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C. B. Nau, M. Malihi, J. W. McLaren, A. J. Sit; Circadian Variation of Aqueous Humor Dynamics in Older Healthy Subjects. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3493.
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Aqueous humor flow decreases during sleep, while in some studies intraocular pressure (IOP) increased or remained the same at night as during the day. This discrepancy suggests that outflow facility might have a circadian rhythm with a decrease at night to prevent the lowering of intraocular pressure. In this study we examined the relationship between outflow facility, intraocular pressure, and aqueous humor flow during sleep.
Twenty-six eyes of 13 healthy subjects (age 47-76 years, mean 58.8 years), with regular circadian sleep patterns, were studied. Sleep patterns were monitored by a wrist actigraph for one week prior to study measurements and were adjusted to match 23:00 sleep and 07:00 wake times. IOP was measured by pneumatonometry in both the seated and supine positions at 16:00 and 04:00. Aqueous humor flow rate was determined from fluorescein clearance with fluorescence measured by fluorophotometry in the cornea and anterior chamber at 14:00 and 16:00, and again at 02:00 and 04:00. Outflow facility was measured by Shiøtz tonography at 16:00 and 04:00. Aqueous humor flow rate, IOP, and outflow facility were compared between day and night by using paired t-tests.
Mean aqueous humor flow rate decreased from 2.4 ± 0.9 µl/min (± SD, n=24) during wake to 1.3 ± 0.6 µl/min (n=22, p<0.001) during sleep. Mean IOP in the seated position decreased from 15 ± 3 mmHg during the day to 14 ± 2 mmHg at night (p<0.001), while IOP in the supine position was not significantly reduced at night (19 ± 3 mmHg during day and 19 ± 2 mmHg at night; p=0.11). Outflow facility was 0.23 ± 0.1 µl/min/mmHg during the day and 0.24 ± 0.2 µl/min/mmHg at night (p=0.79).
At night IOP remains near its daytime pressures even though aqueous humor flow rate decreases by almost 50%. Outflow facility is not lower at night than it is during the day, and its lack of variation through the circadian cycle suggests that circadian changes in another physiologic variable, such as episcleral venous pressure, must explain why the nocturnal IOP is near the daytime IOP. However, it is not known if episcleral venous changes at night.
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