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BLOG: Why should I care about autofluorescence?
If you were at the American Academy of Optometry meeting last month in New Orleans, you’ll no doubt have heard a lot of talks mentioning fundus autofluorescence and the importance of adapting into it practice.
Imagine that looking at a fundus autofluorescence (FAF) image is like looking at a picture painted only with a certain wavelength (around 630 nm) of paint. If the image is black in one area it doesn’t mean there’s nothing there; it means that there’s nothing there that emits light in that certain wavelength. And the main thing that emits light at that wavelength in the fundus is lipofuscin (LF). FAF is essentially a picture of where LF is in the fundus.
LF is important because you can use it to determine if the retinal pigment epithelium (RPE) is healthy, damaged or absent. Sometimes this is obvious on fundus exam, but many times it can be very subtle, and other times it can be completely hidden.
The RPE is responsible for phagocytosis and lysosomal digestion of the outer segments of photoreceptors, which allows photoreceptors to reset their excitability. The miracle of vision is hard work on the RPE: over the course of a lifetime, it’s estimated each RPE cell will phagocytose 3 billion outer segments. As we age, the RPE cells start to incompletely breakdown these outer segments, which causes LF to build up. Certain components of LF like A2E and all-trans retinal are toxic to RPE and retinal cells and can cause apoptosis of these cells past a certain level. An FAF is, at its simplest, a measure of how much lipofuscin is in the patient’s RPE.
So, an older person should have a brighter FAF than a younger person, simply because there is more LF built up in his or her RPE by that age. But there are conditions that cause a change in the FAF signal.
The patient’s right eye in 2010.
Reduction of RPE LF signal: RPE atrophy (obviously because there is no RPE in which to store the LF), intraretinal fluid (which scatters the light signal), hemorrhages, scars, blood vessels, luteal pigment and RPE hypertrophy all block the LF glow.
Increase of RPE LF signal: Lysosomal storage disorders (such as Stargardt’s disease, Best’s disease, pattern and vitelliform dystrophies that have excess LF accumulation), age-related macular degeneration (the edge of geographic atrophy glows, known as the junctional zone), drusen (if they are below the RPE [drusen can block or glow depending on the layer and the type]), choroidal nevi, melanoma and optic disc drusen (often contain LF) can all increase the signal.
FAF of the right eye in 2010.
FAF of the right eye in 2015.
The most obvious application for FAF is to document how a patient with nonexudative AMD progresses. Sometimes RPE atrophy is very difficult to detect on funduscopy. Keep in mind that we label RPE atrophy as “geographic” if it is bigger than 175 microns in diameter (about 1/8 disc diameter), and if there is any geographic atrophy present at all, then the Age-Related Eye Disease Study recommends the patient be started on anti-oxidant supplementation. So, it’s important to look for the presence of GA, and FAF is a nice way to do it.
For instance, take a look at the figure showing the color photo of a patient’s right eye. Most of us can detect the geographic atrophy in the fovea extending temporally. But take a look at the next figure showing the FAF of the right eye in 2010. Did you notice that smaller spot of geographic atrophy just nasal to the fovea? And look at how that spot progresses over the years, nicely shown in the last figure.
A more in-depth article on fundus autofluorescence will appear in a future issue of Primary Care Optometry News. I think it’s important to do FAF and especially worth the small extra effort if you already have it built into your OCT device. Like most things, the more you do it, the more you will discover.