Uveoscleral drainage of aqueous humor: physiology and pharmacology

Uveoscleral outflow can account for 35% to 60% of total outflow in monkeys, although only for a much lesser fraction in cats and rabbits. Accurate assessment of the percentage of outflow via this route in humans is more difficult. In human eyes, this value was found to be in the 5% to 25% range by isotope studies, although calculations based on non-invasive methods have suggested a value of about 35% in normal eyes. In monkey eyes, the rate of uveoscleral outflow is not much affected by IOP but it can be increased by surgical and pharmacological means. Epinephrine, for example, has been found to increase uveoscleral outflow, possibly mediated by endogenously synthesized PGs. In monkeys, observed increases in uveoscleral outflow following administration of very small PG doses equal or exceed the increase that can be achieved with much higher doses of epinephrine. These observations support the concept that PGs may represent an important new approach to the medical management of glaucoma.