Effect of long-term topical betaxolol on tissue circulation in the iris and optic nerve head

The effect of long-term topical betaxolol on tissue circulation in the iris and optic nerve head was studied. Twenty microliters of 0.5% betaxolol ophthalmic solution was instilled in one eye of an albino rabbit and the same amount of physiological saline in the fellow eye twice daily for 20 days. A quantitative index of tissue blood velocity, NB value, was measured in the iris (21 rabbits) and optic nerve head (31 rabbits) of both eyes before the first instillation and 2 hr after completion of instillations, using a laser speckle tissue circulation analyser of which details have been reported previously. The intraocular pressure was also measured at 5-day intervals. The intraocular pressure was lowered by 2-3 mmHg only in the betaxolol-treated eye (P < 0.01). Both in the iris and optic nerve head no significant bilateral difference in the NB was seen before the first instillation, while it was significantly greater in the betaxolol-treated than in the contralateral control eye by about 10% after completion of instillations (P < 0.02-0.05). The difference between the NB after completion of instillations and that before the first instillation was significantly greater in the betaxolol-treated eye than in the contralateral control eye both in the iris and optic nerve head (P < 0.001-0.02). Twenty-day twice daily instillation of 0.5% betaxolol caused relatively small, but significant increase in tissue blood velocity in the ipsilateral iris and optic nerve head in albino rabbits, which may have clinical implications.