Circadian Variation of Aqueous Humor Dynamics in Older Healthy Adults.

Purpose: Intraocular pressure (IOP) in the sitting position changes minimally during sleep, although aqueous humor flow decreases by 50% or more at night. The explanation for this apparent discrepancy has been unclear. This study investigated the roles of outflow facility, episcleral venous pressure (EVP) and uveoscleral outflow in maintaining IOP at night. Methods: Forty-two eyes of 21 healthy subjects (age 47-76 years, mean 59 years) were studied. Aqueous humor flow rate, IOP in the sitting position, outflow facility, and EVP were measured in each eye during the mid-diurnal period (2:00-4:00 PM). Uveoscleral flow was calculated from the other variables by using the modified Goldmann equation. These variables were re-measured during the mid-nocturnal period (2:00-4:00 AM) and compared with those measured during the diurnal period by using generalized estimating equation models. Results: IOP did not change from the mid-diurnal period (13.9±3.0 mmHg) to the mid-nocturnal period (13.0±1.8 mmHg, mean±SD, p=0.07), although aqueous humor flow rate decreased from 2.48±0.96 μL/min to 1.27±0.63 μL/min (p<0.001). Outflow facility decreased from 0.23±0.06 μL/min/mmHg to 0.20±0.06 μL/min/mmHg (p=0.004), and EVP was unchanged from the mid-diurnal period (7.4±1.8 mmHg) to the mid-nocturnal period (7.4±2.2 mmHg, p=0.95). Uveoscleral outflow decreased 93%, from 0.94±1.26 μL/min during the mid-diurnal period to 0.07±0.78 μL/min (p=0.008) during the mid-nocturnal period. Conclusions: The nocturnal decrease in aqueous humor flow rate in older normal subjects is compensated by a small decrease in outflow facility and a large decrease in uveoscleral outflow to maintain a stable IOP.