Effects of corneal thickness, corneal curvature, and intraocular pressure level on Goldmann applanation tonometry and dynamic contour tonometry.

PURPOSE

To compare the measurements of intraocular pressure (IOP) with Goldmann applanation tonometry (GAT) and dynamic contour tonometry (DCT) and the effects of central corneal thickness (CCT), corneal curvature, and level of IOP on these methods.

DESIGN

Cross-sectional population-based study.

PARTICIPANTS

From the Los Angeles Latino Eye Study, 2157 participants of primarily Mexican ancestry.

METHODS

Average GAT measurements were compared to DCT, and both were examined with respect to CCT (< or =500, 501-550, 551-600, >600 microns), corneal curvature (<42, 42-46, >46 diopters), and level of IOP (0-10, 11-20, >20 mmHg).

MAIN OUTCOME MEASURES

Mean GAT and DCT IOP levels were compared for the entire population, and then trends for the CCT, curvature, and IOP groupings were analyzed. The magnitude of the difference of GAT minus DCT was compared for these different strata, with special attention to a difference of +/- 3 mmHg or greater, which was defined as clinically significant.

RESULTS

Mean IOP for the entire population by GAT was significantly lower (14.4+/-3.2 mmHg) compared with DCT (16.0+/-3.6; P<0.0001). Both GAT and DCT IOP levels were lowest for thin CCT and increased stepwise with increasing CCT, but this difference was more pronounced with GAT than DCT (P<0.0001 and P = 0.0012, respectively). The difference between GAT and DCT was largest for thin CCT and decreased for thicker CCT (P<0.0001). After adjusting for CCT, the corneal curvature affected IOP measured by DCT (P = 0.02) but not GAT (P = 0.3) such that mean DCT IOP increased with increasing corneal curvature. After adjusting for the CCT effect on IOP and stratifying by DCT IOP groups, the greatest difference between GAT and DCT was seen in the lowest IOP group (3.55+/-3.1), became negative in the intermediate group (-1.86+/-2.60), and was most negative in the highest IOP group (-3.88+/-3.3; P<0.0001).

CONCLUSIONS

Intraocular pressure measured by GAT was consistently lower when compared with DCT, and this difference was greatest with thinner CCT. Dynamic contour tonometry was also less affected by variations in CCT. Corneal curvature affected IOP measurements with DCT but not GAT, but this effect was less than the CCT effect on GAT. Goldmann applanation tonometry tended to underestimate IOP at higher levels and overestimate it at lower IOP levels when compared to DCT.