Basal Linear Deposit and Large Drusen Are Specific for Early Age-Related Maculopathy

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Abstract
AGE-RELATED maculopathy (ARM)1 is a major cause of vision loss in the elderly.2 Although its causes are poorly understood, it is agreed that the most prominent clinical and histopathologic features of ARM are lesions that involve retinal pigment epithelium (RPE) and Bruch membrane (BM). Bruch membrane is a 5-layer connective tissue sandwich interposed between the basal surface of the RPE and the choriocapillaris, the blood supply to the photoreceptors (Figure 1). Moving outward from the RPE, these layers are the RPE basal lamina (RPE-BL), the inner collagenous layer, the elastic layer, the outer collagenous layer, and the choriocapillaris BL. Debris in inner BM, variably called basal linear (BlinD)3-5 or basal laminar deposits (BlamD),6-10 have figured prominently in hypotheses of ARM pathogenesis for more than 2 decades.3 Green and Enger9 proposed that the terms BlamD and BlinD refer to 2 lesions with distinctive morphologic characteristics and positions relative to the RPE-BL (Figure 1). Basal laminar deposit consists primarily of fibrous long-spacing collagen (FLSC) and an amorphous material similar in electron density and texture to BL3,4,7 located between the RPE and the RPE-BL (ie, internal to the RPE-BL). In contrast, BlinD consists primarily of membranous material located between the RPE-BL and the inner collagenous layer (ie, external to the RPE-BL). Similar membranous debris is also found in soft drusen, ie, large focal deposits with sloping sides that are also external to the RPE-BL.6 Therefore, BlinD is sometimes referred to as diffuse drusen.9 We herein use the generic term "basal deposits" to denote BlamD or BlinD.