Fig. 5.14 Schematic diagram of the pathogenesis of CNV. Increasing lipid deposits within Bruch’s membrane diminish the concentration of growth factors (VEGF) essential for the maintenance of a normal choriocapillaris structure, regression of capillaries ensuing relative hypoxia of the retinal pigment epithelium,

leading to an increased production of growth factors and matrix enzymes, provoking the ingrowth of choroidal capillaries (NV). VEGF vascular endothelial growth factor, BM Bruch’s membrane, NV neovascularization

permeability of Bruch’s membrane for these growth factors leads to a regression of the choroidal capillaries. Hypoxia and lipofuscin-dependent changes in the metabolism of RPE cells follow [102, 103] and can cause enhanced production of growth factors like vascular endothelial growth factor (VEGF). In combination with the altered synthesis and degradation of matrix molecules, this increased supply of VEGF gives rise to an immigration of newly formed choroidal capillaries (Fig. 5.14).

In CNV there are many open questions left. For instance, it is unclear why the preferential manifestation of CNV is in the macular area. Only very rarely are choroidal neovascularizations in the retinal periphery. The vessels mostly develop at the posterior pole of the eye and grow towords the fovea [51]. Higher concentrations of growth factors in the central retina or a preferred destruction of central RPE cells are believed to cause this phenomenon [104]. The density of the photoreceptor cells is highest in the retinal center, thus leading to a high load of metabolic waste in this part of the retina. Correspondingly, the age-related deposition of granules loaded with lipofuscin in the RPE cells is maximal. In addition, the thickening of Bruch’s membrane underlying the macular part is most explicit in this area [7, 94], and drusen are found primarily at the posterior fundus pole [6, 69]. An age-related calcification increase of the fibers in the elastic layer of Bruch’s membrane is observed. Especially in the macular part, the calcified fibers may break which may promote neovascularization [86]. Age-related retinal changes and alterations in the RPE are most prominent in the area of the underlying macula. This may contribute to the preferential occurrence of CNV in that delicate area.

5.4Detachment of the Retinal Pigment Epithelium

During RPE detachment (PED), the complete RPE cell layer with the connected photoreceptor cells and the neural retina is detached from the inner collagenous layer of Bruch’s membrane. The resulting cavity is filled with serous fluid. The detection of a PED is often delayed because loss of vision occurs only at the late stage.

PED is observed in different retinal diseases. The resulting accumulation of fluid in the lumen under the RPE has been described to occur clinically very early when accompanying AMD [77, 105]. About 10% of all patients with exudative AMD suffer from PED [106].

Histologically, the detachment is between the RPE basement membrane and inner collagenous layer of Bruch’s membrane (Fig. 5.15). CNV (see Sect. 5.3) is often associated with the detachment and a disciform scar may form with progressing disease. After a PED apoptosis of RPE cells and cell death of photoreceptor cells may occur, leading to the development of an atrophic area [107–109].

Bruch’s membrane plays a key role in PED. In addition to its thickening by diffuse or localized deposits - as hallmarks of early AMD - its pentalaminar composition becomes indistinct. This is accompanied by an increasing deposition of membranous as well as high-density particles. Bruch’s membrane undergoes structural changes that alter its biochemical composition and hence its physical and physiological qualities. In Sect. 5.2.2, the involved processes of collagen cross-linking, calcification, deposition of lipids

and alterations of protein ingredients are described in detail. These processes also occur during the normal ageing process of Bruch’s membrane, however, preferentially under the macula and are found more often in predisposed people.

There is experimental evidence that PED may be caused by its activity as a fluid pump. Due to the agerelated increased lipid deposition into Bruch’s membrane it is postulated to become more and more of a hydrophobic barrier whilst the amount of fluid to be

Fig. 5.15 Pigment epithelial detachment in histology stained with oil red O (magnification 250-fold) revealing cleavage of the inner collagenous layer of Bruch’s membrane and deposition of serous lipid-containing fluid

transported through the RPE stays consistent. This situation is thought to lead to serous PED [100].

