Purpose: To investigate the possible covariations between diurnal changes in Goldmann applanation tonometry (GAT) estimates of intraocular pressure (IOP), central corneal thickness (CCT), central corneal radius, and ocular response analyzer measures of corneal hysteresis and corneal resistance factor (CRF).
Methods: Twenty-five healthy normal subjects (age, 21 ± 2 years) had central corneal radius, ocular response analyzer, GAT, and CCT measurements performed in 1 eye at multiple times for more than 24 hours. Measurements were taken at 17:00 and repeated every 2 hours until 23:00. Subjects then slept uninterrupted overnight for approximately 8 hours. On awakening, readings were taken at a 20-minute frequency from 7:00 to 9:00 inclusive, followed by 2 hourly intervals until 17:00.
Results: The maximum variation in GAT for more than 24 hours was 2.5 mm Hg, and these fluctuations were predicted by the diurnal variations of CCT (0.37 mm Hg ΔGAT/10 μm CCT, P < 0.01) and CRF (0.68 mm Hg ΔGAT/mm Hg CRF, P < 0.01). The diurnal changes in corneal hysteresis and CRF both covaried with those of CCT (0.16 and 0.30 mm Hg/10 μm ΔCCT, respectively, both Ps < 0.01). None of these associations was statistically different during and after the nominal period of resolution from overnight corneal swelling (2-way interactions, all Ps > 0.05).
Conclusions: If the IOP is assumed to be independent of corneal parameters, the clinically significant 24-hour variation in GAT being partially predicted by the changes in CCT and CRF suggests that the diurnal variation in corneal parameters affects tonometry measurements. CRF may also be dependent on the level of IOP.
*School of Optometry and Vision Science
†Centre for Eye Health, University of New South Wales, Sydney, Australia.
Supported by the UNSW Research Excellence Scholarship (awarded to W.L.).
Sections of this article were presented as a poster at the 2010 annual meeting of the Association for Research in Vision and Ophthalmology (W.L., D.C.P. Invest Ophthalmol Vis Sci 2010;51:ARVO e-abstract 566).
The authors state that they have no proprietary interest in the products named in this article.
Reprints: William Lau, School of Optometry and Vision Science, University of New South Wales, Sydney, NSW 2052, Australia (e-mail: firstname.lastname@example.org).
Received December 9, 2010
Accepted May 14, 2011