Chapter 65 Age-Related Macular Degeneration
Non-neovascular Early AMD, Intermediate AMD, and Geographic Atrophy
Introduction
Age-related macular degeneration (AMD) has been the leading cause of legal blindness in patients aged 65 or over,1 and it has been the most common overall cause of blindness in the western world. Using data from the 2000 census, it has been estimated that in the USA more than 8 million people have specific AMD features that put them at risk for progression to advanced AMD and vision loss.2,3 Over a 5-year period about 1.3 million of these individuals are predicted to develop the advanced forms of AMD, namely neovascular AMD or foveal geographic atrophy (GA).2 In addition, hundreds of thousands of people aged 75 and over are anticipated to join the pool of people at increased risk of developing advanced AMD over subsequent 5-year periods.4 The prevalence of AMD continues to rise as a result of the increasing percentage of elderly persons and the improved management of other eye diseases.3 Prevalence of AMD has also increased steadily in the UK, accounting for approximately 50% of registered blindness in England and Wales that cannot be explained by the increasing age of the population alone.5 In addition, macular degeneration is the commonest reason that patients with lesser handicaps attend low-vision clinics.
Within the past 5 years new and improved treatments for the neovascular form of advanced AMD have been adopted throughout the developed world. Therefore, in the years ahead the overall burden of vision loss associated with AMD is anticipated to decline substantially.6 The same progress has not been made for treatment of advanced non-neovascular AMD (GA); as such, the importance of atrophic AMD as a leading cause of vision impairment is expected to increase.
The advanced forms of AMD are those that are frequently associated with visual acuity loss and they are divided into non-neovascular atrophic (dry) type and neovascular (wet) type. In atrophic AMD, gradual disappearance of the retinal pigment epithelium (RPE) results in one or more patches of atrophy that slowly enlarge and coalesce. Affected areas have no visual function, since loss of the RPE is associated with fallout of photoreceptors. Gass7 applied the term “geographic atrophy of the retinal pigment epithelium” to this presentation, which is the natural end-result of AMD in the absence of clinical evidence of choroidal neovascularization (CNV). This chapter is devoted to the clinical and pathologic features that may lead to this development, as well as their management.
Senile macular degeneration was first reported as a clinical entity in 1885 by Otto Haab,8 who described a variety of pigmentary and atrophic changes in the macular region, causing progressive impairment of central vision in patients over the age of 50. Subsequent observers referred to the different fundus manifestations of the disease as separate entities, resulting in a variety of descriptive eponyms. A review of dominantly inherited drusen9 found, however, that only Doyne’s honeycomb familial choroiditis and malattia levantinese were disorders that could be distinguished from each other by clinical criteria, and these entities are considered to be a separate category. A major step toward a better understanding of the disease was taken when Gass10 clarified that drusen, senile macular degeneration, and senile disciform macular degeneration represented a single disease.
In the 1990s it had been proposed that the features should be termed either early or late age-related maculopathy (ARM),11,12 to suggest that early ARM was not necessarily a pathologic state, with the term age-related macular degeneration (AMD) being reserved for late ARM and encompassing geographic atrophy and neovascular AMD. Since many epidemiologic studies are based on the International Epidemiological Age-related Maculopathy Study Group11 description, it is described here. However, more recent descriptions of AMD from the Age-Related Eye Disease Study Group13 have provided longitudinal information to understand features associated with an increased risk of developing advanced forms of AMD and are used in the description of the clinical management of AMD that follows.
In the International Epidemiological Age-related Maculopathy Study Group11 definitions used in many epidemiologic studies, early ARM was defined as a degenerative disorder in individuals ≥50 years of age, characterized by the presence of any of the following lesions:
• Soft drusen (intermediate >63 µm, ≤125 µm; large >125 µm) drusen. When occurring alone, soft, indistinct drusen are considered more likely to indicate AMD than soft, distinct drusen,4,14,15 and drusen over 125 µm have greater importance than smaller drusen.16,17
• Areas of hyperpigmentation associated with drusen but excluding pigment surrounding hard drusen.
• Areas of depigmentation or hypopigmentation associated with drusen. These areas, which commonly occur as drusen fade, are most often more sharply demarcated than drusen, but do not permit exposure of the underlying choroidal vessels.
• Visual acuity is not used to define ARM or AMD because advanced changes may be present without anatomically affecting the fovea.
This definition of early ARM excluded small, hard drusen alone, pigment changes alone, and even pigment changes surrounding small, hard drusen for two reasons: (1) hard drusen become an almost constant finding in the fifth decade; and (2) a number of diverse processes can cause pigment abnormalities that may not be possible to distinguish from early ARM, so the inclusion of soft drusen makes the definition more specific to ARM and AMD.14 However, eyes with numerous small, hard drusen or eyes with pigment abnormalities in the absence of obvious drusen can also progress to soft drusen.
