Biometrics, Impact, and Significance of Basal Linear Deposit and Subretinal Drusenoid Deposit in Age-Related Macular Degeneration

Abstract

Purpose: Basal linear deposit (BLinD) is a thin layer of soft drusen material. To elucidate the biology of extracellular deposits conferring age-related macular degeneration (AMD) progression risk and inform multimodal clinical imaging based on optical coherence tomography (OCT), we examined lipid content and regional prevalence of BLinD, soft drusen, pre-BLinD, and subretinal drusenoid deposit (SDD) in AMD and non-AMD aged eyes. We estimated BLinD volume and illustrated its relation to type 1 macular neovascularization (MNV).

Methods: Donor eyes were classified as early to intermediate AMD (n = 25) and age-matched controls (n = 54). In high-resolution histology, we assessed BLinD/soft drusen thickness at 836 and 1716 locations in AMD and control eyes, respectively. BLinD volume was estimated using solid geometry in donor eyes, one clinically characterized.

Results: BLinD, drusen, type 1 MNV, and fluid occupy the sub-RPE-basal laminar space. BLinD volume in a 3-mm diameter circle may be as much as 0.0315 mm3. Osmophilic lipid was more concentrated in BLinD/drusen than SDD. In the fovea, BLinD/drusen was prevalent in AMD eyes; pre-BLinD was prevalent in control eyes. SDD was low in the fovea and high in perifovea, especially in AMD eyes.

Conclusions: Although invisible, BLinD may presage type 1 MNV. BLinD volume approaches the criterion OCT drusen volume of 0.03 mm3 for AMD progression risk. BLinD culminates years of subfoveal lipid accumulation. SDD is detected relatively late in life, with currently unknown precursors. Deposit topography suggests one outer retinal lipid recycling system serving specialized cone and rod physiology, and its dysregulation in AMD is due to impaired transfer to the circulation.

Figure 1.

The layers of age-related macular degeneration and outer retinal hyperreflective bands of optical coherence tomography. The vertical dimension is expanded to highlight thin anatomic layers between outer segments and choriocapillaris and incorporated into the fourth outer retinal hyperreflective band of OCT. The trilaminar BrM consists of inner collagenous (ICL), elastic (EL), and outer collagenous (OCL) layers. The RPE as an epithelium rests on a basal lamina (BL) of extracellular matrix (at left). BLamD is a stereotypically thickened extracellular matrix (green, in middle and right) between the RPE plasma membrane and RPE-BL, either replacing or incorporating infoldings of basal RPE. Between the RPE-BL or BLamD and ICL of BrM is the sub-RPE-BL space. In this space accumulates soft drusen material and, in some eyes, type 1 MNV of choroidal origin (up-arrow), with sequelae of fluid, cells, and fibrosis. Soft druse material is found also in BLinD and in basal mounds within BLamD. BLinD may be continuous with, yet distinct from, dome-shaped drusen. Between the RPE and photoreceptors are SDDs (first called reticular pseudodrusen), extracellular material that is reflective on OCT and directly disruptive to photoreceptors. Eyes with SDDs are at risk for type 3 MNV (down-arrow)^, of retinal origin (called retinal angiomatous proliferation). The vertical bars show the third and fourth outer retinal hyperreflective bands of currently available spectral domain OCT, with our interpretations. The third band, called the interdigitation zone (IZ), corresponds to interleaved photoreceptor outer segments and RPE apical processes containing melanosomes. Fourth band components include RPE cell bodies and basal infoldings, RPE-BL, BLamD if present, contents of sub-RPE-BL space if present, and ICL-EL-OCL of BrM. RPE cell bodies contain ∼1400 reflectors in two cushions of similar numerosity: lipofuscin and melanolipofuscin, with some melanosomes in the apical three-fourths, mitochondria in the basolateral three-fourths, and the middle half containing both organelle classes.^, ChC-BL, ChC basal lamina; M, melanosome; ML, melanolipofuscin; Mt, mitochondria; OS, outer segments of photoreceptors.

Figure 2.

Soft drusen are visible in OCT-based multimodal imaging; basal linear deposit is not. (A) Color fundus photograph shows yellowish drusen (yellow arrowhead) in the macular region especially inferior temporal to the fovea. (B) Red-free image shows drusen clearly (yellow arrowhead). (C) OCT B-scan at green arrow in panel A shows several RPE elevations with a medium and homogeneous internal hyperreflectivity representing soft drusen (yellow arrowheads). (D) Corresponding histologic image shows soft drusen (yellow arrowheads) that correspond well with the B-scan. Green frame shows a region magnified in panel E. Retina is artifactually detached at the inner segment myoids (bacillary layer detachment). (E) Three soft drusen with finely granular, lightly osmophilic contents (d) are continuous with BLinD (red arrowhead) containing the same material. Sixty-nine-year-old white old man with early age-related macular degeneration; see Methods for details.

Figure 3.

