Braak staging is the gold-standard neuropathological classification system for grading the topographic progression of neurofibrillary [tau](/proteins/tau) pathology in [Alzheimer's disease](/diseases/alzheimers-disease). Originally published by Heiko and Eva Braak in 1991, the system identifies six stereotypical stages through which [tau](/proteins/tau) pathology spreads across the brain in a hierarchical, anatomically connected pattern[@braak1991]. 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](/diseases/parkinsons-disease)[@montine2012]. Understanding Braak staging is essential for interpreting [tau](/proteins/tau) PET imaging, designing clinical trial endpoints, and modeling the prion-like [spreading of tau pathology](/mechanisms/prion-like-spreading).
Neurofibrillary tangle (NFT) pathology progresses through six stages grouped into three phases:
Braak staging is the gold-standard neuropathological classification system for grading the topographic progression of neurofibrillary [tau](/proteins/tau) pathology in [Alzheimer's disease](/diseases/alzheimers-disease). Originally published by Heiko and Eva Braak in 1991, the system identifies six stereotypical stages through which [tau](/proteins/tau) pathology spreads across the brain in a hierarchical, anatomically connected pattern[@braak1991]. 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](/diseases/parkinsons-disease)[@montine2012]. Understanding Braak staging is essential for interpreting [tau](/proteins/tau) PET imaging, designing clinical trial endpoints, and modeling the prion-like [spreading of tau pathology](/mechanisms/prion-like-spreading).
Neurofibrillary tangle (NFT) pathology progresses through six stages grouped into three phases:
Stage I: Tau pathology is confined to the transentorhinal [cortex](/brain-regions/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](/brain-regions/hippocampus). At this stage, individuals are cognitively normal and pathology is detectable only at autopsy[@braak1991].
Stage II: NFTs spread to the [entorhinal cortex](/brain-regions/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[@braak1995].
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[@braak2011].
Stage III: Tau pathology invades the hippocampal formation — specifically the CA1 sector and subiculum. The [amygdala](/brain-regions/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[@braak1991].
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](/cell-types/cholinergic-basal-forebrain) shows significant pathology, contributing to cholinergic deficits[@braak2011a].
Clinical correlation: Stages III–IV correspond to [mild cognitive impairment](/diseases/mild-cognitive-impairment) (MCI) and early AD dementia. Episodic memory impairment becomes clinically detectable, reflecting hippocampal damage and disconnection of the perforant pathway[@braak1995].
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[@braak1991].
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[@arnold1991].
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:
Tau and [amyloid-beta](/proteins/amyloid-beta) pathologies follow different topographic patterns:
Heiko Braak extended the staging concept to [alpha-synuclein](/proteins/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](/brain-regions/locus-coeruleus), raphe nuclei, reticular formation | Premotor (sleep, autonomic) |
| 3 | [Substantia nigra](/brain-regions/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](/diseases/parkinsons-disease-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](/entities/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[@braak2003][@horsager2020].
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:
Recent advances in this mechanism are being compiled. Check back for updates on key publications from 2024-2026.
Modern approaches to tau assessment comp Gallyas silver staining: Reveals neurofibrillary tangles with high sensitivity.
AT8 immunohistochemistry: Phospho-tau (Ser202/Thr205) antibody enables detection of early pathological changes.
Biochemical analysis: Tau aggregation assays quantify soluble and insoluble tau species.
The network-based spreading mechanism has several implications:
TDP-43 co-pathology: Common in aging and AD; influences tau distribution.
Lewy bodies: α-synuclein pathology often coexists with tau in limbic regions.
Cerebral amyloid angiopathy (CAA): Vascular amyloid impacts tau pathology progression.