In patients with PED, an irregular dispersion of hydrophilic sodium fluorescein in fluorescence angiography can be detected. This may be caused by irregularly deposited lipophilic material in Bruch’s membrane. Drusen with a low fatty acid content are very well visible with this technique, while the specimens rich in lipids cannot be detected [42]. This may provide an explanation for the irregular dispersion of the fluorescent dye. Furthermore, it is well known that protruding hyperfluorescent and hydrophobic soft drusen are risk factors for a PED [60, 110].

Serous PED may be associated with a CNV. In this situation irregular blood vessels grow through Bruch’s membrane and form a membrane composed of fibrovascular tissue (Fig. 5.16). The PED is clearly illustrated and identified in optical coherence tomography as well as in fluorescence angiography (Fig. 5.17).

The increasing lipid content in an ageing Bruch’s membrane reduces the permeability to water-soluble substances. This impedes the exchange of material and fluid between the RPE and choroid, enhancing the barrier function of Bruch’s membrane. Reduced permeability to water passing Bruch’s membrane with increasing age and lipid content was confirmed by several studies

Fig. 5.16 Vascularized pigment epithelial detachment. (a) Fluorescein angiography: large hypofluorescence with hyperfluorescent occult CNV at the nasal edge. (b) Histological section of the same lesion: The RPE appears to be detached together

with basally associated diffuse deposits (open arrow) from a clearly neovascularized (arrow) fibrovascular tissue. Within the subretinal space (asterisk) increasing collagen deposits after serous exudation indicate the healing process

[38, 111]. They revealed a diminished number of pores in Bruch’s membrane and in persons aged 50 years or more. Further, there was a linear relation between a high lipid content and a strong flow-through resistance. In hampering the exchange of material passing Bruch’s membrane, the inner collagenous layer was shown to play the most important role [40].

Other analyses showed a hindered transport of amino acids through the complex of Bruch’s membrane and choroid with increasing age, which may point to a bad nutritional supply in general [112]. With increasing age retinal maintenance is further hampered by a reduced diameter of choroidal capillaries in combination with a lower density of the capillary mesh [56]. The diffusion and exchange of nutrients and metabolites between choroid and RPE cells is essential for a healthy retina [113]. A decline will result in an accelerated retinal undersupply.

So-called integrins - adhesion proteins on the RPE cells - play an essential role in the attachment of the RPE cell layer with its basement membrane to the inner collagenous layer of Bruch’s membrane. In addition to age-related alterations in the expression of different integrin-subunits one knows from blockage experiments that not only the extent of expression but also the correct aligment of the integrin subunits are crucial for adhesion strength [114, 115]. Therefore, an altered integrin expression pattern and rate may be an early sign of PED. A decreased expression and abnormal alignment of integrin subunits may also hamper

reattachment when the fluid under the detachment is absorbed. In addition, the amount of laminin in the retina changes with age; laminin 5 being particularly important for RPE adhesion [117].

The adhesion proteins on the surface of RPE cells bind to extracellular matrix components of Bruch’s membrane. Modification of these components by cross-linked collagen and oxidative damage is another risk factor for PED.

At First it was assumed that serous fluid stems from leaking choroid capillaries or is secreted by abnormal retinal blood vessels growth (Fig. 5.18a) [77, 118]. A directional flow of fluid from the vitreous through the retina to the choroid is physiologically inherent by the net transport of ions passing the RPE and its barrier function [119, 120]. If the PED is accompanied by a CNV or a retinal angiomatous proliferation associated with a penetration of the RPE, these leaking vessels can contribute to the accumulation of fluid. As a PED is not always associated with a CNV, there also have to be other fluid sources [121]. Experiments with isolated RPE layers from dogs showed that even a strong hydrostatic pressure gradient does not change the fluid flow through the RPE [122]. This indicates that the pump activity of the RPE itself may be strong enough to part this cell layer from the underlying Bruch’s membrane, which becomes more and more brittle and hydrophobic with increasing age (Fig. 5.18b) [54]. It can thus be assumed that both, leakage from choroidal neovascular vessels as well as a strong pump activity of the RPE together