Purpose:
Aqueous humor flow decreases during sleep, while IOP in the physiologic positions (sitting during day; supine at night) increases. Previous work suggests that outflow facility decreases slightly at night, but not enough to explain this pattern. However, the contribution of episcleral venous pressure (EVP) and uveoscleral outflow to the nocturnal change in IOP are unknown, due of the lack of objective methods to measure EVP. In this study we used a new method to assess circadian changes in EVP, and combined with measurements of aqueous humor flow and IOP, we examined circadian changes in uveoscleral outflow.
Methods:
Twenty-six eyes of 13 healthy subjects (age 47-76; mean 59 years) were studied in the mid-diurnal and mid-nocturnal periods. IOP was measured by pneumatonometry, aqueous humor flow rate was determined by fluorophotometry, and outflow facility was measured by Shiøtz tonography. EVP was measured by using a custom computerized venomanometer that applies an inflatable chamber to an episcleral vein and objectively determines the pressure required to collapse the vein. Uveoscleral outflow was calculated by using the modified Goldmann equation. IOP (sitting and supine), aqueous humor flow rate, outflow facility, EVP, and uveoscleral outflow rate during the day were compared to the same parameters at night by using paired t-tests.
Results:
At night, aqueous humor flow rate decreased by almost 50%, sitting IOP was slightly lower, and supine IOP was unchanged. EVP was not different at night than during the day, but outflow facility and uveoscleral outflow both decreased significantly at night.
Conclusions:
Decreased aqueous humor flow at night is compensated by decreased outflow facility and uveoscleral flow to maintain a high IOP. EVP remains unchanged, which suggests active regulation. The lack of uveoscleral outflow at night is consistent with the concept of a uveolymphatic drainage system that is pressure-insensitive, but dependent on eye movements.
Keywords: outflow: ciliary muscle • outflow: trabecular meshwork • intraocular pressure