Aqueous Humor Dynamics
An understanding of aqueous humor dynamics is essential for the evaluation and management of glaucoma. Aqueous humor dynamics involves the measurement of parameters that affect IOP. The modified Goldmann equation is a mathematical model of the relationship between IOP and the parameters that contribute to its level in the eye at steady state:
0 = (F − U)/C + Pv
where P0 is the IOP in mm Hg, F is the rate of aqueous humor production in microliters per minute (μL/min), U is the rate of aqueous humor drainage through the pressure-insensitive uveoscleral pathway in microliters per minute (μL/min), C is the facility of outflow through the pressure-sensitive trabecular pathway in microliters per minute per mm Hg (μL/min/mm Hg), and Pv is the episcleral venous pressure in mm Hg. Although there are 5 parameters in the equation, only 4 need to be measured—the fifth parameter can then be calculated. Resistance to outflow (R) is the inverse of facility (C). Figure 2-3 illustrates the effect of reduced outflow facility (C) on IOP.
Figure 2-3 The effect of outflow facility on intraocular pressure (IOP), based on the modified Goldmann equation (assuming a constant aqueous humor production rate of 2.5 μL/min, uveoscleral outflow rate of 35%, and episcleral venous pressure of 7 mm Hg).
(Courtesy of Arthur J. Sit, MD.)
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.