2.1 Diagnostic/Technology Overview
Sodium fluorescein is an orange–red hydrocarbon molecule that circulates in the vasculature, 80% protein bound and 20% nonprotein bound. Nonprotein bound fluorescein will diffuse through fenestrated and leaky capillaries but will not move through the retinal pigment epithelium (RPE) or normal retinal capillaries. Nonprotein bound fluorescein will absorb and become excited by blue light (wavelength: 465–490 nm), causing fluorescent emission of green light (wavelength: 520–530 nm).
To perform fluorescein angiography (FA), white light passes through a blue filter, transmitting blue light to the retina (▶ Fig. 2.1). Fluorescein in the retina tissues and vasculature is excited by the blue light and emits green light. The green light is then detected by fundus photography using a green filter in a standard 30-degree (▶ Fig. 2.2), widefield 50-degree, or ultra-widefield camera (e.g., 200 degrees; ▶ Fig. 2.3).
Fig. 2.1 Diagnostic overview of fluorescein angiography. (1) White unfiltered light passes through a blue filter, producing blue light (2) that will excite fluorescein in the retinal vasculature. (3) Blue light is reflected from the retina and green light is emitted from the excited fluorescein. After passing through a green filter, green light is transmitted (4) and detected by film or digital photography.
Fig. 2.2 Fluorescein angiography of neovascular age-related macular degeneration. (a) Venous phase angiogram showing numerous hyperfluorescent spots (arrow) and a larger hyperfluorescent lesion (dashed arrow). (b) Late phase angiogram showing hyperfluorescent spots without expanding borders (arrow), which correspond to staining of drusen. Additionally, the larger hyperfluorescent lesion (dashed arrow) now demonstrates expanding borders without discrete edges, demonstrating leakage from a choroidal neovascular membrane.
Fig. 2.3 Fluorescein angiography of a branch retinal vein occlusion (RVO). (a) Fundus photograph demonstrating retinal macular hemorrhage (asterisk), a dilated superior temporal retinal vein (between arrows), and a comparatively normal retinal vein (arrowhead). (b) Venous phase angiogram demonstrating complete venous filling in the normal retinal vein (arrowhead) compared to laminar venous filling in the superior temporal retinal vein (between arrows), demonstrating branch RVO. The macular hemorrhage shows blockage of retinal vasculature (asterisk) and blockage of underlying choroidal vasculature (hashtag), suggesting hemorrhage in multiple retinal layers.
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