Braak staging is the gold-standard neuropathological classification system for grading the topographic progression of neurofibrillary tau pathology in Alzheimer's disease. Originally published by Heiko and Eva Braak in 1991, the system identifies six stereotypical stages through which tau pathology spreads across the brain in a hierarchical, anatomically connected pattern[1]. Braak staging is now a mandatory component of the NIA-AA "ABC" neuropathological assessment of AD and has been extended to alpha-synuclein pathology in Parkinson's disease[2]. Understanding Braak staging is essential for interpreting tau PET imaging, designing clinical trial endpoints, and modeling the prion-like spreading of tau pathology.
Neurofibrillary tangle (NFT) pathology progresses through six stages grouped into three phases:
Stage I: Tau pathology is confined to the transentorhinal cortex (Brodmann area 35), specifically in large multipolar neurons of the pre-alpha layer. This region serves as the interface between the entorhinal cortex and the hippocampal formation. At this stage, individuals are cognitively normal and pathology is detectable only at autopsy[1:1].
Stage II: NFTs spread to the entorhinal cortex proper, particularly the layer II stellate (star) cells that give rise to the perforant pathway — the primary cortical input to the hippocampus. The superficial entorhinal cortex is severely affected, while deeper layers remain relatively spared. Subtle neuropil threads appear in CA1 of the hippocampus[3].
Clinical correlation: Braak stages I–II correspond to the preclinical phase of AD. Autopsy studies show that ~50% of cognitively normal individuals over age 70 harbor stage I–II pathology, indicating that early tau accumulation is extremely common in aging[4].
Stage III: Tau pathology invades the hippocampal formation — specifically the CA1 sector and subiculum. The amygdala (particularly the basal and accessory basal nuclei) becomes affected. Neuropil threads are now more abundant than NFTs. The anterior thalamic nuclei and claustrum begin to show involvement[1:2].
Stage IV: The hippocampal pathology intensifies, with severe involvement of CA1 and the subiculum. The insular cortex, anterior cingulate cortex, and temporal pole begin to accumulate tau. The basal nucleus of Meynert shows significant pathology, contributing to cholinergic deficits[5].
Clinical correlation: Stages III–IV correspond to mild cognitive impairment (MCI) and early AD dementia. Episodic memory impairment becomes clinically detectable, reflecting hippocampal damage and disconnection of the perforant pathway[3:1].
Stage V: NFT pathology extends into the neocortical association areas — superior temporal gyrus, prefrontal cortex, parietal association cortex, and occipital association areas. Primary motor and sensory cortices remain relatively spared. The spread follows a laminar pattern, preferentially affecting layers III and V[1:3].
Stage VI: The most advanced stage, with widespread NFT involvement of virtually all cortical areas, including primary sensory cortex (somatosensory area 3, auditory cortex) and eventually the striate cortex (V1). Primary motor cortex is affected last. End-stage pathology involves massive neuronal loss and cortical atrophy[6].
Clinical correlation: Stages V–VI correspond to moderate-to-severe dementia, with progressive impairment in language, visuospatial function, executive function, and eventually basic motor activities.
The stereotypical progression of tau pathology follows principles of selective neuronal vulnerability and anatomical connectivity:
Selective vulnerability: The transentorhinal neurons that accumulate tau first are large, highly metabolically active projection neurons with long axons. These neurons have high expression of MAPT and are particularly susceptible to tau hyperphosphorylation[7].
Connectome-based spread: Tau pathology follows the connectome — spreading along synaptically connected pathways rather than through spatial proximity. Functional MRI and diffusion tensor imaging studies confirm that tau PET signal progresses along structural and functional connectivity networks[8].
Prion-like propagation: Misfolded tau acts as a template to convert normal tau in connected neurons, spreading trans-synaptically via exosomes, direct membrane contact, and synaptic transmission. This prion-like mechanism explains the hierarchical, predictable progression[9].
Network vulnerability: Neurons in the default mode network (DMN) — entorhinal cortex, hippocampus, posterior cingulate, lateral parietal cortex — are preferentially affected across all Braak stages, consistent with their high baseline metabolic activity and synaptic density[10].
Tau and amyloid-beta pathologies follow different topographic patterns:
Heiko Braak extended the staging concept to alpha-synuclein Lewy body pathology in PD in 2003, proposing six stages of ascending progression:
| PD Braak Stage | Region | Clinical Phase |
|---|---|---|
| 1 | Dorsal motor nucleus of vagus, olfactory bulb | Premotor (constipation, anosmia) |
| 2 | Locus coeruleus, raphe nuclei, reticular formation | Premotor (sleep, autonomic) |
| 3 | Substantia nigra pars compacta, pedunculopontine nucleus | Motor symptom onset |
| 4 | Temporal mesocortex, amygdala, basal forebrain | Cognitive decline begins |
| 5 | Prefrontal and parietal association cortex | PD dementia |
| 6 | Primary motor and sensory cortex | Severe dementia |
This bottom-up model proposes that PD pathology begins in the peripheral nervous system (gut, olfactory epithelium) and ascends via the vagus nerve, supporting the "gut-brain axis" hypothesis. However, approximately 40–50% of PD cases do not follow the canonical Braak pattern, particularly those with neocortex-first ("brain-first") pathology[12][13].
The development of tau-selective PET tracers has enabled assessment of Braak-like staging patterns in living patients:
Computational algorithms assign PET-derived Braak stages by thresholding regional SUVRs in anatomically defined regions of interest. Studies show:
Not all cases follow the canonical Braak progression:
The NIA-AA "ABC" scoring system integrates Braak staging into a standardized neuropathological evaluation:
A combined ABC score of A3B3C3 represents high-level AD neuropathological change[2:2].
Recent advances in this mechanism are being compiled. Check back for updates on key publications from 2024-2026.
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Franzmeier N, Rubinski A, Neitzel J, et al. Functional connectivity associated with tau levels in ageing, Alzheimer's, and small vessel disease. Brain. 2019. ↩︎
de Calignon A, Polydoro M, Suárez-Calvet M, et al. Propagation of tau pathology in a model of early Alzheimer's disease. Neuron. 2012. ↩︎
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Pooler AM, Polydoro M, Mapassova-Nowakowska EA, et al. Amyloid accelerates tau propagation and toxicity in a model of early Alzheimer's disease. Acta Neuropathologica Communications. 2015. ↩︎
Braak H, Del Tredici K, Rüb U, et al. Staging of brain pathology related to sporadic Parkinson's disease. Neurobiology of Aging. 2003. ↩︎
Horsager J, Andersen KB, Knudsen K, et al. 'Brain-first versus body-first Parkinson''s disease: a multimodal imaging case-control study'. Brain. 2020. ↩︎
Schöll M, Lockhart SN, Schonhaut DR, et al. PET imaging of tau deposition in the aging human brain. Neuron. 2016. ↩︎
Schwarz AJ, Yu P, Miller BB, et al. Regional profiles of the candidate tau PET ligand 18F-AV-1451 recapitulate key features of Braak histopathological stages. Brain. 2016. ↩︎
Crary JF, Trojanowski JQ, Schneider JA, et al. 'Primary age-related tauopathy (PART): a common pathology associated with human aging'. Acta Neuropathologica. 2014. ↩︎
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