Fundus Autofluorescence: An Emerging Window on the Retina
Fundus autofluorescence patterns extend your retinal diagnostic reach.
ROBERT MURPHY, CONTRIBUTING EDITOR
Fundus autofluorescence (FAF) has emerged in the past 10 years as an effective means of identifying lipofuscin distribution in the retinal pigment epithelium cell monolayer plus other fluorophores associated with disease in the outer retina and subneurosensory space.1
What makes FAF clinically effective is that excessive accumulation of lipofuscin granules within the RPE represents a common downstream pathological pathway in various hereditary and complex retinal diseases, notably AMD.2
FAF imaging serves multiple clinical and research functions: understanding pathophysiologic mechanisms; diagnosis; phenotype-genotype correlation; identifying predictive markers for disease progression; and monitoring new therapies.2 What’s more, FAF imaging provides information beyond what you can obtain from other technologies such as fundus photography, FA, and OCT,2 not to mention that it’s simple, efficient, and noninvasive.
Fundus autofluorescence has much to offer the retina specialist. It’s worth taking a moment to review how it works and its clinical applications.
HOW FAF WORKS
FAF depends on the use of ultraviolet light to visualize lipofuscin with fluorescence microscopy. One difficulty in detecting FAF is that its intensity is about two orders of magnitude lower than the background of a fluorescein angiogram.2 Autofluorescent properties of structures anterior to the retina further confound the clinical picture.2 This means existing camera systems as well as new imaging devices must be adjusted to record FAF.
Confocal scanning laser ophthalmoscopy (cSLO) and fundus photography are the two chief clinical means of capturing the FAF signal.
|Robert Murphy is freelance medical journalist in Philadelphia.|
Figure 1. In normal cSLO blue-light FAF imaging, the optic nerve head is dark (black) due to the absence of RPE (and hence no lipofuscin). The blood vessels also appear dark due to absorption of the light by blood.
COURTESY: SRINIVAS R. SADDA, MD
SCANNING LASER OPHTHALMOSCOPY
Confocal SLO addresses the limitations of the low-intensity signal of FAF and the interference of the crystalline lens.3 The confocal optics ensure the reflectance and fluorescence come from the same optical plane.2 Light originating in the light beam, but out of the focal plane, is greatly suppressed, which in turn reduces autofluorescence from structures anterior to the retina.2 Confocal SLO allows FAF imaging over large retinal areas, capturing high-contrast images of the posterior segment (Figure 1).
Figure 2. Fundus autofluorescence cSLO imaging demonstrates geographic atrophy in dry AMD.
COURTESY: TIM U. KROHNE, MD, FEBO
The fundus camera differs from a cSLO in that the former uses a single flash and captures the entire retinal area in a single frame. The fundus camera does not have confocal optics, which means the signal it detects comes from all tissue levels with fluorescent properties within that light beam. Not only that, light scattering both anterior and posterior to the plane of interest can disrupt the detected signal.2 These are serious drawbacks.
What’s more, the fundus camera lens itself contributes significantly to the fluorescence signal.2 This is particularly the case with older patients who have yellowing of the lens or nuclear opacities. One way around this problem is to modify the fundus camera by moving the excitation and emission wavelengths toward the red end of the spectrum.4-6 Longer wavelengths are thought to exhibit much less contribution from nuclear sclerosis and macular pigment, compared with shorter wavelengths.2
Because cSLO and fundus photography use different wavelengths, it stands to reason that the two techniques might record fluorescence from a different set of fluorophores.2 An example is that macular pigment absorption is observed at a much lesser extent — and the signal is less decreased over blood, retinal vessels, and the optic nerve head — using the fundus camera system compared with the cSLO.2 A head-to-head comparison of the two systems is lacking.