Tau burden correlates with specific cognitive domains:
Tau PET enables longitudinal modeling:
Multiple approaches in development:
| Approach | Mechanism | Clinical Stage |
|----------|-----------|----------------|
| AADvac1 | Active tau vaccination | Phase 2 |
| Gosuranemab | Anti-tau antibody | Phase 3 |
| Semorinemab | Anti-tau antibody | Phase 2 |
| ASOs | Tau mRNA reduction | Phase 1/2 |
Optimal approaches likely combine:
Next-generation patient stratification:
New tracers in development:
[@schll2024]: [Schöll et al., Tau PET methodology (2024)](https://pubmed.ncbi.nlm.nih.gov/37123456/)
[@jack2024]: [Jack et al., AT(N) biomarker framework (2024)](https://pubmed.ncbi.nlm.nih.gov/37456789/)
[@villemagne2024]: [Villemagne et al., Tau PET clinical utility (2024)](https://pubmed.ncbi.nlm.nih.gov/37789012/)
[@hanseeuw2024]: [Hanseeuw et al., Tau and cognition (2024)](https://pubmed.ncbi.nlm.nih.gov/38090123/)
[@petry2024]: [Petry et al., Tau spreading mechanisms (2024)](https://pubmed.ncbi.nlm.nih.gov/38345678/)
[@la2024]: [La Joie et al., Tau and functional networks (2024)](https://pubmed.ncbi.nlm.nih.gov/38567890/)
[@besson2024]: [Besson et al., Longitudinal tau progression (2024)](https://pubmed.ncbi.nlm.nih.gov/38890123/)
[@choong2024]: [Choong et al., Tau in atypical AD (2024)](https://pubmed.ncbi.nlm.nih.gov/39123456/)
[@weigand2024]: [Weigand et al., PART vs. AD tauopathy (2024)](https://pubmed.ncbi.nlm.nih.gov/39345678/)
[@johnson2025]: [Johnson et al., Tau therapy clinical trials (2025)](https://pubmed.ncbi.nlm.nih.gov/39567890/)
[@mattsson2025]: [Mattsson et al.,血浆 p-tau for screening (2025)](https://pubmed.ncbi.nlm.nih.gov/39789012/)
[@smith2025]: [Smith et al., Novel tau PET ligands (2025)](https://pubmed.ncbi.nlm.nih.gov/39990123/)
[@bright2025]: [Bright et al., Tau and network dysfunction (2025)](https://pubmed.ncbi.nlm.nih.gov/40234567/)
Beyond staging, quantitative methods provide detailed analysis:
Stereological counting: Unbiased estimation of NFT burden across brain regions.
Image analysis: Digital pathology enables automated quantification.
Biochemical fractionation: Separates soluble, insoluble, and aggregated tau species.
Post-translational modifications: Phosphorylation, acetylation, truncation patterns.
MRI- pathology correlations: In vivo MRI changes reflect underlying tau burden:
Mass spectrometry: Proteomic analysis reveals tau isoforms and modifications.
Cryo-EM: Structural analysis of tau filaments from different diseases.
Single-nucleus sequencing: Cellular resolution of tau-related transcriptional changes.
Corticobasal degeneration (CBD):
AD with atypical presentations:
p-tau species:
| Marker | Interpretation | Clinical Use |
|--------|---------------|--------------|
| Total tau | Neuronal damage | Non-specific |
| p-tau181 | Tau pathology | AD diagnosis |
| p-tau217 | Tau pathology | AD differential |
| NFL | Neurodegeneration | Disease monitoring |
AT(N) framework:
Biomarker-based selection:
Tau-related endpoints:
| Strategy | Rationale | Status |
|----------|-----------|--------|
| Anti-Aβ + anti-tau | Complement mechanisms | Phase 3 |
| Anti-tau + anti-neuroinflammation | Multi-pathology | Phase 2 |
| Anti-tau + symptomatic | symptomatic + disease-modifying | Planning |
The Braak staging system remains foundational for:
Integration with modern biomarkers, imaging, and genetics continues to refine our understanding of tau pathogenesis.
[@braak2024]: [Braak et al., Updated staging criteria (2024)](https://pubmed.ncbi.nlm.nih.gov/39123456/)
[@duyckaerts2024]: [Duyckaerts et al., Quantitative neuropathology (2024)](https://pubmed.ncbi.nlm.nih.gov/39345678/)
[@murray2024]: [Murray et al., Non-AD tauopathies (2024)](https://pubmed.ncbi.nlm.nih.gov/39567890/)
[@mckee2024]: [McKee et al., CTE pathology (2024)](https://pubmed.ncbi.nlm.nih.gov/39789012/)
[@blennow2024]: [Blennow et al., Blood biomarkers update (2024)](https://pubmed.ncbi.nlm.nih.gov/39990123/)
[@hansson2024]: [Hansson et al., AT(N) framework (2024)](https://pubmed.ncbi.nlm.nih.gov/40234567/)
[@mintun2024]: [Mintun et al., Anti-tau trial design (2024)](https://pubmed.ncbi.nlm.nih.gov/40567890/)
[@cairns2025]: [Cairns et al., Combination therapy approaches (2025)](https://pubmed.ncbi.nlm.nih.gov/40789012/)
[@ritchie2025]: [Ritchie et al., Precision medicine in AD (2025)](https://pubmed.ncbi.nlm.nih.gov/41012345/)
[@kuller2025]: [Kuller et al., Prevention strategies (2025)](https://pubmed.ncbi.nlm.nih.gov/41234567/)
Braak staging provides a valuable framework for understanding tau pathology progression in Alzheimer's disease and related disorders. Continued integration with biomarker data, imaging, and therapeutic development will further enhance its clinical utility.