Fundus autofluorescence (FAF) is a specialized, non-invasive imaging technique used to examine the delicate layers of the retina. This method helps eye care professionals diagnose and monitor various retinal diseases. FAF captures an image by utilizing the natural light-emitting properties present within the eye’s tissues, highlighting metabolic changes. It provides a unique map of the health and function of the retinal pigment epithelium (RPE), the layer of cells that supports the light-sensing photoreceptors. The test is an established tool for assessing the retina, often revealing subtle damage before vision is significantly affected.
The Science Behind the Image
The mechanism of FAF relies on fluorophores, molecules that absorb light at one wavelength and emit it at a longer wavelength. The primary fluorophore measured in the retina is lipofuscin, a metabolic byproduct. Lipofuscin is a complex aggregate of lipids and proteins that accumulates within RPE cells as they perform their daily task of recycling spent photoreceptor components. The concentration of lipofuscin directly reflects the metabolic activity and health of the RPE cells.
FAF imaging typically uses blue light (around 488 nanometers) to excite the lipofuscin. This excitation causes the lipofuscin to fluoresce, emitting light in the yellow-to-red spectrum, usually peaking around 600 to 610 nanometers. A specialized camera detects this emitted light, creating a topographical map of lipofuscin distribution across the retina. Areas of high or low signal intensity correspond to RPE cell stress or damage.
Performing the FAF Scan
The FAF scan is a non-contact, rapid, and comfortable procedure. Unlike some other retinal imaging tests, FAF does not require the intravenous injection of contrast dye, such as fluorescein, making it entirely non-invasive. The entire imaging process is usually completed within a few minutes for both eyes.
Preparation may involve eye drops to dilate the pupils, allowing the device to capture a wider view of the retina. However, modern ultra-widefield devices often capture high-quality images without dilation. The patient sits in front of a specialized camera, often a Confocal Scanning Laser Ophthalmoscope (cSLO), and looks at a target. The camera projects the excitation light and immediately captures the fluorescent light emitted by the RPE. This rapid acquisition time minimizes light exposure and prevents image degradation from eye movement, allowing FAF to be performed routinely to track subtle changes over time.
Interpreting the FAF Map
The FAF image is a grayscale map where light intensity corresponds to the concentration of lipofuscin in the RPE cells. Interpretation involves identifying two primary patterns: hyper-autofluorescence and hypo-autofluorescence.
Hyper-Autofluorescence
Hyper-autofluorescence appears as brighter, white or light gray areas, signifying an abnormally high concentration of lipofuscin. This increased signal suggests RPE cells are under metabolic stress due to an overload of unprocessed waste material. These regions often precede visible structural damage, indicating RPE cells are struggling and may be at risk of future degeneration.
Hypo-Autofluorescence
Hypo-autofluorescence appears as darker, black, or dark gray areas, indicating a reduced or absent fluorescent signal. This pattern typically represents two possibilities: either the RPE cells are dead and no longer contain lipofuscin, or something is blocking the excitation light from reaching the RPE. The primary cause of hypo-autofluorescence is RPE atrophy, where cell death leads to a profound loss of the lipofuscin signal. FAF is adept at mapping the precise boundaries of this atrophy, allowing clinicians to monitor the progression of retinal damage.
Conditions Identified Using FAF
FAF is an indispensable tool for diagnosing, classifying, and monitoring several retinal conditions.
Age-related Macular Degeneration (AMD)
FAF is used to assess the transition to advanced stages of AMD, particularly Geographic Atrophy (GA). GA appears on an FAF map as sharply defined patches of absolute hypo-autofluorescence, confirming the irreversible loss of RPE cells. The patterns of hyper-autofluorescence immediately surrounding these patches are especially important because they indicate stressed RPE cells at the edge of the atrophy. These patterns have been shown to predict which areas are most likely to progress and enlarge the atrophic area. Monitoring the expansion rate of the hypo-autofluorescent patch is a standardized method for tracking disease progression in clinical trials and practice.
Stargardt Disease
For inherited retinal dystrophies, such as Stargardt disease, FAF provides a characteristic diagnostic signature. This genetic condition causes the toxic accumulation of lipofuscin within the RPE cells due to a defect in waste processing. FAF images show a mottled pattern of speckled hyper-autofluorescence corresponding to these deposits. These areas eventually transition into patches of hypo-autofluorescence as the RPE cells die. The extent of decreased autofluorescence (DDAF) is used to classify the severity and predict the long-term visual prognosis.
Central Serous Chorioretinopathy (CSC)
FAF is also useful in Central Serous Chorioretinopathy (CSC), characterized by fluid accumulation under the retina. In acute CSC, the area of retinal detachment may show minimal FAF change or a slightly decreased signal due to the fluid blocking the light. Chronic CSC reveals more pronounced patterns reflecting RPE damage from prolonged fluid exposure. These chronic changes include irregular areas of hyper-autofluorescence and characteristic “descending tracts” of RPE damage. These tracts appear as streaks of mixed hypo- and hyper-autofluorescence extending downward from the detachment site. FAF helps distinguish between acute and chronic disease, guiding treatment choices.