To estimate the relationships between ocular parameters and tonometrically measured intraocular pressure (IOP), to determine the influence of ocular parameters on different instrument measurements of IOP, and to evaluate the association of ocular parameters with a parameter called hysteresis.
Patients presenting at a glaucoma clinic were recruited for this study. Subjects underwent IOP measurement with the Goldmann applanation tonometer (GAT), the TonoPen, and the Reichert Ocular Response Analyzer (ORA), and also measurements of central corneal thickness (CCT), axial length, corneal curvature, corneal astigmatism, central visual acuity, and refractive error. Chart information was reviewed to determine glaucoma treatment history. The ORA instrument provided a measurement called corneal hysteresis. The association between measured IOP and the other ocular characteristics was estimated using generalized estimating equations.
Among 230 patients, IOP measurements from the TonoPen read lowest, and ORA read highest, and GAT measurements were closest to the mean IOP of the 3 instruments. In a multiple regression model adjusting for age, sex, race, and other ocular characteristics, a 10 μm increase in CCT was associated with an increase of 0.79 mm Hg measured IOP in untreated eyes (P<0.0001). Of the 3 tonometers, GAT was the least affected by CCT (0.66 mm Hg/10 μm, P<0.0001). Hysteresis was significantly correlated with CCT with a modest correlation coefficient (r=0.20, P<0.0007).
Among parameters related to measured IOP, features in addition to CCT, such as hysteresis and corneal curvature, may also be important. Tonometric instruments seem to be affected differently by various physiologic characteristics.
*Dana Center for Preventive Ophthalmology, The Johns Hopkins School of Medicine
†Department of Biostatistics, The Johns Hopkins Bloomberg School of Public Health, Baltimore, MD
Supported in part by PHS Research Grants 01765 (Core Facility Grant, Wilmer Institute) and the Leonard Wagner Trust, New York.
The authors' financial interest in the techniques and devices reported on herein is limited to the receipt of loaned equipment without cost from the Reichert Corporation.
Reprints: Aimee Teo Broman, MA, Wilmer 122, Johns Hopkins Hospital, 600 North Wolfe Street, Baltimore, MD 21287 (e-mail: firstname.lastname@example.org).
Received for publication April 26, 2006; accepted March 24, 2007