Grading systems have been devised to permit comparison of severity over time in the size, number, and extent of drusen.11,18–20 Drusen extent is measured by mentally sweeping all drusen present in the area of interest into a condensed zone and estimating the area of that zone. Grids and standard circles prepared on transparent sheets are overlaid to one film image of a pair of stereoscopic color fundus transparencies to assist in the grading process. Alternatively, a digital version of the same grids and circles is superimposed on a digital fundus image. Because of the 3× magnification of the 30-degree fundus camera, 4.7 mm on the grid corresponds to approximately 1500 µm, the diameter of the optic disc in the average fundus. Figure 65.1 illustrates the standard grid used when applying the Wisconsin Age-Related Maculopathy Grading System.20 This same grid is used in most fundus grading systems of AMD from color photographs. The diameters of the circles within the standard grid are respectively 1000 µm, 3000 µm, and 6000 µm. The central and middle circles combined define the inner macula, which is two disc diameters across. The outer circle defines the macula itself. Figure 65.2 illustrates smaller standard circles, which are used to grade the size and area of specified lesions.11
These grading systems are applied to color images and are intended primarily for epidemiologic studies and clinical trials. However, fluorescein angiography often provides additional insight into the natural history of the disease, as do pathologic studies,21–25 which have demonstrated aging and degeneration to be a continuum based on diffuse morphologic changes at the level of the RPE under the macula, as distinct from focal abnormalities such as drusen. These diffuse changes comprise two types of sub-RPE or basal deposits separated by the RPE basement membrane. On the internal aspect lies a layer of abnormal basement membrane material, referred to as the basal laminar deposit (BLD); on the external aspect of the basement membrane is a layer of membranous debris, referred to as the basal linear deposit.22 This latter deposit may build up into a type of soft drusen specific for AMD. However, although significant diffuse changes correlate with a decline in visual acuity,23 they are difficult to see in the fundus, making a histologic definition of AMD based on basal laminar and basal linear deposits unworkable in a clinical setting.
Aging
The aging eye – clinical findings
The normal aged fundus usually demonstrates loss of the foveal and foveolar reflexes. This may be due to fallout of cells from the inner retinal layers, shallowing of the walls of the foveal pit, and enlargement of the capillary-free zone.26 A few small, hard drusen are almost always present.12,14,16,18
Irregularity of retinal pigmentation gives rise to a fine granularity, and the fundus commonly demonstrates a tigroid background. This senile tigroid fundus (see Fig. 65.3A) is increasingly apparent with advancing age but remains compatible with normal vision. It is unrelated to skin pigmentation and differs from the tigroid fundus in youth in that the choroidal vessels become visible under the macula. There is commonly also a peripapillary halo of atrophy in which the exposed vessels may be sheathed and the intervascular spaces appear pale. Studies using blue-field stimulation27 and scanning laser Doppler flowmetry28 have shown a decrease in blood flow in the retinal macular capillaries of older individuals, and a lower number of perifoveal arterioles and venules have also been reported.29 These findings are consistent with enlargement of the capillary-free zone26 and loss of ganglion cells.30
Many aspects of visual function, not just visual acuity, show a decline with age, including dark adaptation, stereopsis, contrast sensitivity, sensitivity to glare, and visual field tests.31,32 Color perception and foveal cone pigment densities show a decline.33 The limits of normal aging are therefore difficult to define in terms of visual performance.
The aging eye – morphologic changes
The evolution of the aging process is easier to appreciate by studying morphologic changes. The RPE, Bruch’s membrane, and choriocapillaris must function efficiently to serve as the nutritional complex for the photoreceptors. In a normal eye (Fig. 65.4A), the complement of photoreceptors is normal, the RPE forms a regular layer, Bruch’s membrane is not unduly thickened, and the choroid consists of the usual three layers of vessels. Each of these tissues has at one time been regarded as primarily at fault in macular degeneration. Therefore it is first necessary to consider the changes developing in these structures during life (Fig. 65.4B).
