Aqueous humor dynamics during the day and night in volunteers with ocular hypertension

Objective: To evaluate the differences in aqueous humor dynamics between nighttime and daytime in participants with ocular hypertension. Methods: Thirty participants (mean [SD] age, 59.2 [11.1] years) with ocular hypertension were enrolled in the study, which included 1 daytime and 1 nighttime visit. During each visit, measurements included central cornea thickness by ultrasound pachymetry, intraocular pressure (IOP) by pneumatonometry, aqueous flow by fluorophotometry, outflow facility by tonography, and blood pressure by sphygmomanometry. Uveoscleral outflow was calculated using the Goldmann equation. Daytime measurements were made only of episcleral venous pressure by venomanometry, anterior chamber depth by A-scan, and outflow facility by fluorophotometry. Repeated-measures analysis of variance and 2-tailed t tests were used for statistical comparisons. Results: Compared with daytime seated IOP (21.3 [3.5] mm Hg), nighttime seated IOP (17.2 [3.7] mm Hg) was reduced (P<.001) and nighttime supine IOP (22.7 [4.6] mm Hg) was increased (P=.03). Central cornea thickness was increased at night from 570 (39) μm to 585 (46) μm (P<.001). There was a 48% nocturnal reduction in aqueous flow from 2.13 (0.71) μL/min during the day to 1.11 (0.38) μL/min at night (P<.001). Uveoscleral outflow was significantly reduced (P=.03) by 0.61μL/min at night when using supine IOP, tonographic outflow facility, and episcleral venous pressure adjusted for postural changes in the Goldmann equation. All other measurements had no significant changes. Conclusions: Significant ocular changes occur at night in individuals with ocular hypertension, including a reduction in seated IOP but an increase in habitual IOP, thickening of the cornea, and decreases in aqueous flow and uveoscleral outflow. Outflow facility does not change significantly at nighttime.