Measurement of Outflow Facility
The facility of outflow (C in the Goldmann equation) is the mathematical inverse of outflow resistance and varies widely in normal eyes, with a mean ranging from 0.22 to 0.30 μL/min/mm Hg. Outflow facility decreases with age and is affected by surgery, trauma, medications, and endocrine factors. Patients with glaucoma and elevated IOP typically have decreased outflow facility. In normal eyes, approximately 50% of conventional outflow resistance is in the trabecular meshwork, while the remainder is in the distal outflow system. However, in primary open-angle glaucoma with elevated IOP or ocular hypertension, most of the pathologic change occurs in the juxtacanalicular tissue of the trabecular meshwork and inner wall of the Schlemm canal.
Tonography is a method used to measure the facility of aqueous humor outflow. In this technique, a weighted Schiøtz tonometer or pneumatonometer is placed on the cornea, acutely elevating the IOP. Outflow facility in μL/min/mm Hg can be computed from the rate at which the pressure declines with time, reflecting the ease with which aqueous humor leaves the eye.
However, tonography depends on a number of assumptions (eg, ocular rigidity, stability of aqueous humor formation, and constancy of ocular blood volume) and is subject to many sources of error, such as poor patient fixation and eyelid squeezing. These problems reduce the accuracy and reproducibility of tonography for an individual patient. In general, tonography is best employed as a research tool and is rarely used clinically.
Kazemi A, McLaren JW, Lin SC, et al. Comparison of aqueous outflow facility measurement by pneumatonography and digital Schiøtz tonography. Invest Ophthalmol Vis Sci. 2017;58(1):204–210.
Rosenquist R, Epstein D, Melamed S, Johnson M, Grant WM. Outflow resistance of enucleated human eyes at two different perfusion pressures and different extents of trabeculotomy. Curr Eye Res. 1989;8(12):1233–1240.
Vahabikashi A, Gelman A, Dong B, et al. Increased stiffness and flow resistance of the inner wall of Schlemm’s canal in glaucomatous human eyes [epub ahead of print December 5, 2019]. Proc Natl Acad Sci USA. doi: 10.1073/pnas.1911837116
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.