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L. Jia, W.O. Cepurna, E.C. Johnson, J.C. Morrison; The TonoLab Rebound Tonometer Reliably Measures Intraocular Pressure (IOP) in Awake Rats . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1252.
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© ARVO (1962-2015); The Authors (2016-present)
Elevated intraocular pressure (IOP) is the glaucoma risk factor most often modeled in experimentally induced glaucoma in small laboratory animals. The utility of these models is dependent on accurate determination of IOP history. By using awake animals, the dramatic and variable IOP lowering produced by anesthetics is avoided. The Tonopen has been used for this purpose, but accurate use of this instrument requires significant operator experience and subjective recognition of valid readings. The purpose of this study is to determine the applicability and accuracy of the TonoLab in determining IOP in awake rats under pertinent experimental conditions.
TonoLab accuracy was evaluated in cannulated eyes of 5 anesthetized Brown Norway rats connected to a pressure transducer by varying actual IOP from 10 to 100 mm Hg (N=5). Then awake IOP characteristics of normal eyes (without either general or topical anesthesia) were determined in (1) the light and dark phases of standard lighting conditions (N=5) and (2) low level constant light (N=39). Finally, IOP histories were collected for four weeks on awake animal eyes with experimental IOP elevation following episcleral injection of hypertonic saline (N=43). Pressures were obtained with both the TonoLab and the Tonopen to allow comparison of the two instruments.
TonoLab calibrations demonstrated a linear correlation with transducer IOP for pressures between 15 and 65 mmHg (R2=0.996, p<0.0001). However, average TonoLab readings over this range were 6.6 ± 1.0 mm Hg below actual IOP. In animals in standard lighting, a distinctly higher pressure during the dark phase (28.3 ± 2.8 mmHg ) than the light phase (14.5 ± 1.6 mmHg ) was found. In low level constant lighting, IOP was stabilized at 20.9 ± 3.3 mmHg. Finally, there was an excellent correlation between TonoLab and Tonopen mean IOP in eyes with experimentally elevated pressure (R2 = 0.89, p<0.0001).
TonoLab measurements correlate well with actual IOP, although it underestimates actual IOP by approximately 7 mmHg. This suggests that individual users must perform calibration studies to validate instrument accuracy in their hands. In awake rats, the TonoLab detected light dependent variations in IOP, similar to those previously demonstrated by Tonopen. Finally, TonoLab measurements demonstrated excellent correlation with those obtained by Tonopen in eyes with experimental IOP elevation. While both the TonoLab and Tonopen are useful for determining IOP in awake rats, we found the TonoLab easier to learn, to use and less subjective.
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