Platelet-fibrin embolism in a rhesus macaque: angiographic and pathologic studies comparing fluorescein and indocyanine green.

A male rhesus macaque was found to have what appeared to be numerous platelet-fibrin emboli in retinal vessels in the perimacular area. Indocyanine green (ICG) dye fluorescence and fluorescein angiograms of the fundus demonstrated leakage of fluorescein, but not ICG, from the involved arterioles. Histopathologic changes in the eyes included occlusion of retinal and choroidal vessels with platelet-fibrin emboli, inner retinal ischemia, ischemic injury to the parafoveal capillary bed distally to occlusion of precapillary arterioles, and retinal exudate limited to the regions of capillary damage. Differential leakage of fluorescein may be explained by the difference in binding affinities of the 2 dyes to blood protein: 20% to 40% of the circulating fluorescein is unbound, and 98% of ICG is bound to serum albumin. Simultaneously or serially performed angiograms with fluorescent probes of different sizes might be used to obtain a qualitative measure of vascular integrity in persons with embolism, diabetic retinopathy, sickle cell retinopathy, vasculitis, and other disorders known to produce focal retinal and choroidal vascular occlusion.