Photoreceptors
The cone density at the foveal center does not appear to alter significantly during the first eight decades.30,34,35 A significant loss beyond the ninth decade has been reported, but it is not invariable.36
In the rods the outer segments become convoluted, possibly as an expression of impaired phagocytosis.37 This may lead to the accumulation of outer-segment material at the apical surface of the RPE.36 Fallout of rods can also be demonstrated, with the fastest rate occurring between the second and fourth decades. Cells in the ganglion cell layer show a similar rate of decrease, so the rod and ganglion cell layer densities maintain a constant ratio.30 Rod photoreceptors and cells in the ganglion cell layer therefore appear to be more vulnerable than cones to loss during aging. In fact, this may be the initial subclinical stage of AMD because the spatial population of parafoveal rods decreases by 30% during adulthood, and AMD often commences in a similar parafoveal distribution.34,38
Retinal pigment epithelium
Each pigment epithelial cell must continue to engulf spent photoreceptor discs on a diurnal basis for life, the rods being digested by day and the cones by night,39 and any undigested residual bodies remain as lipofuscin.40 The RPE must also remove material from other retinal pigment cells or photoreceptors that may be eliminated, a burden that increases sharply once degeneration of these tissues commences. Finally, because the RPE is a nondividing tissue, autophagy alone could lead to the accumulation of lipofuscin in the same way that it builds up in the neurons of the central nervous system, which have no photoreceptors to phagocytose. The RPE is therefore particularly vulnerable to cell encumbrance.
Damage to molecules may occur in the photoreceptor outer segments as a result of free radical chain reactions initiated by radiation or oxygen metabolism. After phagocytosis, the lysosomal enzymes may fail to “recognize” these abnormal molecules, with a consequent failure of molecular degradation41 and accumulation of lipofuscin. Free radicals also damage the cells’ own molecules, and there is evidence that enzymatic inactivation occurs, particularly cathepsin D, which is the main lysosomal protease responsible for rod outer-segment digestion.42 There is also an increase of complex granules of melanolysosomes and melanolipofuscin, which are thought to be melanin granules undergoing repair or degradation.
The accumulation of lipofuscin in the RPE, which can be demonstrated as early as the second decade of life,43 reduces the cytoplasmic space. As the cell volume available to the organelles diminishes, the capacity to deal with photoreceptors is reduced. The issue of whether lipofuscin accumulation has significant deleterious effects on the RPE, and consequently on overall retinal function, continues to be of great interest.44,45 Since lipofuscin is the predominant fluorophore responsible for fundus autofluorescence, the in vivo imaging and mapping of retinal autofluorescence using the confocal scanning laser ophthalmoscope46 or fundus spectrophotometer47 may prove helpful in estimating the risk for progression to AMD.
A certain loss of RPE cells occurs with age, particularly in the periphery. For the fovea this decrease in cell density has been estimated to be about 0.3% per year.48 The ratio of photoreceptors to RPE cells remains the same,30,36 the average cone-to-RPE ratio at the center of fovea being approximately 24 : 1. Photoreceptors and RPE cells therefore show a parallel loss during aging. However, the most notable changes in the RPE develop at the base of the cells, where there is loss of basal infoldings and deposition of patches of abnormal basement membrane material (Fig. 65.4B). (This BLD is described under “Onset and progress of age-related macular degeneration”, below.)
Bruch’s membrane
Although anatomists regard Bruch’s membrane as a five-layered structure, pathologic processes are more readily understood if one uses the definition proposed by Gass49 that excludes the basement membranes of the RPE and choriocapillaris. Bruch’s membrane can then be thought of as a sheet-like condensation of the innermost portion of the choroidal stroma that consists of an inner and outer collagenous zone separated by the elastic layer. In this way the location of drusen, RPE detachments, and sub-RPE neovascular membranes can be described more accurately than by using the all-embracing term “within Bruch’s membrane.” Also, thickening of Bruch’s membrane then refers to the collagenous layers alone, which focuses on a possible etiologic role for Bruch’s membrane in AMD, rather than on the actual manifestations of the disease mentioned above.
