Effect of topical timolol on the human retinal circulation.

The effect of topical timolol maleate 0.5% on the retinal circulation was investigated in 14 normal subjects using laser Doppler velocimetry and monochromatic fundus photography. In a double masked, randomized design, one eye received two drops of timolol maleate 0.5% and the fellow eye received two drops of placebo. Vessel diameter, maximum velocity of red blood cells, and volumetric blood flow rate were determined in a major temporal vein of each eye just prior to the instillation of drops, and then 90 min later. In comparison to the baseline value, there was a significantly larger average percentage increase in maximum velocity of red blood cells and volumetric blood flow rate in the timolol-treated eyes (11.0% and 13.2%, respectively) than in the control eyes (2.2% and 1.5%, respectively, paired t-test, P less than 0.05). No significant change in venous diameter was detected. No significant linear correlations were found between the percentage change in maximum velocity of red blood cells and mean brachial artery blood pressure, and between the percentage change in volumetric blood flow rate and mean brachial artery pressure in the placebo-treated eyes, whereas significant correlations were present between these quantities in the timolol-treated eyes (P less than 0.01 and P less than 0.05, respectively). The lack of such correlations in the control eyes is probably due to autoregulation of the retinal circulation. The presence of correlations in the timolol-treated eyes suggests that the drug may affect the capacity of the retina to autoregulate.