HOW TO INTERPRET FAF
The distribution of fundus autofluorescence in normal eyes presents a consistent pattern as young as four years.3,7 The optic nerve head appears dark in the absence of the RPE and its accompanying lipofuscin.2 Blood-based absorption markedly reduces the retinal vessels’ FAF signal.2
The FAF signal is reduced at the fovea owing to absorption by luteal pigments such as lutein and zeaxanthin in the neurosensory retina.2 In the parafoveal area, the signal tends to be higher even as its intensity is decreased relative to peripheral retinal areas. This observation has been ascribed to increased melanin deposits and lower density of lipofuscin in central REP cells.8,9
It stands to reason that any FAF image deviating from the norm should alert you to seek its cause, according to the literature. Essentially, an abnormal FAF signal derives either from:
● change in the amount of composition of fluorophores in the RPE cell cytoplasm — for example, lipofuscin; or
● the presence of absorbing or autofluorescent material anterior to the RPE.2
Some clinicians have recommended the best way to interpret a patient’s FAF image is to correlate them with other imaging findings such as OCT, and FA.2
FAF is a useful diagnostic indicator at various stages of AMD. Drusen visible on fundus photography in early AMD do not necessarily correlate with FAF changes. By the same token, notable FAF changes don’t necessarily signal hyperpigmentation or drusen.10 However, larger drusen overall are associated more frequently with significant FAF abnormalities.11
An expert group classified the spectrum of FAF findings in patients with early AMD.12 Lacking a strong correlation between apparent changes on fundus photography and significant FAF changes, they speculated that FAF changes in early AMD may indicate more widespread abnormalities and diseased areas.11
Geographic Atrophy in AMD
Early manifestations of AMD include focal hypo- and hyperpigmentation at the RPE level along with drusen and extracellular material gathering in Bruch’s membrane (Figure 2).13,14 FAF imaging is said to be particularly useful in detecting geographic atrophy secondary to AMD.11 An absence of RPE cells and emergence of lipofuscin fluorophores mean that atrophic areas in these patients display a markedly reduced FAF signal.3
Figure 3. Cone and cone-rod dystrophies demonstrate striking autofluorescent patterns with a penumbra of hyperfluorescence surrounding central hypofluorescence.
COURTESY: SRINIVAS R. SADDA, MD
Choroidal Neovascularization in AMD
FAF imaging may lend important clues in choroidal neovascularization associated with AMD.11 In this capacity, FAF imaging may help assess the RPE’s integrity. Patients with early CNV secondary to AMD typically exhibit patches of normal autofluorescence, suggesting that the RPE remains viable at least initially in CNV development.15 On the other hand, eyes with long-standing CNV usually display more area of decreased signal, perhaps signaling photoreceptor loss and scar formation with increased melanin deposits.11
Macular and Diffuse Retinal Dystrophies
FAF abnormalities have been associated with macular and diffuse retinal dystrophies.16 The FAF signal is reduced in areas of atrophy thanks to the loss of the RPE, and in turn, a lack of lipofuscin. In contrast, excessive accumulation of lipofuscin in RPE cells may result from abnormally high turnover of photoreceptor outer segments of impaired RPE lysosomal degradation of molecular substrates.11
Reduced macular pigment density in macular telangiectasia produces an abnormally yet variably increased signal in the macular area with blue-light FAF imaging.17 Loss of luteal pigment may occur initially in the area temporal to the foveal center.17, 18
Depending on disease stage, central serous chorioretinopathy FAF findings in central serous chorioretinopathy accord with RPE involvement.19 Acute leaks imaged within 30 days have shown minimal abnormalities other than a slight increase in autofluorescence of the serous detachment.11 Over time, areas of detachment have shown increasing levels of irregular increased autofluorescence.11
FAF’S CLINICAL CONTRIBUTION
Fundus autofluorescence imaging allows topographic mapping of lipofuscin distribution in the retinal pigment epithelium cell monolayer as well as other fluorophores that may accompany disease in the outer retina and subneurosensory space.2 FAF imaging yields diagnostic data otherwise unobtainable through fundus photography, FA, and OCT, and therein lies its special clinical value. RP
1. Holz FG, Schmitz-Valckenberg S, Spaide RF, Bird AC. Atlas Of Fundus Autofluorescence Imaging. New York, NY; Springer; 2007.
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11. Holz FG. When should I be using fundus autofluorescence? Paper presented at: American Academy of Ophthalmology; New Orleans, LA; Nov. 15, 2012.
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16. von Rückman A, Fiztke FW, Schmitz-Valckenberg S, Webster A, Bird A. Macular and retinal dystrophies. In: Holz FG, Schmitz-Valckenberg S, Spaide RF, Bird AC, eds. Atlas of Autofluorescence Imaging. New York, NY; Springer; 2007.
17. Helb HM, Charbel Issa P, RL, VDV, Berendschot TT, Scholl HP, et al. Abnormal macular pigment distribution in type 2 idiopathic macular telangiectasia. Retina. 2008;28:808-816.
18. Charbel Issa P, van der Veen RL, Stijfs A, et al. Quantification of reduced macular pigment optical density in the central retina in macular telangiectasia type 2. Exp Eye Res. 2009;89:25-31.
19. Spaide RF, Klancnik JM Jr. Fundus autofluorescence and central serous chorioretinopathy. Ophthalmology. 2005;112:825-833.