A linear relationship exists between the thickness of Bruch’s membrane and age, the membrane increasing in thickness from 2 µm in the first decade of life to 4.7 µm by the 10th decade.50 The debris that accumulates within the collagenous and elastic layers, which coincides with the buildup of lipofuscin in the RPE and is similarly first detected early in life on electron microscopy, takes three main forms:
1. A general increase in collagen. The 64 nm banded fibers found in increasing numbers in the collagenous layers with age are believed to be fibrillar type I collagen.51 Clumps of fibrous long-spacing collagen with band periodicity of about 120 nm are found primarily in the outer collagenous layer or embedded in the basement membrane of the choriocapillaris.52,53 Fibrous long-spacing collagen is thought to be a combination of collagen and proteoglycans or glycoprotein and may be formed by depolymerization of native collagen fibrils.52 Other components that have been identified include collagen types III, IV, and V, fibronectin, chondroitin sulfate, dermatan sulfate, and proteoglycans.51,54 A significant linear decline in solubility of Bruch’s membrane collagen occurs with age and may be due to increase in crosslinking.51
2. Rounded, coated membrane-bound bodies (Fig. 65.4B, Fig. 65.5). Since these are found as early as the second decade,55 it has been suggested that this material may result from the shedding of unwanted basal cytoplasm through the basement membrane of the RPE.56 The actual separation of the bodies from the cells appears to have been demonstrated,57 but it is such a rare finding that their derivation remains uncertain. These membrane-bound bodies then rupture, spilling their content of coated vesicles and granular material into Bruch’s membrane and, together with fragments of the coated membrane wall, the resulting debris accounts for most of the thickening of Bruch’s membrane with age.58 However, most of the debris is found in the outer collagenous zone and even on the outer side of the choroidal capillaries, suggesting that it may also be derived from the choroid.53
3. Mineralized deposits, which primarily affect the elastic lamina. The degeneration of elastin may be initiated by actinic damage.24 The corresponding histologic findings in Bruch’s membrane, which become evident in the fifth decade, comprise thickening, hyalinization, and patchy basophilia.23,25 This diffuse deposition in the collagenous zones also extends down the intercapillary pillars and can be correlated with an increase in the lipid content of Bruch’s membrane after the fourth decade.59–61 The lipids consist largely of phospholipids, triglycerides, fatty acids, and free cholesterol. There is little cholesterol ester, which would have been expected to predominate if the lipids had been derived from the bloodstream, suggesting that the source of the material is the RPE.59 However, the specific inclusions seen with electron microscopy cannot be correlated with any particular type of lipid.62
Peroxidized lipids have been identified in Bruch’s membrane, the total amount increasing exponentially with age. The peroxidized lipids identified were derived from long-chain polyunsaturated fatty acids, particularly docosahexanoic acid and linolenic acid, which are polyunsaturated fatty acids found in photoreceptor outer segments. Lipid peroxides have been shown to induce neovascularization by inducing expression of a cascade of angiogenic cytokines.63
Changes in hydraulic conductivity
Hydraulic conductivity is the measurement of the bulk flow of fluid through a test membrane in response to applied pressure. Bruch’s membrane would be expected to show increasing resistance to flow with age because it exhibits a linear increase in thickness50 and a significant accumulation of lipid after the fourth decade.61,62,64 However, studies undertaken on Bruch’s membrane have shown that the decrease in hydraulic conductivity is exponential, being greatest in the first four decades of life.65,66 It is unclear why this occurs before age 40. It has therefore been suggested that remodeling of collagen occurs as a result of increased cross-linkage, and this may cause an increase in rigidity of the membrane and reduced pore size, with entrapment of passing protein molecules.67 After age 40 the increasing lipid content would be expected to have an increasing effect on hydraulic conductivity, while in the 60s a further reduction would result from the diffuse deposits that appear beneath the RPE.
The excimer laser has been used to remove progressively ultrathin shavings of Bruch’s membrane to determine in which layer the major barrier to the flow of water lies. This demonstrated that the greatest resistance throughout life resides within the inner collagenous zone.66 Serial ultrathin sections cut parallel to the plane of Bruch’s membrane to estimate the porosity at sequential levels confirmed that the inner collagenous zone presented the lowest porosity. Calculations based on the pore radii and length further confirmed that the inner collagenous zone also had the lowest flow rate. However, ultrastructural studies would appear to indicate that it is mainly the outer collagenous zone that increases in thickness with age, with the inner collagenous zone remaining constant.58 Clearly, further studies are required, as only a limited number of younger eyes have been examined.
Choroid
A decrease in choroidal blood flow with age can be demonstrated by laser Doppler flowmetry and is mainly due to a decrease in choroidal blood volume rather than in velocity of flow.68 This is consistent with histologic changes in aged eyes. Comparing normal maculas in the first and 10th decades, the density of the choroidal capillaries (combined length of capillary lumina per unit length) decreased in a linear fashion by 45%, and the anteroposterior diameter by 34%.50
The middle layer of medium-sized vessels decreases with age, resulting in a progressive decrease in thickness of the choroid from 200 µm at birth to 80 µm by the age of 90 years.50 The resulting thinning of the choroid throws the remaining larger vessels into greater prominence, accounting for the senile tigroid fundus. This clinical appearance has generally been attributed to unmasking of the choroidal vessels by attenuation and loss of pigment from the retinal pigment cells. However, senile choroidal atrophy appears to contribute more significantly to the increased visibility of the vessels.
Onset and progress of age-related macular degeneration
Clinical features in the absence of drusen
The patient illustrated in Figs 65.3 and 65.6 shows this evolution to geographic atrophy over a 17-year time span. The first change detected was the presence of scattered, small drusen-like dots, 25–50 µm in size (Fig. 65.3B). A ring of small pigment clumps then developed around the fovea (Fig. 65.3C, D), but vision remained 20/20, demonstrating the difficulty of determining when, on the basis of visual acuity alone, pigment changes become pathologic. This is due to the fact that the foveal center is often spared for many years. Hyperpigmentation is accompanied by hypopigmentation, with geographic atrophy (Fig. 65.6) then spreading into the area of attenuated RPE (incipient atrophy).
