Fundus Autofluorescence (FAF)

Definition

Principle / Mechanism

• Principal fluorophore: Lipofuscin, contained within cytoplasmic granules of RPE cells
• Lipofuscin is a complex mixture of lipids, oxidized proteins, and trace metals that accumulates as a byproduct of photoreceptor outer segment phagocytosis by the RPE
• The active fluorescent component of lipofuscin is A2E (N-retinylidene-N-retinylethanolamine), a bis-retinoid formed from the visual cycle
• When excited by blue light (~488 nm) or green light (~532 nm), lipofuscin emits light at ~520–700 nm (detected as autofluorescence)
• Other minor fluorophores include melanin, collagen, elastin, and porphyrins in the choroid and Bruch's membrane

"Autofluorescence imaging records the light emitted by native fluorophores in the retina after excitation by light of a blue or green wavelength. The principal naturally occurring fluorophore is lipofuscin, contained within cytoplasmic granules of retinal pigment epithelial cells." — Bradley and Daroff's Neurology in Clinical Practice

"FAF provides information on RPE function and the integrity of the chorioretinal interface." — Kanski's Clinical Ophthalmology, 10th edition

Imaging Modalities Used

1. Modified fundus camera (with appropriate excitation/barrier filters)
2. Scanning Laser Ophthalmoscope (SLO) — the gold standard; uses confocal optics to reduce background noise and improve contrast
3. SLO-OCT combination machines
4. Wide-field SLO — captures up to ~80% of the retina in a single image

Normal FAF Appearance

A – Normal FAF. B – Geographic atrophy (central hypo-AF). C – Optic disc drusen (hyper-AF foci on optic nerve). (Bradley & Daroff's Neurology)

• Low-intensity background autofluorescence throughout the fundus
• Fovea appears as an area of reduced signal — due to absorption of blue excitation light by macular xanthophyll (luteal) pigment (lutein and zeaxanthin)
• Optic disc and retinal blood vessels appear very dark — no fluorophores present
• RPE melanin causes slight background signal variation

Patterns of Abnormal FAF

1. Hyperautofluorescence (Increased Signal)

• RPE dysfunction → failure to degrade lipofuscin → accumulation → increased signal
• Seen at the leading edges of geographic atrophy (AMD) — reflects dying but not yet dead RPE cells
• Inherited retinal degenerations (Stargardt disease, Best's vitelliform dystrophy, retinitis pigmentosa)
• Inflammatory conditions: MEWDS, serpiginous choroidopathy, AZOOR
• Choroidal melanoma / naevus — patchy increased signal from orange pigment (lipofuscin in overlying RPE)
• Chloroquine/hydroxychloroquine toxicity — bull's eye pattern

2. Hypoautofluorescence (Decreased Signal)

• Geographic atrophy (AMD) — complete RPE loss → well-demarcated dark zone
• RPE atrophy (any cause)
• Subretinal fibrosis
• Retinal blood vessels — normal dark areas
• Dense sub-RPE deposits blocking signal

"Hyperautofluorescence: Many retinal diseases result in RPE dysfunction and an accumulation of RPE lipofuscin… Hypoautofluorescence indicates areas of decreased lipofuscin, decreased RPE density or blockage of fluorescence (e.g. RPE atrophy, fibrosis)." — Kanski's Clinical Ophthalmology, 10th edition

Clinical Applications

"It is particularly useful in the diagnosis and follow-up of patients with inherited retinal degeneration… It can demonstrate more extensive macular disease than is visible clinically (e.g. MEWDS)… In patients with geographic atrophy, FAF shows distinct areas of hyperautofluorescence at the leading edges of lesions that seems to precede retinal demise." — Kanski's Clinical Ophthalmology, 10th edition

"Autofluorescence imaging is also useful for identifying optic disc drusen at or slightly below the surface of the optic nerve head." — Bradley and Daroff's Neurology in Clinical Practice

"After taking a careful history… the main investigations that are usually needed to make a diagnosis of inherited retinal degeneration are OCT and fundus autofluorescence (FAF)." — Kanski's Clinical Ophthalmology, 10th edition

Advantages

• Non-invasive — no dye injection required
• No systemic side effects (unlike fluorescein angiography)
• Provides functional information about RPE health
• Can detect subtle disease before clinical visibility
• Useful for monitoring disease progression over time
• Can be combined with wide-field imaging to assess the peripheral retina

Limitations

• Media opacities (cataract, vitreous haemorrhage) cause artifact and reduce image quality
• Low signal-to-noise ratio in some patients
• Cannot assess subretinal fluid or choroidal circulation (unlike FA/ICGA)
• Requires pupil dilation with conventional fundus cameras (not always with SLO)

"Limitations of FAF include a susceptibility to artifact from media opacities." — Kanski's Clinical Ophthalmology, 10th edition

Note on Sources

"Fundus autofluorescence has become particularly helpful in the diagnosis and management of retinal diseases and the clinical use is likely to expand in the future." — Kanski's Clinical Ophthalmology, 10th edition

Fundus Autofluorescence (FAF)

Definition

"FAF is a non-invasive technique that detects lipofuscin and melanin in retinal pigment epithelium (RPE) cells. This provides information on RPE function and the integrity of the chorioretinal interface." — Kanski's Clinical Ophthalmology, 10th ed.

"Imaging modality that takes advantage of the naturally and pathologically occurring fluorophores in the fundus. Provides sensitive information regarding the health of the retinal pigment epithelium (RPE) and allows early detection and monitoring of a variety of conditions." — Wills Eye Manual

Physical Basis — What is Autofluorescence?

