Purpose: Hyposecretion of aqueous humor has been postulated to adversely affect the health of the trabecular meshwork and outflow resistance. However, the effect of medications that reduce aqueous humor production on outflow facility in living human eyes is unclear. This study evaluated the effect of timolol, an aqueous humor flow suppressant, on outflow facility in healthy eyes. Design: Prospective, before-and-after study. Methods: In a multicenter study, 113 healthy participants over 40 years of age were included. Intraocular pressure (IOP)was measured with the participant in the sitting position by using a pneumatonometer. The outflow facility was measured with the participant in the supine position by 2-minute pneumatonography. After participants self-administered drops of timolol 0.5% for 1 week, twice daily in each eye, both measurements were repeated. Results: Mean IOP decreased from 15.1 ± 3.0 mm Hg at baseline to 12.4 ± 2.4 mm Hg (P < 0.001)after 1 week of timolol use. Mean outflow facility decreased from 0.23 ± 0.08 μL/min/mm Hg at baseline to 0.18 ± 0.08 μL/min/mm Hg (P < 0.001)after timolol. The change in outflow facility was negatively correlated with baseline outflow facility (r = −0.51; P < 0.001). Conclusions: Timolol reduces outflow facility in healthy human eyes, and this effect is greater in eyes with higher baseline outflow facility. This phenomenon may be related to reduced aqueous humor flow, but the precise mechanism remains to be determined.
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