Purpose: To assess the asymmetry of 24-hour intraocular pressures (IOPs) between right and left eyes of untreated open-angle glaucoma (OAG) patients, and its influence on the monocular therapeutic trial. Design: Experimental study with human patients. Participants: Forty-one subjects (40-78 years old) with untreated OAG. Methods: Subjects were housed in a sleep laboratory for 24 hours. Intraocular pressures of both eyes were measured with a pneumatonometer every 2 hours with the patient in the sitting and supine positions from 7 am to 11 pm and in the supine position only from 11 pm to 7 am. Mean, peak, and trough IOPs were compared in right versus left eyes. The strength of association between right and left IOPs was assessed using coefficients of determination (R 2). Validity of the monocular therapeutic trial was assessed by examining residual values from 2 linear regression models of right versus left IOP. Main Outcome Measures: Strength of association between IOPs of right and left eyes, and residual values from linear models of IOP. Results: No statistically significant difference was found between mean, peak, or trough IOPs of right and left eyes over a 24-hour period. The strength of association for mean IOP was only moderate (R2 = 0.421-0.623). Residual values of <3 mmHg were found in 14.0%±12.0% (mean ± standard deviation) of IOP measurements for a symmetric linear regression model, and 8.5%±10.6% of IOP measurements for a best-fit linear regression model over 24 hours. Conclusions: In this group of untreated glaucoma patients, the strength of association between the right and left eye mean IOPs is only moderate. Residual values of <3 mmHg were more common using a symmetric model than with a best-fit model. The current method of performing monocular therapeutic trials in glaucoma patients is unreliable, but it may be possible to improve reliability by using several IOP measurements obtained at different times of the day instead of a single office measurement.
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