Neurodegenerative diseases progress through defined stages of biological and clinical change, from the earliest detectable biomarker alterations through to end-stage neurodegeneration. Each disease has its own staging framework, rooted in distinct neuropathology — amyloid-beta/tau for Alzheimer's Disease, alpha-synuclein for Parkinson's Disease, TDP-43 for ALS and FTD. Yet there are remarkable commonalities: all begin with a prodromal phase of biomarker abnormalities preceding clinical symptoms, progress through characteristic neuroanatomical sequences, and culminate in widespread neuronal loss. Understanding these staging mechanisms is essential for early diagnosis, clinical trial design, and therapeutic intervention timing.
This synthesis maps disease progression frameworks across the major neurodegenerative diseases, compares staging architectures, identifies cross-disease patterns, and highlights the therapeutic implications of staging-based intervention windows.
The 2018 NIA-AA Research Framework[@jack2018niaaa] defines Alzheimer's disease by biomarker profiles rather than clinical syndrome, enabling biological staging independent of symptom onset. The AT(N) classification categorizes biomarkers into three pathologic domains:
| Stage | A | T | (N) | Clinical | Description |
|---|---|---|---|---|---|
| A−T−(N)− | No | No | No | Normal | Normal AD biomarkers |
| A+T−(N)− | Yes | No | No | Preclinical AD | Isolated amyloid pathology |
| A+T+(N)− | Yes | Yes | No | Preclinical AD | Amyloid + tau, cognitively normal |
| A+T+(N)+ | Yes | Yes | Yes | MCI or dementia | Confirmed AD pathophysiology |
The preclinical AD phase (A+T−(N)− and A+T+(N)−) can persist for 15–20 years before symptom onset[@villain2012displaying]. Tau PET studies show that tauopathy spreads in a predictable spatial pattern: from entorhinal cortex → hippocampus → limbic regions → isocortical association areas[@schwarz2016comparing].
For patients with symptoms, the Clinical Dementia Rating (CDR) scale provides a 0–3 staging of dementia severity:
The Thal amyloid staging system[@thal2005phases] defines five sequential phases of amyloid plaque deposition observable at autopsy:
Biomarker studies establish a characteristic temporal ordering[@villain2012displaying][@blennow2005gamma]:
The Braak staging system[@braak2003staging][@braak2004neuroanatomy] describes the stereotypic propagation of alpha-synuclein (a-syn) pathology through the nervous system:
| Stage | Primary Sites | Clinical Correlation |
|---|---|---|
| Stage 1 | Olfactory bulb, enteric nervous system | Preclinical, anosmia possible |
| Stage 2 | Lower brainstem (dorsal motor nucleus, raphe nuclei, coeruleus) | Prodromal PD, RBD onset |
| Stage 3 | Upper brainstem (substantia nigra pars compacta) | Motor symptoms emerge (PD diagnosis) |
| Stage 4 | Limbic system (amygdala, hippocampus) | Cognitive changes appear |
| Stage 5 | Neocortex (associative areas) | Dementia, hallucinations |
| Stage 6 | Primary sensory/motor cortex | Advanced PD, severe cognitive decline |
This staging suggests that PD begins in the periphery (olfactory and enteric nervous system) and spreads centripetally to the CNS. The gut-first hypothesis posits that alpha-synuclein pathology initiates in the enteric nervous system, propagating via the vagus nerve to the dorsal motor nucleus[@sanchez2018gut].
The Movement Disorder Society (MDS) Clinical Staging of PD[@goetz2008movement]:
The prodromal phase of PD encompasses a decade or more before motor diagnosis[@siderowf2018clinical]:
Molecular imaging (DAT-SPECT) shows dopaminergic denervation 3–5 years before motor diagnosis[@hughes1992accuracy].
ALS progresses through a clinically-defined staging system[@balendra2024staging]:
| Stage | Criteria | Functional Status |
|---|---|---|
| Stage 1 | Diagnosis | Functional independence, 1-2 regions affected |
| Stage 2 | 3 body regions OR first nutritional/respiratory failure | Multiplying regions, beginning support needs |
| Stage 3 | Second intervention required | Wheelchair-dependent,PEG/RIP needs |
| Stage 4 | One intervention fulfilled | Advanced disease, comprehensive care |
| Stage 5 | Both interventions fulfilled | End-stage, hospice/palliative |
The King's staging is based on the El Escorial criteria regions (bulbar, cervical, thoracic, lumbar) and the need for nutritional (PEG) or respiratory (non-invasive ventilation) support.