• Excitation: Blue (~488 nm) or green (~532 nm) light
• Emission: ~520–700 nm (detected as the autofluorescence signal)

"Autofluorescence imaging records the light emitted by native fluorophores in the retina after excitation by light of a blue or green wavelength." — Bradley & Daroff's Neurology in Clinical Practice (Ryan's Retina chapter)

The Principal Fluorophore: Lipofuscin

• Lipofuscin is a complex mixture of lipids, oxidized proteins, and trace metals — the indigestible byproduct of photoreceptor outer segment phagocytosis by the RPE
• Its active fluorescent component is A2E (N-retinylidene-N-retinylethanolamine), a bis-retinoid of the visual cycle
• As the RPE ages or becomes dysfunctional, lipofuscin accumulates progressively

"The principal naturally occurring fluorophore is lipofuscin, contained within cytoplasmic granules of retinal pigment epithelial cells." — Bradley & Daroff's Neurology in Clinical Practice

"Lipofuscin is a fluorescent pigment that accumulates in retinal pigment epithelial cells following photoreceptor degradation." — Bradley & Daroff's Neurology in Clinical Practice

Imaging Equipment

"Images may be captured by a properly equipped fundus camera, an SLO-OCT machine, or a wide-field SLO." — Bradley & Daroff's Neurology in Clinical Practice

"Imaging of FAF using an enhanced fundus camera or scanning laser ophthalmoscopy permits visualization of accumulated lipofuscin in the retinal pigment epithelium." — Kanski's Clinical Ophthalmology, 10th ed.

Normal FAF Appearance

Fig: A – Normal FAF with low background signal. B – Geographic atrophy in AMD appearing as well-demarcated hypo-AF zone. C – Optic disc drusen appearing as hyper-AF foci. (Bradley & Daroff's Neurology)

• Background: Low-intensity, uniform autofluorescence across the posterior pole
• Fovea: Area of reduced signal — macular xanthophyll pigment (lutein + zeaxanthin) absorbs blue excitation light
• Optic disc and retinal vessels: Appear very dark — contain no fluorophores
• Choroid: Largely masked by overlying RPE

Abnormal FAF Patterns

1. Hyperautofluorescence (Increased signal)

• RPE dysfunction → impaired lysosomal degradation → lipofuscin build-up
• Stressed but viable RPE at the margin of advancing atrophy

• Geographic atrophy (AMD) — hyper-AF ring at leading edge precedes RPE death
• Inherited retinal degenerations (Stargardt, Best's, retinitis pigmentosa)
• Inflammatory conditions: MEWDS, serpiginous choroidopathy, AZOOR
• Choroidal naevus / melanoma (from overlying RPE lipofuscin)
• Central serous chorioretinopathy

"Many retinal diseases result in RPE dysfunction and an accumulation of RPE lipofuscin, which causes abnormal patterns of hyperautofluorescence on FAF imaging. It is particularly useful in the diagnosis and follow-up of patients with inherited retinal degeneration. In patients with geographic atrophy, FAF shows distinct areas of hyperautofluorescence at the leading edges of lesions that seems to precede retinal demise." — Kanski's Clinical Ophthalmology, 10th ed.

"Pronounced autofluorescence can be detected in patients with MEWDS, serpiginous choroidopathy, and AZOOR." — Kanski's Clinical Ophthalmology, 10th ed.

2. Hypoautofluorescence (Decreased signal)

• Complete RPE atrophy (geographic atrophy) — well-demarcated dark zone
• Subretinal fibrosis
• Dense sub-RPE deposits blocking signal
• Retinal vessels (normal finding)

"Hypoautofluorescence indicates areas of decreased lipofuscin, decreased RPE density or blockage of fluorescence (e.g. RPE atrophy, fibrosis)." — Kanski's Clinical Ophthalmology, 10th ed.

Clinical Applications

"After taking a careful history and undertaking a clinical examination, the main investigations that are usually needed to make a diagnosis of inherited retinal degeneration are OCT and fundus autofluorescence (FAF)." — Kanski's Clinical Ophthalmology, 10th ed.

"FAF shows an abnormal perimacular ring of hyperautofluorescence (increased RPE lipofuscin secondary to RPE dysfunction) and diffuse, patchy hypoautofluorescence in the mid-periphery. Using FAF as a stand-alone test, RP can be distinguished from a normal fundus in 95% of cases." — Kanski's Clinical Ophthalmology, 10th ed. (on Retinitis Pigmentosa)

"FAF is useful in the evaluation of certain ocular tumors, specifically, choroidal nevi and melanomas." — Wills Eye Manual

"Autofluorescence imaging is also useful for identifying optic disc drusen at or slightly below the surface of the optic nerve head." — Bradley & Daroff's Neurology in Clinical Practice

Advantages

• No dye injection — no risk of anaphylaxis or systemic side effects
• Non-invasive and repeatable
• Detects subtle RPE dysfunction before visible clinical changes
• Provides functional mapping of RPE health across the entire posterior pole
• Useful for monitoring disease progression (e.g., rate of geographic atrophy growth)
• Can be combined with wide-field imaging for peripheral assessment

Limitations

• Media opacities (cataract, vitreous haze) cause artifacts and signal degradation
• Low signal-to-noise ratio — requires image averaging
• Does not image choroidal circulation or subretinal fluid dynamics (unlike FA/ICGA)
• Cannot distinguish between different fluorophores causing similar signals

"Limitations of FAF include a susceptibility to artifact from media opacities." — Kanski's Clinical Ophthalmology, 10th ed.

Summary

"Fundus autofluorescence has become particularly helpful in the diagnosis and management of retinal diseases and the clinical use is likely to expand in the future." — Kanski's Clinical Ophthalmology, 10th ed.

Sources Available in Library