Fig. 65.6 Same patient as in Fig. 65.3. (A and B, red-free) At age 81, patient fixing between two small areas of atrophy that have developed. Pigment clumping and drusen-like dots are spreading outward. This surrounding incipient atrophy corresponds to the area of geographic atrophy that developed subsequently (D); vision is still 20/30. (C) At age 82, atrophy involves fixation, and dots have faded. Vision has dropped to 20/200. (D) At age 84, area of atrophy has almost doubled; vision is 20/400. Choroidal atrophy causes exposed vessels to appear white. Patient died at age 85. Pathology of this eye is shown in Fig. 65.8.
(Reproduced with permission from Sarks JP, Sarks SH, Killingsworth M. Evolution of geographic atrophy of the retinal pigment epithelium. Eye 1988;2:552–77.)
Morphologic changes
The morphologic alterations (see Fig. 65.4C) considered thus far in the photoreceptors, RPE, Bruch’s membrane, and choroid are progressive throughout life. However, by the seventh decade other changes have appeared at the base of the RPE that have no counterpart in earlier life. These comprise the deposition of basement membrane-like material and shedding of membranous debris. Although these changes first develop in a patchy distribution while the fundus and vision are still normal, their diffuse occurrence is the principal feature of AMD.23,69,70
Basal laminar deposit – early form
The BLD lies beneath the RPE, between the plasma membrane and the basement membrane, in contrast to typical drusen, which lie external to the basement membrane. It can be demonstrated consistently by the seventh decade,23 but has been found even in the fifth decade.25 It first appears in a patchy distribution over thickened or basophilic segments of Bruch’s membrane, over intercapillary pillars, or over small drusen, suggesting a potential response to altered filtration at these sites. It can be quantified histologically25 as class 1 (small, solitary patches), class 2 (a thin continuous layer), and class 3 (a thick layer, at least half the height of the RPE).
Histologically the deposit exists in two different forms, early and late, according to the stage of degeneration. The early BLD is a pale-staining eosinophilic material that stains blue with picro-Mallory and shows faint anteroposterior striations (Fig. 65.7). On electron microscopy the BLD consists of three phenotypes: fibrillar, amorphous, and polymerized. The fibrillar phenotype appears to be the earliest manifestation and may only be detected by electron microscopy as irregular nodules lying on the original basement membrane. The polymerized form resembles the fibrous long-spacing collagen seen in Bruch’s membrane and is also found in the cornea, trabecular meshwork, and other tissues in the body.53 It projects internally from the original RPE basement membrane71 (Fig. 65.8) and accounts for the striations, or bush-like appearance, seen histologically.
Fig. 65.8 Electron micrograph illustrating changes developing between retinal pigment epithelium and choriocapillaris (CC) in age-related maculopathy corresponding to Fig. 65.4C. Horizontal yellow arrows indicate the basal plasma membrane of the RPE. Early-type basal laminar deposit (BLD) projects inward from the RPE basement membrane (white arrows) and comprises mainly banded material resembling fibrous long-spacing collagen. Coiled membranes with a bilayered structure of lipids lie among the clumps of BLD and appear to pass through the basement membrane to lie between it and the inner collagenous zone (ICZ), as well as filtering into the membrane itself. Identifiable structures within Bruch’s membrane include fragments of coated membrane (CM) and fibrous long-spacing collagen (FLSC) (×11 780).
(Reproduced with permission from Killingsworth MC, Sarks JP, Sarks SH. Macrophages related to Bruch’s membrane in age-related macular degeneration. Eye 1990;4:613–21.)
The similarity of the BLD to basement membrane and its proximity to rough endoplasmic reticulum at the base of the cells suggest it is a secretory product of the RPE.71,72 It reacts with antibodies against type IV collagen, heparan sulfate proteoglycans, and laminin,54,55,72,73 but the BLD is biochemically distinct from the RPE basement membrane, and a faulty, degradative process rather than enhanced synthesis may account for its accumulation in aged maculas.54,55
Membranous debris
Coiled membrane fragments continuous with the plasma membrane of the RPE appear together with the BLD, but they are not found unless BLD is also present24 (Fig. 65.8). This material has the bilayered structure of phospholipids and is not to be confused with the coated membrane-bound bodies described earlier in Bruch’s membrane. Whereas by light microscopy the BLD was regarded as the hallmark of macular degeneration, by electron microscopy it is this membranous debris that correlates more closely with the degree of degeneration. These membranes are found in three locations, as described in the following paragraphs.
