Ocular blood flow and associated functional deviations in diabetic retinopathy

Diabetic retinopathy is among the leading causes of blindness in the industrialised nations. The clinical course of diabetic retinopathy has been described in detail [1] and a grading system for the disease has been elaborated [2, 3]. The mechanisms underlying this severe complication of Type I (insulin-dependent) and Type II (non-insulin-dependent) diabetes mellitus are, however, still not clear. Whereas some of the mechanisms which lead to proliferation in the late stages of retinopathy have been explained, the early processes, which lead to the onset of the disease are still obscure. Several hypotheses have been proposed including biochemical dysfunction linked to hyperglycaemia, oxidative stress, humoral or genetic factors, thickening of the capillary basement membrane and rheological and haemodynamic factors. The concept that altered retinal blood flow may have a role in the development of diabetic retinopathy was emphasised more than 20 years ago. Since then haemodynamics of the retina in patients with diabetes mellitus as well as in animal models of diabetes has attracted considerable research interest. Much less attention has been paid to the choroidal circulation in diabetes, which is surprising as much of the retinal oxygen is supplied from this vascular bed. In part this may arise from the difficulties in assessing choroidal blood flow in humans. A role for the choroidal circulation in the pathogenesis of diabetic retinopathy has been proposed previously, based on histological studies of the diabetic choroid [4±6]. Excessive basement membrane thickening in the choroid has been shown in human and experimental diabetes [4, 5]. The exact nature of ocular blood flow abnormalities in the different stages of diabetic retinopathy is still a matter of controversy. This may at least partially be caused by the variety of techniques used to assess haemodynamics of the eye. Hence we will briefly review the different techniques used to assess ocular haemodynamics in animals and humans and discuss results from clinical and animal studies in diabetes.