6.1 Diagnostic/Technology Overview

Fundus autofluorescence (FAF) is a noninvasive imaging modality which is used to record the retinal and retinal pigment epithelial (RPE) autofluorescence (AF). Lipofuscin, which is a mixture of fluorophores, is considered to be the most intense source of AF in the posterior eye. Consequently, FAF is thought to be an effective surrogate for lipofuscin concentration in the RPE.

The RPE, which is composed of a single layer of hexagonal cells, separates the choroid from the neurosensory retina. An essential function of the RPE cells is the phagocytosis and degradation of shed photoreceptor outer segment discs with subsequent release of degraded material at the basal side of the cell. Lipofuscin, which forms in the RPE cells, is composed of several different molecules (the most important is A2E: N-retinyl-N-retinylidene ethanolamine) and derives primarily from phagocytized photoreceptor outer segments. Lipofuscin is not completely recognized by lysosomal enzymes and is thus incompletely removed and accumulates in the lysosomes. Strong evidence suggests that lipofuscin is not an inert by-product but that it may interfere with cell function, as well as may increase in quantity with age as well as in the setting of RPE dysfunction.

AF is the natural emission of light by biological molecules (fluorophores) following absorption of a higher energy shorter wavelength of light. FAF imaging uses the fluorescent properties of lipofuscin to generate a grayscale image, where dark pixel values correspond to relative low intensities of emission and bright pixel values correspond to high intensities of fluorescence emission. Commercially available FAF systems include flash fundus cameras, confocal scanning laser ophthalmoscopes (cSLO), and ultra-widefield imaging devices. Moreover, the AF of the retina and RPE can be excited across a broad range of wavelengths in the blue and green spectrum (blue- and green-light FAF, BAF, and GAF, respectively).

The naturally occurring AF of the ocular fundus is known to be of low intensity and to be characterized by a specific pattern: (i) the optic nerve head typically appears dark due to the absence of lipofuscin in this area; (ii) the areas with retinal vessels are characterized by a reduced FAF signal due to the absorption by blood; and (iii) FAF signal is also reduced in the central macular area, due to the absorption of blue light by the luteal pigment. In contrast to BAF imaging, GAF cSLO imaging is not as significantly affected by macular pigment due to a reduced amount of absorption (▶ Fig. 6.1). Causes for a reduced AF signal may include RPE loss, presence of intraretinal fluid or subretinal fibrosis, reduction in RPE lipofuscin density, or presence of luteal pigment. Causes for an increased AF signal may include loss of luteal or photopigment, increased RPE lipofuscin, or presence of subretinal AF material.

6.2 Key Applications

6.2.1 Age-Related Macular Degeneration

There is strong evidence that RPE cells play an important role in age-related macular degeneration (AMD) pathogenesis. This evidence is also supported by the fact that drusen composition includes incompletely degraded material from autophagy and phagocytosed shed photoreceptor outer segment discs.

Early/Intermediate Age-Related Macular Degeneration

The FAF signal may be normal, decreased, or increased in corresponding drusen areas (▶ Fig. 6.2). This may reflect the variable composition of drusen, including other fluorophores, as well as different reactive alterations in the overlying RPE cell monolayer. In 2005, the International FAF Classification Group defined eight phenotypic FAF patterns associated with early nonneovascular AMD: normal, minimal change, focal increase, patchy, linear, lace-like, reticular, and speckled. Patchy, linear, and reticular patterns are associated with an increased risk for choroidal neovascularization.

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