Objectives: To investigate the daytime vs nighttime differences in intraocular pressure (IOP), aqueous humor dynamics, central cornea thickness, and blood pressure among a cohort of healthy volunteers. Methods: Thirty healthy volunteers (mean [SD] age, 57.0 [8.6] years) were enrolled in the study. Individuals underwent 1 daytime visit and 1 nighttime visit for the measurement of aqueous humor dynamics. Measurements included IOP by pneumatonometry, aqueous flow by fluorophotometry, outflow facility by fluorophotometry and tonography, uveoscleral outflow by mathematical calculation, central cornea thickness by pachymetry, and blood pressure by sphygmomanometry. Results between visits were compared by appropriate t test. Dependence of the pneumatonometer probe results on position was tested in enucleated rabbit eyes at set pressures and probe positions. Results: Compared with daytime seated IOP, nighttime seated IOP was reduced by 16%, whereas nighttime supine IOP was increased by 17% (P<.001 for both). The IOP changes were independent of the pneumatonometer probe position. Central cornea thickness was increased at nighttime from a mean (SD) of 560 (37) μm to a mean (SD) of 574 (37) μm (P<.001). Compared with daytime aqueous flow, nighttime aqueous flow was reduced by 49% (P<.001). During the night, fluorophotometric outflow facility was reduced by 45% (P=.05), and tonographic outflow facility was reduced by 17% (P<.01). Uveoscleral outflow at night was decreased when calculated using tonographic outflow facility but not fluorophotometric outflow facility in the Goldmann equation. All other measurements were unchanged. Conclusions: Significant changes in aqueous humor dynamics at night in healthy mature humans include reductions in aqueous flow, outflow facility, and possibly uveoscleral outflow. Nocturnal changes in IOP are independent of the pneumatonometer probe position and are dependent on an individual's posture during the measurement.
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