Reliability and sensitivity of the TonoLab rebound tonometer in awake Brown Norway rats

To compare the sensitivity of the TonoLab rebound tonometer with the Tono-Pen in awake Brown Norway rats and to compare their ability to predict optic nerve damage induced by experimental IOP elevation.

TonoLab and Tono-Pen tonometers were calibrated in cannulated rat eyes connected to a pressure transducer. The TonoLab was used in awake animals housed in standard lighting to measure IOP during light and dark phases. Both instruments were used to monitor chronically elevated IOP produced by episcleral vein injection of hypertonic saline. Measured IOPs were correlated with quantified optic nerve damage in injected eyes.

Although they were lower than transducer and Tono-Pen measurements at all levels, TonoLab readings showed an excellent linear fit with transducer readings from 20 to 80 mm Hg (R(2) = 0.99) in cannulated eyes. In awake animals housed in standard lighting, the TonoLab documented significantly higher pressures during the dark phase (27.9 +/- 1.7 mm Hg) than during the light phase (16.7 +/- 2.3 mm Hg). With elevated IOP, correlation between TonoLab and Tono-Pen readings (R(2) = 0.86, P < 0.0001) was similar to that in cannulated eyes. Although both instruments provided measurements that correlated well with optic nerve injury grade, only the Tono-Pen documented significant IOP elevation in eyes with the least amount of injury (P < 0.05).

The TonoLab is sensitive enough to be used in awake Brown Norway rats, though instrument fluctuation may limit its ability to identify significant pressure elevations in eyes with minimal optic nerve damage.