Effect of central corneal thickness on intraocular pressure with the rebound tonometer and the applanation tonometer in normal dogs.

OBJECTIVE

To evaluate the effect of central corneal thickness (CCT) on the measurement of intraocular pressure (IOP) with the rebound (TonoVet(®)) and applanation (TonoPen XL(®)) tonometers in beagle dogs.

ANIMAL STUDIED

Both eyes of 60 clinically normal dogs were used.

PROCEDURES

The IOP was measured by the TonoVet(®), followed by the TonoPen XL(®) in half of the dogs, while the other half was measured in the reverse order. All CCT measurements were performed 10 min after the use of the second tonometer.

RESULTS

The mean IOP value measured by the TonoVet(®) (16.9 ± 3.7 mmHg) was significantly higher than the TonoPen XL(®) (11.6 ± 2.7 mmHg; P < 0.001). The IOP values obtained by both tonometers were correlated in the regression analysis (γ(2) = 0.4393, P < 0.001). Bland-Altman analysis showed that the lower and upper limits of agreement between the two devices were -0.1 and +10.8 mmHg, respectively. The mean CCT was 549.7 ± 51.0 μm. There was a correlation between the IOP values obtained by the two tonometers and CCT readings in the regression analysis (TonoVet(®) : P = 0.002, TonoPen XL(®) : P = 0.035). The regression equation demonstrated that for every 100 μm increase in CCT, there was an elevation of 1 and 2 mmHg in IOP measured by the TonoPen XL(®) and TonoVet(®), respectively.

CONCLUSIONS

The IOP obtained by the TonoVet(®) and TonoPen XL(®) would be affected by variations in the CCT. Therefore, the CCT should be considered when interpreting IOP values measured by tonometers in dogs.