Alteration in choroidal blood flow produced by local pressure

Background: To investigate how transient pressure applied to the retinal pigment epithelium (RPE) layer and choroid affects choroidal blood flow in rabbits. Methods: Twelve rabbits underwent vitrectomy and local retinectomy. In nine of the rabbits a glass rod was used to exert brief pressure on the RPE layer and choroid. Three of the rabbits had no pressure indentation and were considered to be controls. The choroidal circulation was studied by indocyanine green (ICG) angiography. The retina and choroid were studied by postmortem histology. Results: Pressure on the RPE layer and choroid caused nonfluorescence in segments of retinal arteries and veins and reduced fluorescence in adjacent choroidal capillaries, producing a black region at the pressure site in the angiograms. The size of this region decreased during the angiogram, often accompanied by the appearance of fine channels considered to be flow through the partially blocked vessels; the obstructed ends of the vessels became increasingly hyperfluorescent. These changes lasted for about 24 h before the choroidal circulation recovered. Histology showed evidence of thrombotic-like material in choroidal arteries and veins at the areas of absent perfusion. After local retinectomies, there was no evidence of thrombosis in control eyes where no pressure had been applied. Conclusions: Brief pressure on the RPE and choroid causes immediate reduction in flow through choroidal vessels, which appears to be due to local thrombosis in small segments of these vessels that resolves slowly. This may reflect a tendency for thrombi to form rapidly in choroidal vessels; it may also depend on neural reflexes causing vasoconstriction. The long time course of recovery could result in retinal ischemia and may underlie the pathophysiology of other pressure insults to the choroid.