AMD updated - page 81

81
Fundus autofluorescence in
age-related macular degeneration 8
1.1 Definitions
Fluorescence is the capability of absorbing light at a spe­
cific wavelength and releasing it at a longer, less energetic
wavelength. This phenomenon raises an especial interest
when the released radiation is found within the spectrum
of visible light permitting its visualization, recording and
measurement.
Autofluorescence is the spontaneous fluorescence that
some substances present naturally.
Fluorophore is the part of a molecule that makes it
fluo­resce.
The human eye contains autofluorescent substances
in the retina, especially within the retinal pigmentary
epi­thelium (RPE). The main autofluorescent compo-
nent of the RPE is lipofuscin (LF), containing at least
ten dif­ferent fluorophores presenting discrete emission
spectra within the green, golden-yellow, yellow-green,
and or­ange-red emitting range(1). When LF granules are
stimu­lated with light within the blue range, a character-
istic yellow fluorescence is emitted(2).
1.2 Basic considerations on fundus autofluorescence
The RPE plays an important role in the physiopathol­
ogy of age-related macular degeneration (AMD)(3, 4). The
study of RPE can help to achieve a better understand­
ing of AMD and to find new ways of early diagnosis as
well as new prognostic and progression markers for this
condition.
LF originates from the constant phagocytosis of the shed
outer segment disks of the photoreceptors and is accu­
mulated in the cytoplasm of RPE cells(5,6). This accumu­
lation is considered to be a hallmark of RPE aging(7).
Re­cent experimental studies have addressed the molecu-
lar mechanisms of the interaction of excessive LF with
the normal cellular functions of RPE(8,9). According to
these studies, A2-E (N-retinylidene-N-retinylethanol-
amine) has been identified as the main fluorophore of
LF. A2-E may play a toxic effect including phototoxic
and deter­gent actions, as well as an inhibitory effect on
lysosomal function(10,11). It has been suggested that the
photooxi­dation by products related to LF may trigger
the com­plement cascade, thus contributing to the path-
ological chronic inflammation of the macular area(12).
New stud­ies have been designed to improve our under-
standing of potential underlying molecular mechanisms.
The autofluorescence of LF, its distribution in post-
mitotic human RPE cells and its accumulation with age
have been extensively studied in post-mortem eyes with
fluorescence microscopy(14,15). In the past few years we
have started to study the autofluorescence in vivo.
Ultraviolet light is frequently used to visualize LF by flu­
orescence microscopy ex vivo, since the absorption prop­
erties of the eye limit the transmission of ultraviolet light
within the retina in human living eyes. However, due
to the wide range of excitation of LF (from 300 nm to
600 nm), visible light can be used to visualize its fluores-
cence in vivo. The emitted spectrum ranges from 480 to
800 nm and peaks within the range of 600 to 640 nm(15).
Fundus autofluorescence (FAF) imaging of the human
living eye is a relatively new imaging method that pro­
vides a topographic map of the distribution of LF in the
RPE.
1. Introduction
Authors: Jose M Ruiz-Moreno, MD, PhD1,2
Javier A Montero, MD, PhD2,3
Virginia Bautista Ruescas, MD1
1Department of Ophthalmology, Vissum Alicante & CHUA. Spain.
2Alicante Institute of Ophthalmology, VISSUM, Vitreo-Retina Unit. Alicante. Spain.
3 Pio del Rio Hortega Hospital, University of Valladolid. Valladolid. Spain.