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D. Selvadurai, A. Sit; Aqueous Outflow Facility Changes With Posture. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1578. doi: https://doi.org/.
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Intraocular pressure (IOP) varies with body position, with a significant increase occurring from the seated to the supine position. Previous research has indicated that the IOP increase cannot be explained by changes in episcleral venous pressure alone. As well, previous research has indicated that aqueous flow rate is independent of body position. The purpose of this project was to determine if aqueous humor outflow facility varied with changes in posture.
Nine healthy volunteers aged 18-45 years were recruited from employees and students at the Mayo Clinic (Rochester, MN) and the local area. Initial steady-state IOPs were measured by pneumatometer (Reichert, Inc. Depew, NY). Tonography was performed using an electronic Schiotz tonometer (Berkeley Bio-Engineering, Inc., San Leandro, CA) with a 5.5g weight and a four minute tracing in each eye in both the supine and sitting positions. The sitting position was simulated by elevating the chair 70 degrees above horizontal with patient’s neck extended until the anterior aspect of the cornea was level with the floor. Greater than 30 minutes was allowed between tonography measurements in the sitting and supine positions to allow the eye to return to steady state. Aqueous humor outflow facility was calculated using standard tables and normograms. Statistical significance was determined using Generalized Estimating Equation (GEE) models to control for correlations between right and left eyes. Our study was able to detect a difference between sitting and supine outflow facilities of 0.092 µl/ml/mmHg, with a power of 80%, α=0.05, assuming a normal supine outflow facility to be 0.27 µl/ml/mmHg.
Eighteen eyes from 9 subjects (5 females, 4 males) were included in the study. The average age of the subjects was 28 years old (range 24 to 34 years old). The mean IOP in the supine and sitting positions was 19.9mmHg and 17.7mmHg respectively. The mean outflow facility in the supine and sitting positions was 0.28 ± 0.09 µl/ml/mmHg and 0.31 ± 0.09 µl/ml/mmHg, respectively. The difference in outflow facility between supine and sitting positions was 0.037 ± 0.13 µl/ml/mmHg, but was not statistically significant (p=0.22).
Aqueous humor outflow facility did not vary significantly between the supine and sitting positions in our subjects. However, a trend towards a small non-significant difference was found. This difference was smaller than the detection threshold for our study. Further testing will be required to determine if a real difference exists.
Clinical Trial: :
pending IRB # 07-006662 00
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