Ocular and optic nerve blood flow at normal and increased intraocular pressures in monkeys ( Macaca irus): a study with radioactively labelled microspheres including flow determinations in brain and some other tissues

Abstract

The effects of moderate increments in the intraocular pressure on blood flow rates in the various tissues of the eye were studied in monkeys. Blood flow rates were determined with radioactively labelled microspheres, 15 μm in diam. One eye had its spontaneous intraocular pressure while the other eye had its pressure stabilized at a higher level. The mean values for the intraocular pressures in the two eyes were 13 and 41 cm H 2O respectively. In eyes with spontaneous intraocular pressure mean blood flow through the retina, the iris, the ciliary body, and the choroid were 25, 17, 89, and 607 mg/min respectively. Blood flow through the ciliary processes was 222 and through the ciliary muscle 153 g/min/00 g tissue respectively. In eyes with increased intraocular pressure there were statistically significant reductions in blood flow through the choroid and through the prelaminar part of the optic nerve by 29 and 30% of the mean blood flow through control eyes respectively. The changes in blood flow through the retina, the iris, the ciliary processes and the ciliary muscle were not statistically significant. They ranged from a reduction by 8% to an increase by 19% in eyes with increased intraocular pressure. The results suggest that even moderate increments in intraocular pressure cause clear reductions in the blood flow through the choroid and through the prelaminar part of the optic nerve, while blood flow through the retina outside the optic disc and through the different parts of the anterior uvea is efficiently autoregulated. It is suggested that the susceptibility of the optic disc to increments in intraocular pressure is due to the deficient autoregulation of blood flow through the optic disc, which in turn might be explained by the choroidal origin of the optic disc vessels which interferes with normal autoregulatory mechanisms. Determinations were also made of blood flow through various parts of the brain and some other organs. Two different doses of microspheres were used to determine blood flow rates in the brain and in the eye. The larger dose seemed to reduce blood flow by 25% or more in the brain and in the iris, while no such effect was seen in the retina, the choroid or the ciliary body.