Absolute retinal blood flow in healthy eyes and in eyes with retinal vein occlusion

<bold>Purpose: </bold>To measure non-invasively retinal venous blood flow (RBF) in healthy subjects and patients with retinal venous occlusion (RVO). <bold>Methods: </bold>The prototype named AO-LDV (Adaptive Optics Laser Doppler Velocimeter), which combines a new absolute laser Doppler velocimeter with an adaptive optics fundus camera (rtx1, Imagine Eyes (R), Orsay, France), was studied for the measurement of absolute RBF as a function of retinal vessel diameters and simultaneous measurement of red blood cell velocity. RBF was measured in healthy subjects (n = 15) and patients with retinal venous occlusion (RVO, n = 6). We also evaluated two softwares for the measurement of retinal vessel diameters: software 1 (automatic vessel detection, profile analysis) and software 2 (based on the use of deep neural networks for semantic segmentation of vessels, using a M2u-Net architecture). <bold>Results: </bold>Software 2 provided a higher rate of automatic retinal vessel measurement (99.5 % of 12,320 AO images) than software 1 (64.9 %) and wider measurements (75.5 +/- 15.7 mu m vs 70.9 +/- 19.8 mu m, p < 0.001). For healthy subjects (n = 15), all the retinal veins in one eye were measured to obtain the total RBF. In healthy subjects, the total RBF was 37.8 +/- 6.8 mu l/min. There was a significant linear correlation between retinal vessel diameter and maximal velocity (slope = 0.1016; p < 0.001; r(2) = 0.8597) and a significant power curve correlation between retinal vessel diameter and blood flow (3.63 x 10(-5) x D-2.54; p < 0.001; r(2) = 0.7287). No significant relationship was found between total RBF and systolic and diastolic blood pressure, ocular perfusion pressure, heart rate, or hematocrit. For RVO patients (n = 6), a significant decrease in RBF was noted in occluded veins (3.51 +/- 2.25 mu l/min) compared with the contralateral healthy eye (11.07 +/- 4.53 mu l/min). For occluded vessels, the slope between diameter and velocity was 0.0195 (p < 0.001; r(2) = 0.6068) and the relation between diameter and flow was Q = 9.91 x 10(-6) x D-2.41 (p < 0.01; r(2) = 0.2526). <bold>Conclusion: </bold>This AO-LDV prototype offers new opportunity to study RBF in humans and to evaluate treatment in retinal vein diseases.