MITOS is a simpler 0-4 staging based on functional progression[@chio2009validation]:
Key biomarkers for ALS staging and progression tracking:
FTD encompasses multiple clinical variants and underlying pathologies, complicating unified staging[@rack:FTDstaging]:
| Stage | Clinical Features | Biomarker Changes |
|---|---|---|
| Preclinical | Asymptomatic, possible biomarker changes | CSF p-tau217, NfL elevated years before |
| Prodromal | Subtle personality changes, mild executive dysfunction | FDG-PET hypometabolism in frontal/temporal |
| Manifest | Clear behavioral/language syndrome | MRI cortical atrophy in affected regions |
| Advanced | Global cognitive decline, motor neuron involvement | Widespread atrophy, biomarker normalization |
The FTD-ALS spectrum represents a continuum[@lillo2018overlap]:
The NINDS PSP criteria define phenotypic variants and disease progression[@hoglinger2017clinical]:
| Stage | Dominant Features | Progression Pattern |
|---|---|---|
| PSP-P | Richardson's syndrome: vertical gaze palsy, early falls | Cortical-subcortical involvement |
| PSP-PAGF | Pure akinesia with gait freezing | Isolated motor dysfunction |
| PSP-PGF | Progressive gait freezing | Frontal executive preserved longer |
| PSP-CBS | Corticobasal syndrome features | Asymmetric cortical involvement |
Corticobasal syndrome is clinically heterogeneous[@ripley2020corticobasal), with progression varying by underlying pathology (CBD pathology vs. AD pathology vs. PSP pathology):
Despite distinct pathologies, all major neurodegenerative diseases share staging principles:
| Principle | AD | PD | ALS | FTD | PSP |
|---|---|---|---|---|---|
| Biomarker prodromal phase | 15-20 years | 5-10 years | 1-2 years | 5-15 years | 2-5 years |
| Neuroanatomical sequence | EC → HC → Limbic → Cortex | ENS → Brainstem → SN → Limbic → Cortex | Lower motor → Upper motor → Bulbar → Cognitive | Frontal/Temporal → Widespread | Subcortical → Cortical |
| Clinical threshold | CDR 0.5 | MDS Stage 1 | King's Stage 1 | Clinical diagnosis | NINDS Stage 1 |
| Primary pathology | Aβ plaques + tau NFTs | α-Syn inclusions | TDP-43 inclusions | TDP-43 or tau | 4R tau |
| Propagation mechanism | Templated misfolding, prion-like | Prion-like, transsynaptic | Prion-like, motor neuron-to-neuron | Prion-like, transsynaptic | Unknown templating |
| Disease | Typical Duration | Prodromal to Diagnosis | Diagnosis to Death |
|---|---|---|---|
| AD | 8-15 years | 15-20 years | 8-12 years |
| PD | 15-25 years | 5-10 years | 10-15 years |
| ALS | 2-5 years | 1-2 years | 2-5 years |
| FTD | 6-12 years | 5-15 years | 3-10 years |
| PSP | 5-9 years | 2-5 years | 3-7 years |
Several biomarker classes serve as cross-disease staging markers:
The staging frameworks reveal critical windows for therapeutic intervention:
| Disease | Optimal Intervention Window | Rationale |
|---|---|---|
| AD | Preclinical (A+ T−) to early MCI | Prevent tau spread, preserve neuronal networks |
| PD | Prodromal (RBD, anosmia) to Stage 1 | Prevent alpha-synuclein propagation beyond brainstem |
| ALS | At diagnosis (King's Stage 1) | Rapid progression leaves minimal window |
| FTD | Genetic carriers, prodromal | C9orf72, GRN, MAPT carriers identifiable pre-symptomatically |
| PSP | Early (within 2 years of onset) | More responsive to tau-targeting before widespread atrophy |
| Endpoint Type | Application | Staging Phase |
|---|---|---|
| Biomarker (CSF, PET) | Primary endpoint | Preclinical, prodromal |
| CDRSB, MDS-UPDRS | Co-primary with biomarker | Symptomatic |
| ADCS-ADL, ALSFRS-R | Functional endpoints | All stages |
| Survival / time-to-event | Regulatory approval | Advanced disease |
Unified neurodegenerative staging: No cross-disease staging framework exists. Developing a common language would facilitate trial design and comparison across diseases.
Prodromal biomarker validation: While Aβ PET and dopamine imaging are validated, better peripheral biomarkers (blood-based) are needed for prodromal detection in PD and ALS.
Staging-linked therapeutic response: Do specific disease stages respond preferentially to certain therapies? Anti-Aβ antibodies show greater effect in early-stage patients (Clarity AD, TRAILBLAZER)[@van2024donanemab].
Individual variation in staging: Why do some patients deviate from the stereotypic Braak/Thal staging patterns? Patient-specific factors (genetics, comorbidities, reserve) modulate staging.
Retinal and other peripheral staging markers: Retinal OCT imaging is emerging as a non-invasive proxy for CNS staging in AD and PD[@chehelomomi2023retinal].
Tau staging variability in 4R tauopathies: PSP and CBS show more variable topographic progression compared to AD's stereotyped Braak-like tau spread[@mccluskey2022tau].
The analysis reveals several unifying principles: