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S. L. Galata, S. Fan, C. Gangahar, M. Zhao, J. J. Hejkal, C. B. Camras, C. B. Toris; Comparison of the Effects of Timolol on Aqueous Humor Dynamics During the Day and the Night. Invest. Ophthalmol. Vis. Sci. 2010;51(13):147.
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The purpose of this study was to compare the daytime and nighttime effects of timolol on aqueous humor dynamics in patients with ocular hypertension.
Thirty ocular hypertensive volunteers were enrolled in this study after washout of all ocular drugs. Study measurements were made during the day and night, at baseline and after twice-daily timolol (0.5%) for two weeks. Daytime measurements included blood pressure, seated intraocular pressure (IOP) central corneal thickness (CCT), anterior chamber depth, episcleral venous pressure (Pev), aqueous flow (Fa) and outflow facility by fluorophotometry (Cfl) and tonography (Cton). These measurements were repeated at night (starting at 56 hours after completion of the daytime measurements) with the exception of Pev and Cfl. Additionally, IOP at night was taken in the supine position. Uveoscleral outflow was calculated mathematically using either Cfl or Cton in the Goldmann equation.
Compared to baseline, timolol significantly lowered IOP by at least 24% at 9AM and 18% at noon (p<0.001 for both), but at no time at night. During the day timolol reduced aqueous flow by 25% (p=0.001), but at night the aqueous flow reduction was no different than at baseline. There was a significant (p=0.035) increase (0.86 µl/min) in daytime uveoscleral outflow calculated with tonographic data but not fluorophotometric data. No other parameter was significantly changed by timolol treatment.
Timolol effectively lowers IOP during the day by decreasing aqueous flow. It is ineffective at night because it cannot reduce aqueous flow below its normally low nocturnal rate. The finding that IOP is reduced at 9AM, 12 hours after the previous dose, suggests that timolol blocks the normal aqueous flow rise in the morning. The effect of timolol on uveoscleral outflow is an inconsistent finding that should be viewed with caution.
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