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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.

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.

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.

The present possibilities to measure blood flow and other magnitudes of retinal microcirculation in vivo are discussed. On this basis, a simple model designed for retinal microcirculation is presented as a tool for the clinical interpretation of measuring results. Based on the experience of over 200 measurements of retinal microcirculation magnitudes, subsequent reports give a clinical interpretation of measuring results, utilizing the measuring models presented. Thus, it is intended to demonstrate the actual clinical significance of such measurements.

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.

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.

A combined measuring unit for the Retinophot is described which can be used to determine the blood flow through the great retinal vessels. This method is possible by combination of the two-point photometry with length measurements by means of rocking-plate micrometry. The first measuring results are presented.

An additional unit which can be used for length measurements at the fundus of the human eye is described for the ophthalmoscope. The principle of measurement is based on optical displacement of the measuring object to itself. The optical displacement of picture points is made possible by the use of a tilting flat glass plate in the intermediate image plane of the eyepiece. The relation between the optical displacement of picture points and the measuring movement is sufficiently linear for the described device.