Circadian variation of aqueous humor dynamics in older healthy adults.
OBJECTIVE
Intraocular pressure 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 middiurnal period (2:00-4:00 PM). Uveoscleral flow was calculated from the other variables by using the modified Goldmann equation. These variables were remeasured during the midnocturnal period (2:00-4:00 AM) and compared with those measured during the diurnal period by using generalized estimating equation models.


RESULTS
Intraocular pressure did not change from the middiurnal period (13.9 ± 3.0 mm Hg) to the midnocturnal period (13.0 ± 1.8 mm Hg, 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/mm Hg to 0.20 ± 0.06 μL/min/mm Hg (P = 0.004), and EVP was unchanged from the middiurnal period (7.4 ± 1.8 mm Hg) to the midnocturnal period (7.4 ± 2.2 mm Hg, P = 0.95). Uveoscleral outflow decreased 93%, from 0.94 ± 1.26 μL/min during the middiurnal period to 0.07 ± 0.78 μL/min (P = 0.008) during the midnocturnal period.


CONCLUSIONS
The nocturnal decrease in aqueous humor flow rate in older, healthy subjects is compensated by a small decrease in outflow facility and a large decrease in uveoscleral outflow to maintain a stable IOP.
Department of Ophthalmology, Mayo Clinic, Rochester, Minnesota.
Cherie B
CB
Nau
Mehrdad
M
Malihi
Jay W
JW
McLaren
David O
DO
Hodge
Arthur J
AJ
Sit
D000368:Aged; D001082:Aqueous Humor / Q000378:metabolism*; D001794:Blood Pressure / Q000502:physiology; D002940:Circadian Rhythm / Q000502:physiology*; D005260:Female; D015590:Fluorophotometry; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D011187:Posture; D012590:Sclera / Q000098:blood supply; D014065:Tonometry, Ocular; D014690:Venous Pressure / Q000502:physiology
aqueous flow; aqueous humor dynamics; episcleral venous pressure; outflow facility; uveoscleral pathway