Age and the relation of common neuropathologies to dementia in Brazilian adults

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Background: Community-based clinical-pathologic studies have been instrumental to examine the association of Alzheimer's disease and related disorders (AD/ADRD) with age and dementia in very-old non-Latino Whites. Here, we show the age distribution of four AD and three additional common neuropathologies across the adult lifespan and examine their relation to dementia and cognitive impairment in old and young Brazilian adults. Methods: We examined 5,376 brains from decedents age 18 years or older (52.5% male, 39.8% Black), from the Pathology, Alzheimer's and Related Dementias Study (PARDoS), collected between July 2021 and September 2025. Clinical diagnoses were rendered by a clinician who reviewed the Informant Questionnaire on Cognitive Decline in the Elderly (IQCODE), informant-based Clinical Dementia Rating (CDR) Scale, and other selected data. Four indices of AD including β-amyloid deposits (Thal stage), PHF-tau tangles (Braak stage), neocortical phosphorylated plaques and AD neuropathologic change (ADNC), and three other common neuropathologies, i.e., Lewy-body disease (LBD), chronic gross infarcts, and cerebral amyloid angiopathy (CAA) were assessed. Logistic regression was used for associations of pathologies with clinical diagnoses, adjusting for demographics. Results: Intermediate to high ADNC were first found as early as the fourth decade. Chronic gross infarcts were found in one-fifth of the brains of young adults. Intermediate to high ADNC, limbic and neocortical LBD, chronic gross infarct and moderate to severe CAA were associated with dementia and cognitive impairment (CI) in older adults with mixed pathologies being the most common. Intermediate to high ADNC was associated with CI but not dementia in young adults, whereas, chronic gross infarcts were associated with both CI and dementia in young adults; overall, mixed pathologies were a small minority. Conclusion: In a community-based, clinical-pathologic study including 5300+ brains from diverse Brazilians, we show that AD and other common pathologies frequently begin in young adulthood. In older adults, mixed pathologies are most commonly associated with dementia, whereas in young adults a single pathology, most commonly chronic gross infarcts rather than ADNC is related to dementia.

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