Manometric calibration and comparison of TonoLab and TonoPen tonometers in rats with experimental glaucoma and in normal mice

Purpose: To compare the TonoPen and TonoLab tonometers to each other and to manometrically set intraocular pressure (IOP) in the eyes of normal mice, normal rats, and rats with chronic IOP elevation. Methods: The measurement of IOP by the TonoPen and TonoLab tonometers was made in 21 normal rat eyes, 10 normal mouse eyes, and 16 rats that had either 2 or 4-week experimental glaucoma. IOP was varied from 10 to 50 mm Hg in steps of 10 mm Hg under conditions in which the eye was either open or closed to the reservoir controlling IOP. Results: In normal rat eyes, TonoPen overestimated manometric IOP at 10 min Hg and underestimated it by up to 6 mm Hg at higher IOP, whereas the TonoLab matched set IOP within I mm Hg. In glaucoma rat eyes, the TonoLab accurately reflected manometric IOP under open stopcock conditions (linear regression: y = 0.99x - 0.62, R-2 = 0.98), whereas in the closed stopcock condition, IOP measured lower at the higher IOP levels (P = 0.0059, paired t test). In uncarmulated rat glaucoma eyes, the tonometer used first gave higher IOP [paired t test, P = 0.015 (TonoLab first); P = 0.005 (TonoPen first)]. In normal mouse eyes under the open stopcock condition, the TonoLab nearly matched manometric IOP (linear regression: y = 0.98x+ 1.57, R-2 = 0.98). Conclusions: In mouse and rat eyes, including rats with chronic IOP elevation, the TonoLab accurately reflected manometrically set IOP in an efficient manner.