Effect of corneal thickness on dynamic contour, rebound, and goldmann tonometry.

PURPOSE

To identify correlations among intraocular pressure (IOP) measurements obtained using the rebound tonometer (RBT), the dynamic contour tonometer (DCT), and the Goldmann applanation tonometer (GAT). The effects of corneal thickness on the measures obtained using each of the 3 tonometers also were examined.

DESIGN

Cross-sectional study.

PARTICIPANTS

One hundred forty-six eyes of 90 patients with ocular hypertension or glaucoma.

METHODS

Intraocular pressure measurements were obtained in all patients using RBT, DCT, and GAT. Central corneal thickness was determined by ultrasound pachymetry. Patients were divided randomly into 6 groups to vary the order in which the tonometers were used. All IOP measurements were made by the same examiner, who was masked to the readings obtained.

MAIN OUTCOME MEASURES

Intraocular pressure and central corneal thickness.

RESULTS

There was good correlation between IOP readings obtained using the RBT and GAT (r = 0.864; P<0.0001), between DCT and GAT (r = 0.871; P<0.0001), and between RBT and DCT (r = 0.804; P<0.0001). Rebound tonometer and DCT readings consistently were higher than GAT measurements (RBT-GAT median difference, 1.4+/-2.7 mmHg; DCT-GAT median difference, 4.4+/-2.6 mmHg). A Bland-Altman plot indicated that the 95% limits of agreement between RBT and GAT were -4.3 to 6.4 mmHg (slope = 0.056; P = 0.218), those between DCT and GAT were -0.7 to 9.5 mmHg (slope = 0.016; P = 0.717), and those between RBT and DCT were -3.1 to 9.8 mmHg (slope = -0.041; P = 0.457). Using RBT, the point that best discriminated between patients with an IOP 21 mmHg or less and more than 21 mmHg as determined by GAT was >23.3 mmHg (sensitivity, 66.7%; specificity, 92.1%); using DCT, this point was >22.7 mmHg (sensitivity, 95.6%; specificity, 71.3%). In terms of pachymetry, GAT and RBT behaved similarly. Using these instruments, differences of approximately 3 mmHg were detected between the groups of patients with the thinnest (<531 mum) and thickest (>565 mum) corneas, whereas a significantly lower difference (0.5 mmHg) was noted for the DCT.

CONCLUSIONS

Measurements obtained both with the RBT and DCT show excellent correlation with those provided by applanation tonometry. Both tonometers tend to overestimate the IOP measured with the GAT, particularly the DCT. This last tonometer seems to be less affected by the corneal thickness.