In seven diabetics of type 1 and seven of type 2, the flow physiologic magnitudes were measured in a retinal quadrant before and after photocoagulation. The segmental blood flow, the arterial flow velocity, and the diameters of artery and vein are smaller after photocoagulation than before. Investigations into the time course of the flow-physiologic parameters following photocoagulation show that the flow-physiologic values are stationary about 2 weeks after photocoagulation.
Six hypotheses relating to the mode of action of photocoagulation, based on comprehensive data gathered from the literature, are put forward and discussed. Particular emphasis is given to the question of achieving a lasting break through of the barrier represented by Bruch's membrane and the pigment epithelium and thus to the altered diffusion conditions between the choriocapillaries and the retina. In addition a peculiar marginal fluorescence in fluorescein serial angiograms of old light coagulation foci is described and compared with data from the literature. The authors put forward a new hypothesis concerning the mode of action of light coagulation; they believe that particularly in the marginal areas affected by photocoagulation, completely new conditions are created for the metabolic exchange between choroid and retina, including the formation of anastomoses and shunts.
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 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.
The generalized capillary dilatation demonstrable at the early stage of diabetic retinopathy can now, on the basis of recent results in biochemistry, be assessed as an attempt at autoregulation in relative hypoxia of tissue. In patients with diabetic retinopathy, an elevated HbA1c concentration in the blood can be observed. The DPG level, however, is reduced. Both facts cause reduced oxygen transmission from blood to tissue. Rheologic changes such as augmented aggregation of erythrocytes, decreased deformability of erythrocytes and increased viscosity of blood and plasma finally lead to disturbances of microcirculation in the terminal vascular system. From these latest findings concerning the development of microcirculation disorders in diabetic retinopathy possible consequences for new therapeutic procedures can be derived.
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