Regional variation in capillary hemodynamics in the cat retina

PURPOSE. The behavior of the retinal microcirculation and its role in the progression of ocular disease is of considerable interest, yet few details are known about the flow of blood through the capillary networks of the retina. Although retinal vessels may be viewed through the pupil using standard optics, the optical limitations of the cornea and lens prevent the resolution of retinal features smaller than approximately 10 μm in size. Because red blood cells are smaller than this, fluorescent techniques such as angiography, specific cell labeling, and fluorescein-encapsulated liposomes have typically been used to observe the retinal microcirculation in vivo. Here the authors report a study of in vivo retinal capillary hemodynamics using white light GRadient INdex of refraction (GRIN) lens endoscopy. METHODS. GRIN lens endoscopy and robotic manipulation were used to directly observe and record the motion of erythrocytes within retinal capillary networks. Video images from the endoscope were analyzed to determine the regional variation of erythrocyte velocity and normalized optical density (an index of relative capillary hematocrit) in the superficial retinal capillaries of the cat. RESULTS. A significant decrease in mean retinal capillary velocity coupled with a corresponding increase in red blood cell density was observed in peripheral regions of the retina when compared with regions of the retina near the optic disc. Stasis or intermittent flow was not observed in the unstained retina, nor were capillaries noted that contained only plasma. CONCLUSIONS. Quantification of the bloodflow in retinal microcirculation was possible using GRIN lens endoscopy and showed significant regional heterogeneity in the cat retina.