When a fundus autofluorescence (FAF) image is being evaluated, any deviation from a normal recording should be recognized, and a potential cause should be sought for the abnormal findings. The characteristics of a FAF image without disease-related abnormalities include the following (see Fig. 8.1, top row):
The optic nerve head typically appears dark because of the absence of retinal pig- ment epithelium (RPE) and, thus, autofluorescent lipofuscin.
The retinal vessels are associated with a markedly reduced FAF signal because of absorption phenomena from blood contents.
In the macular area, the FAF signal is most prominently reduced at the fovea.
From the foveal center, a distinct increase in the signal can be observed at about the margin of the fovea, followed by a further gradual increment toward the outer mac- ula. This is caused by absorption from luteal pigment (i.e., lutein and zeaxanthin) in the neurosensory retina and possible spatial differences in melanin deposition (see chapter 2). There is marked interindividual variation with regard to the topographic distribution of luteal pigment (see Chaps. 6 and 7).
Outside these areas with decrements in FAF intensity, the FAF signal appears evenly distributed. Advances in the optical and software development for the confo- cal scanning laser ophthalmoscope (cSLO) technique now even allow for delineation of the polygonal RPE-cell monolayer in the presence of clear media and optimal pa- tient cooperation (Fig. 8.1, middle row). This delineation is possibly due to the spatial orientation of the lipofuscin and melanin granules in the RPE cell cytoplasm (see Fig. 8.1, bottom row). While there is typically a predominant location of lipofuscin granules at the peripheral cell margin, absorbing melanin granules are oriented to- ward the apical cell center.
Identification of abnormalities in the FAF image is very much dependent on the quality of the recorded image (see Chap. 4). Any opacities in the vitreous, the lens, the anterior chamber, or the cornea may affect the detection of pathological alterations at the level of the RPE and the neurosensory retina. Lens opacities particularly have an impact on the image quality, with yellowish alterations of the nucleus with age leading to absorption of the blue excitation light used for FAF imaging. Images recorded with fundus cameras using equivalent filters are affected more by such lens changes com- pared with recordings from cSLOs. However, in the presence of advanced lenticular
Evaluation of Fundus Autofluorescence Images
Frank G. Holz, Monika Fleckenstein, Steffen Schmitz-Valckenberg, Alan C. Bird
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changes, it may be impossible to obtain an image with sufficient quality to allow for a reasonable interpretation of the FAF findings (see Chap. 4, Fig. 4.4).
In essence, abnormal FAF signals either derive from a change in the number or composition of fluorophores in the RPE cell cytoplasm (i.e., lipofuscin) or from the presence of absorbing or autofluorescent material anterior to the RPE monolayer (Figs. 8.2 and 8.3). In addition, abnormal tissue with fluorophores with spectral char- acteristics similar to RPE-lipofuscin at the level of the choroid may cause a corres- ponding increased FAF signal (Fig. 8.4).
As outlined in the previous chapters and demonstrated in the clinical part of this book, it must be noted that FAF imaging provides information over and above con- ventional imaging techniques. Therefore, the identification of a pathological change on funduscopy such as drusen under the RPE may not give a clue as to the associated autofluorescence characteristics of these changes. For example, the FAF signal may be normal, decreased, or increased, depending on the molecular composition of the drusen material (more or fewer fluorophores) and the corresponding alteration of the overlying RPE (more or less flattening and reduction in lipofuscin granule density; see Chap. 11). While drusen in association with monogenetic juvenile macular dystro- phies tend to be associated with an increased FAF signal, this is usually not the case for drusen in the context of complex, multifactorial age-related macular degeneration.
For the evaluation and interpretation of a FAF image in a individual patient, it may be helpful to correlate the findings with those obtained with reflectance images of the same excitation wavelength and other imaging methods, including fundus photo- graphs, optical coherence tomography, and fluorescein angiography.
Below are the major causes and pathophysiologic categories for increased or re- duced FAF signals.
Causes for a reduced FAF signal:
Reduction in RPE lipofuscin density
• RPE atrophy (such as geographic atrophy)
• Hereditary retinal dystrophies (such as RPE65 mutations)
Increased RPE melanin content
• e.g., RPE hypertrophy
Absorption from extracellular material/cells/fluid anterior to the RPE
• Intraretinal fluid (such as macular edema)
• Migrated melanin-containing cells
• Crystalline drusen or other crystal-like deposits
• Fresh intraretinal and subretinal hemorrhages
Causes for an increased FAF signal:
Acknowledgements
The authors thank Adnan Tufail and Andrew R. Webster at Moorfields Eye Hospital for their help in the collection of images.
