title: Intravenous administration of clonidine reduces intraocular pressure and alters ocular blood flow.
abstract: OBJECTIVE
  To evaluate the effect of intravenously administered clonidine on ocular blood flow in healthy volunteers.
  
  
  METHODS
  A randomised, double-masked, placebo-controlled, two-way crossover study was performed in 12 healthy young volunteers. Clonidine (0.2 microg/kg/min) or placebo was administered intravenously over 10 minutes. The effects of clonidine were studied at baseline and up to 150 minutes after infusion. Ocular haemodynamics were measured using laser Doppler flowmetry, laser Doppler velocimetry and a retinal vessel analyser.
  
  
  RESULTS
  Clonidine significantly decreased mean arterial pressure (MAP) and intraocular pressure (IOP). Calculated ocular perfusion pressure decreased significantly by -8.7+/-8.7% after infusion of clonidine (p<0.01 vs placebo). Retinal arterial diameters increased by +4.4+/-2.7% (p = 0.012 vs placebo), whereas no significant change was observed in retinal veins. Red blood cell velocity decreased by -16+/-14% (p<0.01 vs placebo) after infusion of clonidine. Hence, calculated retinal blood flow decreased by -14+/-12% (p = 0.033 vs placebo). Choroidal blood flow increased by +18+/-19% (p<0.01 vs placebo) and optic nerve head blood flow increased by +16+/-23% (p = 0.046 vs placebo) 30 minutes after administration of clonidine but both returned to baseline thereafter.
  
  
  CONCLUSIONS
  The short-time increase in choroidal and optic nerve head blood flow indicates a transient vasodilatory effect of clonidine due to an unknown mechanism. The decrease in retinal blood flow indicates clonidine-induced vasoconstriction in the retinal microvasculature.
authors.affiliation: Department of Clinical Pharmacology, Medical University of Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria.
authors.forename: Günther
authors.initials: G
authors.lastname: Weigert
authors.forename: Hemma
authors.initials: H
authors.lastname: Resch
authors.forename: Alexandra
authors.initials: A
authors.lastname: Luksch
authors.forename: Herbert A
authors.initials: HA
authors.lastname: Reitsamer
authors.forename: Gabriele
authors.initials: G
authors.lastname: Fuchsjager-Mayrl
authors.forename: Leopold
authors.initials: L
authors.lastname: Schmetterer
authors.forename: Gerhard
authors.initials: G
authors.lastname: Garhofer
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage*; D001794:Blood Pressure / Q000187:drug effects; D002829:Choroid / Q000098:blood supply; D003000:Clonidine / Q000008:administration & dosage*; D018592:Cross-Over Studies; D004311:Double-Blind Method; D005123:Eye / Q000098:blood supply* / Q000187:drug effects; D006339:Heart Rate / Q000187:drug effects; D006801:Humans; D007262:Infusions, Intravenous; D007429:Intraocular Pressure / Q000187:drug effects*; D017078:Laser-Doppler Flowmetry / Q000379:methods; D009898:Optic Disk / Q000098:blood supply; D012039:Regional Blood Flow / Q000187:drug effects; D012171:Retinal Vessels / Q000187:drug effects
chemical_list: D000316:Adrenergic alpha-Agonists; D003000:Clonidine