Aqueous humor outflow facility in human eyes: History, techniques, and clinical implications

Outflow facility is one of 4 parameters of aqueous humor dynamics that together determine the intraocular pressure (IOP). Specific abnormalities in tissues in the drainage pathways that reduce outflow facility are the main causes of pathologic IOP elevation. As most new treatments for elevated IOP target these tissues, accurate assessment of outflow facility is of crucial importance in understanding glaucoma and efficacy of treatments. Outflow facility is measured noninvasively in humans primarily by a tonographic method and less often by a fluorophotometric method. These clinical research methods are used to study and understand the physiology and pathology of the eye and could be applied to personalize management and therapy for patients with glaucoma. This review discusses these two methods, summarizes important clinical findings from this outflow facility data, discusses the need for standardizing the methods, and provides thoughts on future approaches for clinical adoption.