Effects of multiple dosing of epinephrine on aqueous humor dynamics in human eyes

Numerous studies have provided conflicting evidence to explain the ocular hypotensive mechanism of action of epinephrine. Although epinephrine has been shown consistently to increase outflow facility, its effects on aqueous flow and uveoscleral outflow are not as clear. The purpose of this study was to clarify the effects of multiple doses of topical epinephrine on aqueous humor dynamics in human eyes. This was done by evaluating the four main parameters that determine steady state intraocular pressure. These parameters were assessed at baseline and after a week of twice-daily treatment of epinephrine hydrochloride 2% to one eye. Twenty-six human volunteers were enrolled in the study. Intraocular pressure was measured by pneumatonometry, aqueous flow and trabecular outflow facility by fluorophotometry, episcleral venous pressure by venomanometry and uveoscleral outflow by mathematical calculation. In epinephrine-treated eyes compared to baseline, intraocular pressure and aqueous flow were reduced from 21.2 +/- 0.3 to 17.1 +/- 0.2 mmHg (19%, p = .01) and 3.3 +/- 0.2 to 2.9 +/- 0.2 microl/min (12%, p = .03), respectively. Trabecular outflow facility obtained by fluorophotometry was increased from 0.18 +/- 0.02 to 0.26 +/- 0.03 microl/min/mmHg (44%, p = .02). Topical epinephrine did not significantly affect uveoscleral outflow or episcleral venous pressure. In conclusion, multiple topical doses of epinephrine lowered intraocular pressure in human volunteers by reducing aqueous humor formation and increasing trabecular outflow facility. The increase in uveoscleral outflow suggested by other studies was not observed.