Reduced vascular amyloid burden at microhemorrhage sites in cerebral amyloid angiopathy

Abstract

Microhemorrhages are strongly associated with advanced cerebral amyloid angiopathy (CAA). Although it has been frequently proposed that the deposition of Aβ in the walls of cortical vessels directly causes microhemorrhages, this has not been studied in great detail, mainly because the ruptured vessels are often missed on routine histopathologic examination. Here, we examined histopathological data from studies targeting microhemorrhages with high-resolution ex vivo 7 T MRI in nine cases with moderate-to-severe CAA, and assessed the presence of Aβ in the walls of involved vessels. We also assessed the density of Aβ positive cortical vessels in areas surrounding microhemorrhages compared to control areas. In seven out of 19 microhemorrhages, the presumed involved vessel could be identified on the histopathological section. Only one of these vessels was positive for Aβ at the site of rupture. Moreover, the density of Aβ positive cortical vessels was lower (1.0 per mm²) within a range of 315 µm surrounding the microhemorrhage, compared to control areas (2.0 per mm²; p < 0.05). These findings question the widely held assumption that the deposition of Aβ in the walls of cortical vessels directly causes microhemorrhages.

Keywords: Amyloid β; MRI; Microbleeds; Small vessel disease.

Fig. 1

Vascular Aβ was absent in the walls of six out of seven vessels that were involved in microhemorrhages in cerebral amyloid angiopathy. A recent microhemorrhage was identified on an H&E-stained section, characterized by the presence of intact erythrocytes (arrows) in the parenchyma, surrounding a penetrating cortical vessel (a, inset shows enlargement of boxed area; adapted from [22] with permission). The adjacent Aβ-stained section revealed that the involved vessel was negative for Aβ (broken arrows), whereas neighboring vessels did contain Aβ (arrows) (b, inset shows enlargement of boxed area, broken arrows indicate absence of Aβ in the wall of this vessel). In another case, an old microhemorrhage was identified on an H&E-stained section, characterized by the focal deposition of hemosiderin (blue deposits in inset) in the parenchyma, surrounding a cortical vessel (broken arrow in inset) (c, inset shows Perl’s Prussian blue staining of the area outlined in the box for enhanced detection of hemosiderin). The adjacent Aβ-stained section revealed that the involved vessel was negative for Aβ (broken arrows in inset), as well as vessels in close proximity to the microhemorrhage (broken arrows), whereas vessels further away did contain Aβ (arrows) (d, inset shows enlargement of boxed area). All scale bars indicate 250 µm

Fig. 2

Vascular Aβ was absent in the walls of vessels with fibrinoid necrosis in cerebral amyloid angiopathy. Two severely dilated vessels with fibrinoid necrosis (arrows, also confirmed by phosphotungstic acid/hematoxylin stain, not shown) were identified on H&E (a, b) (b was adapted from [21] with permission). The adjacent Aβ-stained section revealed that these vessels were negative for Aβ (broken arrows), whereas neighboring vessels did contain Aβ (c). Boxed areas in c represent areas contained in a and b respectively

Fig. 3

Density (vessels/mm²) of Aβ positive cortical vessels was lower in the shell directly adjacent to a microhemorrhage, compared to shells further away. The first shell adjacent to a microhemorrhage contained a lower density of Aβ positive cortical vessels compared to control areas (upper graph). This effect was not observed in shells further away from the lesion. Moreover, the first shell adjacent to a microinfarct did not contain a lower density of Aβ positive cortical vessels compared to control areas (lower graph). This suggests that the observed lower density of Aβ positive cortical vessels adjacent to a microhemorrhage was not caused by tissue damage in that shell. *p < 0.05 (paired samples t test). Error bars represent SEM. The graphs depict the first three shells. The width of one shell was set at 50 pixels, which equals 315 μm. Microhemorrhages: n = 17; total number of CTRL areas: n = 34; microinfarcts: n = 8