Unclassifiable senile plaques and extensive cerebral amyloid angiopathy involving spinal and bridging vessels in autopsied patients with Down syndrome

Abstract

Background: Individuals with Down syndrome (DS) face markedly increased risk of premature aging and age-related pathological changes, particularly Alzheimer's disease (AD)-like neuropathology. By the fourth decade of life, virtually all individuals with DS develop the hallmark AD features such as senile plaques (SPs) and neurofibrillary tangles (NFTs). The aim of this study was to characterize the topographical distribution of cerebral amyloid angiopathy, the morphology of senile plaques, and the spectrum of co-existing aging-related proteinopathies in autopsied DS patients, with reference to age-matched and elderly controls.

Methods: Nine autopsied DS patients (aged 0.5–68.0 years at death) were examined alongside age-matched controls. Immunohistochemical staining was performed for amyloid-β (Aβ), phosphorylated tau, α-synuclein, and phosphorylated TDP-43. In addition, silver impregnation using the Gallyas method and Congo red staining were performed. Aging-related pathologies were assessed using established criteria for NFTs, Aβ deposits, cerebral amyloid angiopathy (CAA), and other neurodegenerative changes.

Results: All four DS patients aged ≥ 28 years (D6–D9) showed moderate-to-severe AD neuropathological changes, whereas none of five age-matched controls (23.1–68.4 years old) did. In DS patients with AD, unclassifiable SPs were predominant, and NFTs with both 3-repeat and 4-repeat tau were observed. The distribution and progression of the latter were similar to those of sporadic AD patients. CAA was observed in three DS patients and, owing to systematic sampling, could be documented in the spinal arteries and subdural/subarachnoid bridging vessels—sites not routinely evaluated in autopsy series of sporadic CAA. All three DS cases with CAA reached Thal stage 3 CAA, contrasting with a maximum of stage 2 in CAA-positive sporadic AD and elderly control cases. Notably, two of three DS patients with CAA had a documented clinical history of subdural hemorrhage (SDH); both showed marked cerebral atrophy at autopsy, precluding definitive attribution of SDH to CAA. The high frequency of SDH suggests increased hemorrhagic risk in DS patients due to extensive vascular amyloid deposition.

Conclusions: This study demonstrates accelerated ADNC development in DS, with characteristic unclassifiable SPs and extensive CAA representing unique features that distinguish DS from common aging patterns. The clinical history of SDH in DS patients with CAA, together with the histological extension of CAA to subdural bridging vessels, may warrant attention when considering the vascular safety of emerging anti-amyloid therapies in this population. However, causality between CAA and SDH could not be established from the present autopsy data. These findings provide crucial insights into AD pathogenesis and highlight the importance of developing targeted therapeutic strategies while considering safety implications.