BLinD and soft drusen are two forms of the same deposit, and pre-BLinD is the precursor. (A) A large soft druse (d) is located in the sub-RPE-basal laminar space, with a granular internal structure that stains gray and continuous with BLinD to the left. The crack is artifact. Seventy-six-year-old woman. (B) Undulating layer of BLinD (red arrowheads) and flat layer of pre-BLinD (yellow arrowhead) are observed in the same sub-RPE-basal laminar space and have a similar internal structure and staining characteristics as soft drusen. Cone pedicles with dark staining synaptic complexes (fuchsia arrowhead) indicate good tissue fixation. Teal frame shows BLinD and pre-BLinD magnified in panel C. Artifactual retinal detachment was digitally approximated to the RPE (Photoshop). (C) BLinD and pre-BLinD consist of finely granular gray-staining material (red and yellow arrowheads, respectively). Pre-BLinD stains darker than BLinD. Ninety-year-old woman. Ch, choroid; HFL, Henle fiber layer; IS, inner segment; ONL, outer nuclear layer; OS, outer segment. Green arrowheads, external limiting membrane.

Figure 4.

Continuity of basal linear deposit and soft drusen with type 1 neovascularization and exudation. (A) Panoramic view of a section that passes through the edge of the foveal floor of the right eye, judging from the rod-free zone and single row of ganglion cell bodies. Teal frame shows a region magnified in panel B. (B) One continuous compartment contains BLinD and soft druse (frame C), fluid (frame D), and type 1 neovascularization (frame E), magnified in panels C, D, and E, respectively. (C) Druse continuous with BLinD (yellow arrowheads) and fringe of BLinD persists at a site of artifactual separation of BLamD from the inner collagenous layer of BrM. Yellow asterisk, basal mound. (D) Fluid in the same sub-RPE-basal lamina compartment (fuchsia arrowheads). (E) Choroidal neovessels with patent lumens (fuchsia asterisks) pass through a break in BrM (orange arrowheads), accompanied by pericytes (teal arrowheads) and fibrous material. An 81-year-old female donor. INL, inner nuclear layer. Green arrowheads, ELM, external limiting membrane. Scale bar in D applies to C and D.

Figure 5.

Basal linear deposit thickness in age-related macular degeneration eyes. The horizontal axis represents eccentricity, that is, distance from the foveal center (zero point). The vertical axis represents the thickness of BLinD. Blue dots represent BLinD thickness assessed at 168 different locations, as described in the Methods. Data from all retinal quadrants are collapsed onto one axis. Eccentricities were collapsed into bins as described in the Methods and the median (M) and third quartile thickness value (Q3) in each bin calculated. Two trendlines were created (M, red line; Q3, green line) and used to estimate volumes of BLinD shown in Table 2.

Figure 6.

Sub-RPE-BL lipid accumulation is topographically organized in aged and age-related macular degeneration fundus. One foveal and three perifoveal fundus regions were defined in central and superior sections, as described in the Methods. (A) Percentage of 836 locations with BLinD/drusen and pre-BLinD in different regions in 25 AMD eyes. (B) Percentage of 1716 locations with BLinD/drusen or pre-BLinD in different fundus regions in 54 control eyes. In AMD eyes and control eyes, the percentage of locations with BLinD/drusen or pre-BLinD is highest in the fovea. The percentage of locations with any sub-RPE-BL lipid in AMD eyes is 1.14 times, 1.27 times, 2.45 times, and 0.99 times that of the control eyes in fovea, nasal, superior, and temporal regions, respectively. In the fovea, AMD and control eyes have a similar total number of locations with sub-RPE-BL lipid, with proportionally more BLinD and less pre-BLinD in AMD eyes.

Figure 7.

Subretinal drusenoid deposit is less lipid rich than basal linear deposit. (A) The sub-RPE-BL space is normally devoid of lipid. (B) 60 µm away from panel A. BLinD (red arrowheads) is continuous with and thicker than pre-BLinD (yellow arrowheads) in the sub-RPE-BL space. BLinD is operationally distinguished from pre-BLinD by its undulating profile. SDD (orange arrowhead) is observed on top of wavy RPE, with a finely granular texture, several RPE melanosomes, and minimal osmophilia relative to BLinD. A choroidal lipid globule (g) is deep gray and serves as a positive control for osmophilic lipids. Eighty-seven-year-old man. Green arrowheads, external limiting membrane. Scale bar in B applies to A and B.

Figure 8.

Subretinal drusenoid deposit is topographically organized in aged and in age-related macular degeneration fundus. Yellow columns indicate percentage of 836 locations with SDDs in different macular subregions in 25 AMD eyes. Gray columns indicate percentage of 1716 locations with SDDs in different macular subregions in 54 control eyes. The percentage of locations with SDDs was lower in the fovea than in the other subregions in both groups. The percentage of locations with SDDs in AMD eyes is higher than in control eyes by factors of 5.2, 2.5, 6.0, and 24.7 in the fovea, nasal, superior, and temporal subregions, respectively. Percentages are overall lower than those for sub-RPE-BL lipid in Figure 6 (note different scaling on Y-axis).