🔬 Causes & Pathology
Dementia results from the convergence of multiple interacting pathological pathways. No single mechanism is sufficient — the emerging consensus is that dementia is a multifactorial syndrome where genetics, environment, and lifestyle factors collectively determine who crosses the threshold into clinical disease.
The Amyloid Cascade Hypothesisstrong evidence
The dominant theory since 1992 (Hardy & Higgins) proposes that accumulation of amyloid-beta protein fragments in the brain triggers a cascade leading to neurodegeneration. Autosomal dominant AD mutations (APP, PSEN1, PSEN2) directly affect amyloid pr…
Tau Pathologystrong evidence
Tau protein normally stabilizes microtubules in neurons. In disease, it becomes hyperphosphorylated, forming neurofibrillary tangles that spread in a prion-like fashion through connected brain regions (Braak staging). Tau burden correlates more close…
Neuroinflammation & the Immune Systemstrong evidence
Microglia (brain immune cells) become chronically activated in dementia, releasing pro-inflammatory cytokines. The TREM2 receptor on microglia is the second-strongest genetic risk factor after APOE. The NLRP3 inflammasome is a central mediator — acti…
The Infection Hypothesismoderate evidence
Growing evidence links chronic infections to dementia risk. HSV-1 (herpes simplex): meta-analysis shows 20-32% increased AD risk, with up to 2.44x higher odds with co-infections. P. gingivalis (periodontal bacteria): gingipains found in >90% of AD br…
Gut-Brain Axismoderate evidence
Gut dysbiosis (reduced beneficial bacteria like Bifidobacterium and Akkermansia, increased pro-inflammatory Proteobacteria) leads to "leaky gut" — bacterial endotoxins (LPS) enter systemic circulation, promoting chronic inflammation that compromises …
Metabolic Dysfunction — "Type 3 Diabetes"moderate evidence
Brain insulin resistance may be a core driver of AD. The mechanism chain: insulin resistance leads to hyperinsulinemia, which saturates insulin-degrading enzyme (IDE) — the same enzyme that degrades amyloid-beta. Reduced PI3K/Akt signaling activates …
Vascular Contributionsstrong evidence
Cerebral small vessel disease is present in most elderly brains. Blood-brain barrier breakdown allows toxic proteins and immune cells into brain tissue. White matter hyperintensities on MRI correlate with cognitive decline. Hypertension is the most t…
Geneticsstrong evidence
APOE4 is the strongest genetic risk factor for late-onset AD (3-12x risk, dose-dependent). APOE4 raises risk 81% in women vs 27% in men. TREM2 variants (2-4x risk) affect microglial function. Protective variants exist: APOE2 (~40% reduced risk) and t…
The APOE4 Sex Disparitystrong evidence
APOE4 raised dementia risk 81% in women but only 27% in men (Alzheimer's Research UK 2024, Stanford 2026). Preliminary research suggests DNA near the APOE gene has an estrogen binding site — the presence or absence of estrogen may directly impact APO…
Estrogen's Neuroprotective Rolemoderate evidence
Before menopause, estrogen provides protection against APOE4-related brain changes through: enhanced amyloid-beta clearance, blood-brain barrier integrity maintenance, and neuroinflammation suppression. This explains why the APOE4 sex disparity is mo…
The Perimenopause Windowmoderate evidence
Perimenopause represents a "critical window of emerging vulnerability" to Alzheimer's-related brain changes. Estrogen decline triggers cascading effects on brain metabolism, inflammation, and protein clearance. The HRT debate remains unresolved — ear…
Sex Differences in Pathology
Women show faster tau accumulation, different neuroinflammatory profiles, and may have different biomarker thresholds. Men tend to present with more behavioral symptoms; women with more memory decline. Incident loneliness increases dementia risk diff…
Sleep & the Glymphatic Systemmoderate evidence
The glymphatic system clears amyloid-beta and tau from the brain during NREM sleep via CSF oscillations driven by norepinephrine fluctuations. A 2026 randomized crossover trial (n=39) confirmed sleep-driven clearance. However, there's a 2025 debate —…
Air Pollutionmoderate evidence
PM2.5 exposure is associated with 14-17% increased dementia risk. The strongest links are to black carbon and organic matter from agriculture and wildfires. PM2.5 can traverse the blood-air barrier and promote neuroinflammation, microglial activation…