Circadian variation of aqueous dynamics in young healthy adults

Abstract

Purpose: Recent research indicates that intraocular pressure (IOP) does not decrease significantly during the nocturnal period, although aqueous humor flow decreases by 50% or more at night. This study was undertaken to investigate whether changes in outflow facility, episcleral venous pressure, or uveoscleral flow at night could account for the nocturnal IOP.

Methods: Sixty-eight eyes of 34 healthy subjects (age, 18-44 years; mean, 29) were studied. Aqueous humor flow rate, IOP, and outflow facility were measured with pneumatonometry, anterior chamber fluorophotometry, and Schiotz tonography respectively, in each eye during the mid-diurnal (2-4 PM) and mid-nocturnal (2-4 AM) periods. Nocturnal IOP, flow rate, and outflow facility were compared to the same variables during the diurnal period. Mathematical models based on the modified Goldmann equation were used to assess the conditions under which these results could be reconciled.

Results: Supine IOP decreased slightly from 18.9 +/- 2.7 mm Hg in the mid-diurnal period to 17.8 +/- 2.5 mm Hg in the mid-nocturnal period (mean +/- SD, P = 0.001). Aqueous flow rate decreased from 2.26 +/- 0.73 to 1.12 +/- 0.75 microL/min (mean +/- SD, P < 0.001). There was a nonsignificant trend toward a nocturnal decrease of outflow facility (diurnal, 0.27 +/- 0.11 microL/min/mm Hg; nocturnal, 0.25 +/- 0.08 microL/min/mm Hg; mean +/- SD, P = 0.13).

Conclusions: Outflow facility measured by tonography does not decrease enough during the nocturnal period to compensate for the decreased aqueous humor flow rate. Modeling results indicate that the experimental results could be reconciled only if nocturnal changes in episcleral venous pressure and/or uveoscleral flow occurred.

FIGURE 1

Histogram of the experimentally measured diurnal-to-nocturnal outflow facility change in individual eyes, showing an approximately Gaussian distribution (mean ± SD = −0.02 ± 0.10 µL/min/mm Hg, n = 68, P = 0.13).

FIGURE 2

The diurnal-to-nocturnal change in outflow facility necessary to reconcile the experimentally measured IOP and aqueous flow rate results, assuming constant uveoscleral outflow fractions ranging from 0 to 0.85.

FIGURE 3

The diurnal-to-nocturnal change of EVP necessary to reconcile the experimentally measured IOP and aqueous flow rate results, assuming constant uveoscleral outflow fractions ranging from 0 to 0.90.

FIGURE 4

Calculated EVPs during the diurnal and nocturnal periods using measured IOP, aqueous flow rate, and outflow facility, assuming constant uveoscleral flow fractions ranging from 0 to 1.

FIGURE 5

The diurnal-to-nocturnal change in uveoscleral outflow fraction necessary to reconcile the experimentally measured IOP and aqueous flow rate for diurnal uveoscleral outflow fractions ranging from 0.1 to 1.0, assuming constant EVP.