Measurement of Aqueous Humor Production
The rate of aqueous humor production by the ciliary processes cannot be measured noninvasively, but it is assumed to be equal to the aqueous humor outflow rate in an eye at steady state. The most common method used to measure the rate of aqueous humor outflow is fluorophotometry. For this test, fluorescein is administered topically, its gradual dilution in the anterior chamber is measured optically, and the change in fluorescein concentration over time is then used to calculate aqueous flow. As previously noted, the normal flow rate is approximately 2–3 μL/min while awake, and the aqueous volume is turned over at a rate of approximately 1% per minute.
The rate of aqueous humor formation varies diurnally and is reduced by approximately half during sleep. It also decreases with age. The rate is affected by a variety of factors, including the following:
integrity of the blood–aqueous barrier
blood flow to the ciliary body
neurohumoral regulation of vascular tissue and the ciliary epithelium
Aqueous humor production may decrease after trauma or intraocular inflammation and after the administration of certain systemic drugs (eg, general anesthetics and some systemic hypotensive agents), as well as aqueous humor suppressants used for treatment of glaucoma. Carotid occlusive disease may also decrease aqueous humor production.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.