Studies on Retinal Circulation in Experimental Animals, Healthy Human Eyes and Eyes with Diabetic Retinopathy

The retina is a highly metabolically active tissue with large demands on the supply of nutrients. Disorders affecting the retina often include some vasculopathy with an impact on retinal circulation. Studies of retinal haemodynamics could thus help to detect, differentiate and diagnose diseases, to monitor changes in disease as well as progression and efficiency of the therapy. The present studies were an attempt to validate and determine the clinical usefulness of a newly developed technique for studying the retinal circulation in human eyes. We used different techniques to evaluate different parameters of retinal circulation. We examined how leukocyte velocity determined with Blue Field Simulation and transit times, mean transite time (MTT) and arterio-venous passage (AVP), and vessel diameter, determined from fluorescein angiograms, together reflects the retinal circulation. MTT was determined with a method based on an Impulse-Response technique, MTTIR. In a study on monkeys we compared our method, together with two conventional methods, with an absolute measurement of retinal blood flow (RBF) determined with labelled microspheres. There was a weak, but not statistically significant, correlation between retinal blood flow and MTTIR (r2 = -0.60, p = 0.06), but no useful correlation between retinal blood flow and either of the other two measures of transit times. In a study on healthy eyes we determined the effect of a physiological provocation, changes in arterial blood gases, on retinal circulation. Breathing pure oxygen or increased level of carbon dioxide in inspired air had no effect on MTT, but oxygen reduced leukocyte velocity and vessel diameter and carbon dioxide increased leukocyte velocity significantly. We concluded that unchanged transit time trough the retinal tissue was not due to a lack of effect of the gas provocation but a result due to concomitant changes in volume and flow. In a study on eyes of patients with diabetic retinopathy we investigated the relation between the extent of retinal circulation changes and the severity of the diabetes retinopathy (DRP). Transit times were relatively unaffected until proliferative DRP (PDRP) developed. In eyes with PDRP both MTTIR and AVP were increased. After panretinal photocoagulation treatment MTTIR returned to normal levels and vessel diameters tended to decrease while leukocyte velocity and AVP remained unchanged. We concluded that the increase in MTTIR in eyes with PDRP is at least partly explained by vessel dilation, causing an increased volume of the retinal vascular bed