Daytime and nighttime effects of brimonidine on IOP and aqueous humor dynamics in participants with ocular hypertension

PURPOSE:: The effects of brimonidine on daytime and nighttime intraocular pressure (IOP) and aqueous humor dynamics were evaluated in volunteers with ocular hypertension (OHT). PATIENTS AND METHODS:: Thirty participants with OHT (58.6±1.7 years old, mean±SEM) were enrolled into this randomized, double-masked, cross-over study. For 6 weeks, participants self-administered 0.2% brimonidine or placebo 3 times daily. During daytime and nighttime visits, measurements included aqueous flow (Fa) by fluorophotometry, outflow facility (C) by tonography, episcleral venous pressure (Pev) by venomanometry, and seated and supine IOP by pneumatonometry. Uveoscleral outflow (U) was calculated mathematically. RESULTS:: When treated with placebo, nighttime supine Pev (11.2±0.25 mm Hg) was higher (P<0.05) compared with daytime seated Pev (10.2±0.25 mm Hg), and Fa and U were significantly reduced at night. Brimonidine significantly lowered seated IOP at 9:00 AM and 11:00 AM, 9:00 PM and 11:00 PM and supine IOP at 9:00 AM and 11:00 AM. Brimonidine increased U at 9 AM and 11 AM (P<0.01) and had no effect on daytime and nighttime Fa, C, or Pev. CONCLUSIONS:: In subjects with OHT, brimonidine treatment for 6 weeks significantly reduces seated IOP during the day by increasing uveoscleral outflow. The lack of IOP effect at night can be explained by failure to overcome a normal nighttime reduction of uveoscleral outflow.