The root cause of Alzheimer's disease remains contested, but I can tell you where the evidence points and where it frays.
The dominant framework for three decades has been the **amyloid cascade hypothesis**—that accumulation of amyloid-beta peptides triggers a cascade leading to tau pathology, synaptic loss, and neurodegeneration. This hypothesis generated enormous research investment and several targeted therapies. Yet the repeated failure of anti-amyloid interventions to meaningfully halt cognitive decline in clinical trials suggests either we've been targeting the wrong amyloid species, the wrong stage of disease, or that amyloid is better understood as a *consequence* of earlier upstream dysfunction rather than its origin.
My own bias, shaped by watching neurons fail in other contexts, is that the most parsimonious account points to a convergence of insults: **synaptic vulnerability** sits at the center. Specific neuronal populations—layer 2/3 excitatory neurons and inhibitory interneurons—are exceptionally metabolically demanding and highly interconnected. When these circuits are stressed by:
- Age-related decline in mitochondrial function
- Chronic neuroinflammation (microglial dysregulation)
- Vascular compromise reducing metabolic delivery
- Possibly persistent pathogens or viral reactivation
...the system reaches a tipping point where amyloid deposition and tau spread become self-reinforcing rather than initiating.
The dissatisfying honest answer is that "root cause" may be multiply determined—Alzheimer's likely represents several pathophysiological routes converging on a final common phenotype of network collapse. The field is slowly moving toward recognizing this heterogeneity, which explains why monotherapy approaches have disappointed.
What we desperately need are biomarkers that capture the *earliest* cellular disruptions—before plaques are visible—to actually identify the trigger, not just the downstream debris.