Internal to the retinal pigment epithelium basement membrane
The coils appear to be extruded from the base of the cells, which have lost their infoldings, although they may alternatively result from a free-energy process in which lipid molecules are deposited by the RPE into Bruch’s membrane and coalesce. The membranes form layers and then basal mounds internal to the RPE basement membrane (Fig. 65.4C, Fig. 65.9), which may account for the drusen-like dots noted clinically (Fig. 65.3B). The membranes are not demonstrated in conventional histologic sections, since the mounds manifest only as small, unstained spaces within the BLD (Fig. 65.10). As the mounds enlarge and fuse, the RPE shows more derangement and cell dropout.
External to the RPE basement membrane (basal linear deposit 22)*
Membranes appear to pass through the basement membrane to form a layer between the basement membrane and the inner collagenous layer of Bruch’s membrane (Fig. 65.8). In this location the debris may build up into the soft drusen specific for AMD73 (see Fig. 65.26). The debris also appears to disturb the normal attachment of the RPE to Bruch’s membrane, creating a cleavage plane, and it is in this plane that RPE detachments due to blood and serous fluid lie and into which early choroidal new vessels grow.74 The membranes even appear to percolate into the collagenous zones of Bruch’s membrane.
At the apex of the retinal pigment epithelium
Morphologically similar membranous debris is also found over the apex of the RPE, lying in the subretinal space and presumably representing outer-segment material that has not been phagocytosed (Fig. 65.11).
Fig. 65.11 Same eye as in Figs 65.5 and 65.6. Section passes through the temporal margin of the area of geographic atrophy. (A) Photoreceptors become fewer and outer segments wider and stunted as they approach the edge. Vacuolated appearance under the RPE is due to disappearance of membranous debris. Collections of membranous debris can be seen on the internal (apical) surface of RPE (arrow), possibly due to failure of phagocytosis (×150). (B) Hyperpigmented edge noted clinically corresponds to a double layer of RPE, the inner layer representing necrotic hyperpigmented cells in the process of being eliminated. Late amorphous form of BLD lies internal to striated form and has a multilaminar appearance, suggesting formation in successive waves according to the level of RPE (see Fig. 65.4D). Photoreceptors disappear, and external limiting membrane terminates on BLD (methylene blue and basic fuscin; ×500).
(Reproduced with permission from Sarks JP, Sarks SH, Killingsworth M. Evolution of geographic atrophy of the retinal pigment epithelium. Eye 1988;2:552–77.)
Basal laminar deposit – late form (diffuse thickening of the internal aspect of Bruch’s membrane)
With progressive degeneration of the RPE, another form of basal laminar material appears. On light microscopy it forms a thick, hyalinized layer that stains red with picro-Mallory, similar to hyalinized Bruch’s membrane, and that is more periodic acid–Schiff-positive than the earlier, banded form. Being a later development, it forms a distinct layer on the internal surface of the earlier form (Fig. 65.4D) and may approximate the thickness of the normal RPE, occasionally displaying nodular elevations on its internal surface75 (Fig. 65.12).
On electron microscopy, the later form of the BLD has a flocculent appearance and consists mainly of amorphous material. It may be uplifted with the attenuated RPE over the membranous mounds and appears to be formed in waves according to the level of the base of the cell at the time (Fig. 65.11). It indicates the altered metabolism of a severely stressed RPE and occurs typically over regressing drusen (see Fig. 65.30).
Retinal pigment epithelium and photoreceptors
Lipofuscin and complex melanolipofuscin granules continue to accumulate in the retinal pigment cells, which enlarge and lose their regular shape. The external or basal surface of the cell shows loss of the basal infoldings (with a consequent reduction in surface area) and becomes increasingly separated from its basement membrane by thickening of the BLD and more membranous debris. Occasional cells undergo lipoidal degeneration.76 Finally the hyperpigmented cells resulting from this phagocytic overload round off, so only a few stubby apical microvilli remain, and they lose their ability to phagocytose. Lipofuscin is packed into large degenerate retinal pigment cells or membrane-bound bodies and shed (Fig. 65.4D).
The corresponding pigment abnormalities in the fundus may be classified11 as increased pigmentation (or hyperpigmentation) and depigmentation (or hypopigmentation). Focal hyperpigmentation correlates histologically with localized areas of RPE cell hypertrophy, which may be accompanied by clumps of hyperpigmented cells in the sub-RPE space, in the subretinal space (see Fig. 65.10), and even migrating to the outer nuclear layer. With the advent of spectral domain (SD) ocular coherence tomography (OCT)77 and ultrahigh resolution (UHR) OCT,78,79 deposits have been demonstrated, in vivo, with moderate to intense hyperreflectivity in these various planes and correspond to hyperpigmentation on clinical examination or on fundus photographs. It is common for eyes with drusen to have these intraretinal deposits directly above the drusen, and although the outer nuclear layer is more common, some migration has even occurred into more anterior retinal layers.79 Focal hypopigmentation correlates with attenuated, depigmented RPE cells surrounding the hyperpigmented cells.75 A careful review of SD OCT images in areas of hypopigmentation show attenuated signal from the RPE layer.
