLat, Helsinki, Finland). Solid black line: Perfect agreement between the TonoLab and manometric-set IOP. Error bars, 95% CI for each data point.

Figure 5.

IOP of bead-injected and fellow control eyes. Experimental glaucomatous eyes had significantly higher IOP at each measured time point after baseline (P < 0.004 or greater, t-test). Animals measured under anesthesia with TonoLab. Number of mice at each time point in each group was: 29 eyes for 3 days, 1 and 2 weeks, 28 eyes at 3 weeks, 18 at 4 weeks, and 21 at 5 weeks. Data are the mean ± SEM.

Figure 6.

Representative images of optic nerve cross sections. (A) A section from a bead-injected, young DBA mouse with an axon count of 49,758, showing no damage. (B) A section showing moderate damage from an older C57/Bl6 mouse with 36,367 fibers remaining. An optic nerve from an aged DBA/2J mouse with spontaneous glaucoma and only 7880 fibers remaining is shown in (C). (B, arrowheads) degenerating axons; (C, *) areas of severe atrophy. Bar, 10 μm.