Topical phenylephrine decreases blood velocity in the optic nerve head and increases resistive index in the retinal arteries

Abstract

To investigate the effects of topical phenylephrine on circulation in the optic nerve head (ONH), posterior choroids, or retina in rabbits and healthy humans. Tissue blood velocity in the ONH and posterior choroid was measured using the laser speckle method as normalized blur (NB(ONH) and NB(CH)) in 28 anaesthetized albino rabbits. NB and intraocular pressure (IOP) in both eyes were measured for 180 min after unilateral single instillation of 5% phenylephrine and contralateral physiological saline as a control. In 11 normal volunteers aged 26.0+/-2.7 years, NB(ONH) was measured for 180 min after unilateral three drops of 5% phenylephrine and contralateral physiological saline in a double-masked manner. In the other 17 normal volunteers aged 25.5+/-2.4 years, blood velocity and blood flow in a major branch of the central retinal artery were measured using the laser Doppler blood flow metre and IOP and systemic circulatory parameters were monitored in similar fashion. Analysis of variance was applied for intergroup comparisons. NB(ONH) and NB(CH) decreased significantly only in the phenylephrine-treated eyes in rabbits by maximum of 9 and 20%, respectively (P=0.0046 and 0.0009), despite IOP decrease. In humans, NB(ONH) decreased significant by maximum of 13% (P=0.0047) and resistive index in the retinal arteries increased by 10% (P=0.0067) unilaterally; whereas IOP, diameter, blood velocity, or blood flow of the arteries was not significantly changed. Topical phenylephrine could exert a significant unfavourable effect on circulation in the ONH, choroidal tissue, and retinal arteries in rabbits and normal young humans.