Excessive RPE lipofuscin accumulation
• Lipofuscinopathies, including Stargardt disease, Best disease, pattern dystro- phy, and adult vitelliform macular dystrophy
• Age-related macular degeneration, such as RPE in the junctional zone pre- ceding enlargement of occurrence of geographic atrophy
Occurrence of fluorophores anterior or posterior to the RPE cell monolayer
• Intraretinal fluid (such as macular edema)
• Subpigment epithelial fluid in pigment epithelial detachments
• Drusen in the subpigment epithelial space
• Migrated RPE cells or macrophages containing lipofuscin or melanolipofus- cin (seen as pigment clumping or hyperpigmentation on funduscopy)
• Older intraretinal and subretinal hemorrhages
• Choroidal vessel in the presence of RPE and choriocapillaris atrophy, such as in the center of laser scars or within patches of RPE atrophy
• Choroidal nevi and melanoma
Lack of absorbing material
• Depletion of luteal pigment, such as in idiopathic macular telangiectasia type 2
• Displacement of luteal pigment, such as in cystoid macular edema Optic nerve head drusen
Artefacts
• Fibrosis, scar tissue, borders of laser scars
• Retinal vessels
• Luteal pigment (lutein and zeaxanthin)
• Media opacities (vitreous, lens, anterior chamber, cornea)
Fig. 8.1 Top row: Normal fundus autofluorescence (FAF) images obtained with a confocal scanning laser ophthalmoscope (Heidelberg Retina Angiograph). The FAF image shows the spatial distribution of the intensity of the FAF signal for each pixel in gray values. By definition, low pixel values (dark) il- lustrate low intensities, and high pixel values (bright) illustrate high intensities. In the normal subject, dark-appearing retinal vessels due to absorption from blood contents, a dark optic nerve head due to absence of autofluorescent material, and an increased signal in the macular area secondary to absorp- tion from luteal pigment (lutein and zeaxanthin) can be observed. The relative distribution of FAF inten- sities can also be illustrated in pseudo-three-dimensional reconstruction techniques (right). Areas with low FAF intensities are shown in blue and can be distinguished from the normal background signal (red). Middle row: Modern imaging systems allow visualization of more details (in the presence of clear media and optimal patient cooperation) and the ability to image even larger retinal areas with up to a 55° field. Note the interindividual differences in macula pigment absorption between these examples from normal subjects.
Fig. 8.2 Top row: Example of increased fundus autofluorescence (FAF) signals (right) corresponding to funduscopically visible (left) yellowish lesions in a patient with Stargardt disease. Middle row: Ex- ample of increased FAF signal corresponding to funduscopically visible yellowish and in part hyper- pigmented, reticular lesions in a patient with pattern dystrophy. Bottom row: Example of increased FAF image signal corresponding to macular edema in the presence of diabetic maculopathy. Exudates and hemorrhages are characterized by low FAF levels due to absorption phenomena. Additional areas with decreased intensity can be observed and appear to be normal on fundus photography
Fig. 8.1 (Continued) Bottom row: Confocal scanning laser microscope image of the polygonal retinal pigment epithelium cell monolayer (left) and schematic drawing (right) showing a predominant loca- tion of lipofuscin granules (red) at the peripheral cell margin, while absorbing melanin granules are oriented toward the apical cell center and the cell nucleus at the basal side
Fig. 8.3 Example of decreased fundus autofluorescence (FAF) corresponding to retinal pigment epithelium atrophy and crystalline drusen in a patient with geograph- ic atrophy secondary to age-related macular degeneration. Levels of increased FAF can be observed surrounding the atrophy as retinal areas with excessive lipofuscin accumula- tion. For comparison, near-infrared reflect- ance (lower left) and blue-light reflectance (lower right) are il- lustrated as well. Note also prominent re- ticular drusen outside atropic patches (see chapter 11.4)
Fig. 8.4 Example of marked ey decreased fundus autofluores- cence (FAF) intensity over the whole posteri- or pole in the presence of severe retinal pig- ment epithelium (RPE) atrophy in a patient with Usher syndrome.
Underlying choroidal vessels that are usually obscured by the RPE can now be observed on FAF imaging by levels of increased FAF.
Note that optic disc drusen are present, showing increased FAF. For comparison, near-infrared reflect- ance (lower left) and blue-light reflectance (lower right) are illus- trated as well
