title: Effects of brimonidine on aqueous humor dynamics in human eyes.
abstract: OBJECTIVE
  To evaluate the mechanism by which brimonidine, a selective alpha 2-adrenergic agonist, lowers intraocular pressure (IOP) in humans.
  
  
  METHODS
  Twenty-one volunteers with ocular hypertension.
  
  
  METHODS
  Brimonidine tartrate (0.2%) was given topically twice daily for 1 week to one eye in a randomized, double-masked study. The fellow eye was similarly treated with brimonidine vehicle. Before (baseline) and after 1 week (day 8) of dosing, IOP, aqueous flow, episcleral venous pressure, and tonographic outflow facility were directly measured. Fluorophotometric outflow facility and uveoscleral outflow were calculated. Brimonidine-treated eyes were compared with vehicle-treated contralateral control eyes and with baseline measurements after 1 week of dosing.
  
  
  RESULTS
  Brimonidine significantly (P < .001, Student's two-tailed t test) reduced IOP mean +/- SE of 4.7 +/- 0.7 and 4.2 +/- 0.4 mm Hg compared with the baseline day and with the vehicle-treated contralateral control eyes, respectively. Compared with the baseline day, aqueous flow was reduced by 20% (P = .002) and uveoscleral outflow was increased (P = .04). A slight contralateral decrease in IOP of 1.2 +/- 0.6 mm Hg (P = .05) and in aqueous flow of 12% (P = .05) was noted. No significant difference was seen in the outflow facility values or episcleral venous pressure compared with the baseline day or with the contralateral control eye.
  
  
  CONCLUSIONS
  The brimonidine-induced reduction in IOP in humans is associated with a decrease in aqueous flow and an increase in uveoscleral outflow. The decrease in IOP and aqueous flow in the contralateral control eye on day 8 compared with the baseline day suggests a mild contralateral effect.
mesh_terms: D000287:Administration, Topical; D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage / Q000494:pharmacology*; D000328:Adult; D000368:Aged; D000959:Antihypertensive Agents / Q000008:administration & dosage / Q000494:pharmacology*; D001082:Aqueous Humor / Q000187:drug effects* / Q000378:metabolism; D000068438:Brimonidine Tartrate; D004311:Double-Blind Method; D005260:Female; D015590:Fluorophotometry; D006801:Humans; D007429:Intraocular Pressure / Q000187:drug effects; D008297:Male; D008875:Middle Aged; D009798:Ocular Hypertension / Q000188:drug therapy*; D009883:Ophthalmic Solutions; D011810:Quinoxalines / Q000008:administration & dosage / Q000494:pharmacology*
chemical_list: D000316:Adrenergic alpha-Agonists; D000959:Antihypertensive Agents; D009883:Ophthalmic Solutions; D011810:Quinoxalines; D000068438:Brimonidine Tartrate