Page 253 - Computational Retinal Image Analysis

2 Histopathological lesions and clinical classification 251

SD-OCT images show the absence of the ONL, ELM, and the ellipsoid layer, and
either absence or marked attenuation of the RPE and Bruch’s membrane complex.
Due to the absence of these layers there is enhanced choroidal reflectivity [33]. On
fluorescein angiography, GA appears as well demarcated areas of hyperfluorescence
that appear during the transit phase of the angiogram, sometimes with visualization
of the choroidal vessels. A rim of blocked fluorescence from pigment may surround
the edge. In late-phase, the hyperfluorescence decreases in intensity; some staining
may persist.

2.3 Lesions of neovascular AMD

Neovascular AMD refers to the proliferation of new vessels either under the RPE,
breaking through the RPE, or within the retina layers. Exudation of fluid, lipids and
blood leads to the death of photoreceptors and subsequently fibrosis. Untreated pro-
gression may be rapid resulting in permanent legal blindness. Choroidal neovascular
membrane (CNV) may appear as a gray-green elevation of tissue deep to the retina
with overlying detachment of the neurosensory retina. Other signs visible on biomi-
croscopy and color photography are lipid exudates and blood (which may be subreti-
nal, intraretinal or pre-retinal). CNV can be classified by anatomical location. Type 1
membranes are beneath the RPE (usually proliferating within the inner aspect of an
abnormally thickened Bruch’s membrane). Type 2 membranes are located between
the RPE and the photoreceptors. Retinal angiomatous proliferation (RAP or type 3
membranes) are characterized by intraretinal neovascularization. RPE tears (rip or
dehiscence) are a recognized complication associated with CNV, particularly when
fibrovascular PED. Tears occur at the junction of attached and detached RPE. The
free edge of the RPE retracts and rolls toward the fibrovascular tissue.
CNV may also be classified on the basis of stereoscopic fluorescein angiography
(FA) imaging (see Fig. 5). FA can determine the pattern of fluorescence (classic or
occult and the proportions of each), boundaries (well defined or poorly defined) and
location of the neovascular lesions with respect to the center of the foveal avascular
zone (FAZ). The fluorescein angiographic appearance of classic CNV is an area of
hyperfluorescence identified in the early phase of the angiogram that progressively
intensifies throughout the transit phase, with leakage of dye obscuring the boundaries
of this area by the late phases of the angiogram [34]. Two types of occult CNV are
described. Fibrovascular PED is characterized by irregular elevation (best seen with
stereoscopic view) of the RPE with stippled or granular irregular fluorescence first seen
early in the angiogram, usually by 1–2 min (see Fig. 6). There is progressive leakage
from these regions, with a stippled hyperfluorescent pattern. Late leakage of undeter-
mined origin is characterized by regions of fluorescence at the level of the RPE that are
best appreciated in the late phases of an angiogram; they do not correspond to classic
CNV or to areas of irregular elevation of the RPE during the early or mid-phases of the
angiogram [34]. Angiographic features of RAP lesions include a retinal vessel directed
into the lesion and retinal-retinal anastomosis. ICG imaging is essential for identify-
ing polypoidal choroidopathy. Video ICG may demonstrate pulsatile choroidal polyps.