The sequence of events leading to pigment disturbance and, ultimately, atrophy seems to be the same irrespective of the cause. When a retinal pigment cell dies, the products are phagocytosed by its neighbors. These cells in turn become filled with lipofuscin and round off, losing their ability to phagocytose. As the cells are discarded, the nearby cells migrate and increase in surface area in an attempt to maintain the integrity of the blood–retinal barrier. This results in thinned, hypopigmented cells adjacent to focal hyperpigmentation. Finally, these cells can no longer stretch to fill the gap and atrophy results. Hyperpigmentation therefore precedes hypopigmentation, and this in turn is the prelude to the development of patches of atrophy.80
Progressive derangement of the RPE is accompanied by dropout of photoreceptors, with a reduction in the number of nuclei in the outer nuclear layer. The inner segments tend to become shorter and more bulbous. The outer segments may terminate in collections of membranes over the apical surface of the RPE (Fig. 65.11).
Bruch’s membrane and choroid
Macrophages, giant cells, fibroblasts, and occasional lymphocytes are found in relation to the outer surface of Bruch’s membrane in the space formerly occupied by the choroidal capillaries.81 Segments of the membrane begin to thin, and cell processes are occasionally observed splitting off and even enveloping small fragments of the membrane.71 The choroidal capillaries in the vicinity may show signs of activation, and new vessels still confined entirely to the choroid have been identified.82 This chronic, low-grade inflammatory reaction, which possibly develops in response to the membranous debris liberated by degenerating RPE, is often found in the choroid near breaks in Bruch’s membrane,83 and it appears to be a link in the chain of events leading to CNV. As such mechanisms that decrease inflammation are actively being investigated as a means of altering both the atrophic and neovascular pathways of advanced AMD.
Drusen
Clinical grading
Drusen size
Clinically, drusen size can be compared to the width of a major vein at the disc edge (approximately 125 µm). Small drusen are those less than 0.5 vein width (<63 µm), and since size and morphology are generally correlated, these are considered to be hard.12,18 Drusen ≥125 µm (Fig. 65.2, circle C1) are large, and these are typically considered to be soft based on the appearance of their perimeter. Drusen between ≥63 µm and <125 µm may be termed medium or intermediate in size and are more frequently classified as soft drusen as well.
Extent of fundus involvement
This may be assessed by noting drusen number, the area of fundus involved,20,84 and the density of drusen (discrete, touching, or confluent). The area occupied by drusen yields important prognostic significance as it is a cornerstone in the Age-Related Eye Disease Study (AREDS) severity scale. Drusen area is identified by mentally condensing all drusen located in the zone of interest into a single area and estimating the size of that area by comparison to standard grid areas measured in disc area (DA) equivalents.84 Automated detection of total drusen area on digital fundus images remains under development and may eventually replace, partially or in full, grading by trained readers.
Drusen distribution
Detailed natural history studies have focused on the drusen characteristics for drusen that are located within 1 or 2 disc diameters of the foveal center (see Fig. 65.1).12,84–88 Different patterns of drusen distribution have been reported,15 and it is the superior and temporal quadrants that have been associated with greatest area of drusen involvement and highest prevalence of soft, indistinct drusen or reticular drusen.
Drusen symmetry
Comparisons of the distribution, number, and type of drusen between the two eyes of an individual tend to show a remarkable symmetry, which often leads to similar outcomes in both eyes.15,17,25,89 The drusen type which are most commonly present in both eyes of an individual are reticular drusen and soft, indistinct drusen.17
Clinical grading of AMD severity
Despite their apparently significant role in the evolution of AMD, diffuse deposits detected on histopathology are difficult to study clinically. However, prognostic significance is ascribed to the clinical presence of drusen, the white-to-yellow clinically apparent deposits described above that lie deep to the retina, representing accumulation of the materials described in the preceding sections of this chapter. While it was previously recognized that patients with at least medium-size, soft, or confluent drusen are predisposed to develop advanced stages of AMD,19,87 more recent natural history data from the AREDS has suggested a somewhat more detailed description of non-neovascular AMD to characterize prognosis. In addition, this terminology is also relevant to the management of non-neovascular AMD.13 In this classification features are evaluated within 3000 µm of the center of the macula (Fig. 65.1) and eyes can be classified into one of four groups:
Group 1: An eye is graded as no AMD if there are no drusen or only a few (~5–15) small drusen in the absence of any other stage of AMD.
