Accuracy and reproducibility of the TonoVet rebound tonometer in birds of prey.

Objective: To examine the accuracy and reproducibility of intraocular pressure (IOP) measurements obtained by the TonoVet rebound tonometer. Animals studied  Freshly enucleated healthy eyes of 44 free-ranging birds of prey out of the species Haliaeetus albicilla, Accipiter gentilis, Accipiter nisus, Buteo buteo, Falco tinnunculus, Strix aluco, Asio otus and Tyto alba euthanized because of unrelated health problems.

Procedures: IOP readings from the TonoVet were compared with a manometric device, with IOP being set from 5 to 100 mmHg in steps of 5 mmHg by adjusting the height of a NaCl solution reservoir connected to the eye. Reproducibility of the TonoVet readings was determined by repeated measurements.

Results: TonoVet and manometer values showed a strong linear correlation. In the Accipitridae, the TonoVet tended to increasingly overestimate IOP with increasing pressure, while in the other families, it increasingly underestimated it. In the Sparrowhawk, the values almost represent the ideal line. Reproducibility of TonoVet values decreases with increasing pressure in the clinically important range from 5 to 60 mmHg.

Conclusion: IOP values measured with the TonoVet demonstrated species specific deviation from the manometric measurements. These differences should be considered when interpreting IOP values. Using the regression formulae presented, corrected IOP values could be calculated in a clinical setting.