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A.J. Sit, J.H. K. Liu, R.N. Weinreb; Asymmetry in Right versus Left IOP Measurements in Healthy and Glaucomatous Eyes . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4873.
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Purpose: To evaluate the interocular asymmetry of intraocular pressure (IOP) measurements over a 24 hour period in healthy and glaucomatous eyes. Methods: Fifty–three older healthy individuals (age 40–74), and 41 older glaucoma patients (age 40–78) were housed in a sleep laboratory for 24 hours. IOP of both eyes was measured every two hours in the sitting and supine positions during the diurnal period, and in the supine position during the nocturnal period (11 PM to 7 AM) using a pneumatonometer. Mean IOP was compared in the right vs left eyes. Asymmetry was calculated as the right minus left IOP (may be positive or negative). The range of fluctuation in IOP asymmetry was calculated as the largest asymmetry minus the smallest asymmetry. Linear regression analysis between right and left IOP readings was performed. Results: In habitual body positions (diurnal sitting and nocturnal supine), the mean 24–hour IOP of the right eye was found to be higher than the mean IOP of the left eye in older healthy subjects (18.3 ± 2.1 vs 17.9 ± 2.2 mmHg, p<0.001). This difference was accentuated in the supine only positions (21.3 ± 2.1 vs 20.8 ± 2.2 mmHg, p<0.001). No difference was found between right and left IOPs for older glaucoma patients in the habitual (21.1 ± 3.2 vs 21.2 ± 3.6, p=0.85) or supine (23.6 ± 3.2 vs 23.6 ± 3.4, p=0.93). The range of right minus left IOP fluctuations over a 24–hour period in the habitual position was 6.1 ± 2.2 mmHg in older healthy patients, and 7.5 ± 2.8 mmHg in glaucoma patients. Linear regression best–fit slopes for right versus left eyes was 0.83 ± 0.32 (r2 = 0.68 ± 0.22) in healthy individuals and 0.67 ± 0.35 (r2 = 0.47 ± 0.28) in glaucoma patients in the habitual position over a 24 hour period. Conclusions: The IOP in the right eye of older healthy individuals was found to be consistently higher than in left eyes. This may be due to a systematic effect related to measuring the right IOP first. However, this difference was not found in glaucoma patients. Correlations between the right and left IOPs are higher in healthy subjects than in glaucoma subjects. Neither group has a best fit slope of 1 corresponding to a right:left ratio of 1:1 commonly assumed in clinical practice. As there is poor correlation between right and left eyes of glaucoma patients, the use of monocular therapeutic trials may be unwarranted.
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