Group 2: An eye is considered to have early stage AMD if there are extensive (>15) small drusen, or a few (approximately <20) medium-size indistinct drusen (soft borders) or pigment abnormalities (increased pigmentation or depigmentation but not geographic atrophy) and no other stage of AMD.
Group 3: The intermediate stage of AMD refers to the presence of at least one large druse, but can also be applied to the eye with numerous medium-size drusen (approximately 20 or more when the drusen boundaries are amorphous and approximately 65 or more when the drusen boundaries are distinct, sharp or hard) or to the presence of geographic atrophy that does not extend under the center of the macula (noncentral GA).
Group 4: The advanced stage of AMD is reserved for the presence of geographic atrophy extending under the center of the macula or presence of neovascular AMD.
In addition to providing this simplified classification of AMD (no AMD, early AMD, intermediate AMD, and advanced AMD), the AREDS investigators also devised a simplified clinical scale defining risk categories for development of advanced AMD.88 A scoring system tabulates a person score by assigning 1 risk factor to each eye of an individual for the presence of at least 1 large druse and 1 risk factor for the presence of any pigment abnormality. Drusen are to be scored only within 2 disc diameters of the foveal center, and pigment abnormalities consist of either increased pigment thought to be attributed to AMD, RPE depigmentation, or areas of noncentral geographic atrophy. Risk factors are summed across both eyes, resulting in a 5-step scale (0–4) on which the 5-year risk of developing advanced AMD in at least one eye can be approximated. Risk of progression escalates as follows: total score 0, 0.5% risk; 1 factor, 3%; 2 factors, 12%; 3 factors, 25%, and 4 factors, 50%. Modifications of the scale award persons without any large drusen 1 risk factor if medium-size drusen are present in both eyes and individuals with advanced AMD in their first eye receive a score of 2 for that eye when tabulating the person score to estimate the risk for their fellow eye.
Grading in scientific studies
While the grading of drusen described above is relevant to management and relatively easy to apply in clinical practice, a means of more specific grading of drusen for scientific studies is desirable, ideally without requiring fluorescein angiography. The system proposed by the International Epidemiology Study Group,11 which is based on stereoscopic color fundus photographs, grades for the predominant drusen type, the most severe drusen type, drusen numbers, largest drusen size, area involved by drusen, drusen confluence, and drusen disappearance (see Fig 65.1, 65.2).
A more recent detailed fundus photographic severity scale to be used in research settings has been presented by the AREDS investigators.84 Baseline photographs and annual photographs beginning at year 2 of follow-up from AREDS participants were graded for drusen characteristics (size, type, area), pigmentary abnormalities (increased pigment, depigmentation, geographic atrophy) and presence of abnormalities consistent with neovascular AMD. Relationships between various baseline characteristics and development of advanced AMD at the 5-year exam were explored to develop a 9-step severity scale that sorts the 5-year risk of advanced AMD from less than 1% in step 1 to about 50% in step 9. About half the eyes that had at least a 3-step progression between baseline and the 5 year exam showed stepwise progression through intervening severity levels at intervening visits. The second Age-Related Eye Disease Study (AREDS2), which is presently underway, aims to validate this scale in a separate cohort of patients at high risk for AMD progression. If validated, progression along this scale may be considered as a surrogate outcome for progression to advanced AMD in future studies.
Imaging of drusen
Fluorescence of drusen
Drusen have a variety of constituents that range from being hydrophobic to those that are hydrophilic. Fluorescein is a hydrophilic dye, which diffuses into hydrophilic areas. As such, some drusen routinely bind the dye and hyperfluoresce in late stage angiography. About 50% of drusen present within an eye will stain with fluorescein.90 Although drusen that stain with fluorescein appear across the spectrum of drusen sizes, drusen that stain are, on average, larger than drusen that do not stain. When considering fluorescein-stained drusen, drusen area appears similar to that which is appreciated on color photographs.
During indocyanine green angiography, hard drusen become hyperfluorescent 2–3 minutes after dye administration, and this persists through the middle and late phases. Soft drusen are either hypofluorescent (darker than the background fluorescence) throughout the angiogram with a thin hyperfluorescent rim or remain isofluorescent (indistinguishable from background fluorescence).91,92
Autofluorescence
On autofluorescence imaging, large drusen may or may not be apparent depending upon the alterations in the RPE overlying the druse.64 There may be a pattern of decreased autofluorescence in the center of the druse often surrounded by a ring of subtle increased autofluorescence.93 However, areas of emerging RPE atrophy in eyes with drusen may be more apparent on autofluorescence as compared to clinical examination, as regions in which autofluorescence is completely absent.64
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