Noninvasive measurement and quantitative analysis of the Bayliss effect in human retinal vessels by means of the RVA is possible. Analysis of retinal arterial autoregulation may provide valuable insight into pathologic conditions such as diabetic or hypertensive retinopathy.

Diabetic retinopathy is the most common cause of blindness in adults in all industrialised countries. A much better pathophysiological understanding of the disease has been achieved during the last years. There is evidence that a functional deficit in haemodynamics proceeds the morphological changes of the retina. Modern technologies for measuring haemodynamic parameters and their correlation to pathophysiology in diabetic retinopathy are discussed. In the near future it will be possible to use these parameters as an early sign of worsening before morphological changes occur. Measurement of the therapeutic success, e. g., in laser coagulation would seem to be possible.

Beginning as early as the mid-4th decade of life, an age-dependent reduction in retinal blood flow has to be considered an essential risk factor for disorders of retinal microcirculation. Blood flow measurements are a valuable, but not the sole criterion of microcirculatory disorders. While in retinal occlusive diseases blood flow is a valuable indicator of the severity of microcirculatory disorders, the latter can also occur in the presence of normal and elevated blood flow values. Shifts of the metabolic activity of the retina and changes in the metabolic activity of the retina and changes in the metabolic conditions have to be taken into account when a clinical interpretation is given.

Based on the pathomechanism of diabetic retinopathy, the effect of photocoagulation on retinal microcirculation as assessed by measurements of retinal microcirculation magnitudes is discussed. The clinico-experimental results support the opinion that an essential effect of photocoagulation is due to an improvement in the flow conditions of the noncoagulated central retinal area with regard to metabolic exchange and stasis situation by the coagulation-induced occlusion of peripheral capillary areas. Further clinico-experimental measurements of retinal microcirculation magnitudes for the assessment of the behavior of microcirculation after photocoagulation in venous occlusive diseases of the retina are presented. It is shown that retinal microcirculation immediately after photocoagulation reacts by transitory instationary flow conditions; in this period it is quite possible that an additional blood flow reduction over several days will occur. In particular, the effect of photocoagulation in venous occlusive diseases with narrow arterial vessels is discussed. It is emphasized that with regard to the therapeutic effect of photocoagulation as well as to the discussion of pathomechanisms, apart from blood flow, the differentiated behavior of single retinal vascular areas, such as arteries, arterioles, capillaries, venules and veins, requires more attention. Particularly with this in mind, substantial improvements in differential diagnosis and treatment of retinal circulatory disturbances may be expected from measurements of retinal circulation magnitudes.

Based on results from measurements of arterial blood velocity, arterial and venous diameters of major segmental retinal vessels in normal persons and in patients with venous occlusive diseases and in continuation of the two preceding parts of this series of articles, further possibilities for the differential diagnosis of measurements of retinal microcirculation magnitudes are discussed. Whereas the measurement of blood velocity is an important criterion for the assessment of the stasis conditions and the arterial involvement in an occlusive disease, the diameters of the vessels offer essential suggestions to local regulative processes. In this connection, a dependence on pH of the contraction state of the smooth vascular musculature detected in porcine coronary arteries is presented. By its transmission to the arterial retinal vessels, it is possible to unequivocally clarify the local regulative and pathological behavior of arterial retinal vessels in terms of flow physiology.