Chronic physiological hypoxia has been implicated in the pathogenesis of multiple system atrophy (MSA), a fatal neurodegenerative disorder of unknown etiology. This prospective, multicenter, observational cohort study (Phase II of the High-Altitude Neurodegeneration Cohort [HANC] study) aims to validate the association between chronic hypoxia exposure and incident MSA risk. A total of 20,000 Han Chinese participants aged 40-75 years will be enrolled from 23 sites across China spanning an altitude gradient from 4 m to 4,500 m. All participants will undergo standardized in-person assessment including questionnaires, physical examination, blood collection, and 3-night consecutive nocturnal pulse oximetry monitoring. Participants are to be followed for incident MSA over 12 months. The primary outcome is newly diagnosed MSA (probable or definite per Gilman consensus criteria), adjudicated by an independent panel of movement disorders specialists. Secondary outcomes include the association between altitude strata and MSA incidence, the association between mean nocturnal SpO₂ and MSA incidence, and incidence rates across MSA subtypes (MSA-P and MSA-C).

Background: Multiple system atrophy (MSA) is a rapidly progressive synucleinopathy characterized by glial cytoplasmic inclusions in oligodendrocytes. Although most cases are considered sporadic, an environmental trigger has not been established. Epidemiological observations have reported disproportionately high MSA prevalence at high altitudes, but these have been dismissed as ascertainment bias. Chronic hypoxia stabilizes hypoxia-inducible factors (HIFs), which regulate mitochondrial gene expression and oxidative stress pathways.

Hypothesis: Chronic physiological hypoxia is an independent causal risk factor for MSA, operating through a HIF-1α-dependent mitochondrial lipid peroxidation cascade.

Study Design: Phase II is a prospective validation cohort designed to replicate findings from the retrospective Phase I (N=284,756). Unlike the retrospective Phase I which relied on healthcare claims data, Phase II collects primary data prospectively using standardized protocols.

Altitude Strata: Participants were enrolled from four altitude categories: (1) Lowland: <500 m (8 sites); (2) Intermediate: 500-2,000 m (7 sites); (3) Highland: 2,000-3,500 m (5 sites); (4) Extreme altitude: >3,500 m (3 sites).

Exposure Assessment: Residential altitude was verified through national identity registry cross-linkage. Nocturnal peripheral oxygen saturation (SpO₂) was measured using Nonin WristOx2 devices sampled at 1 Hz for three consecutive nights.

Outcome Adjudication: All potential MSA cases identified during follow-up will be adjudicated by a panel of five board-certified movement disorders specialists using the Gilman second consensus criteria. Adjudication will be supplemented by brain MRI review and video examination where available. Only probable and definite MSA cases are included in primary analyses.

Statistical Analysis: Cox proportional hazards models will be used to estimate hazard ratios for MSA incidence by altitude category and SpO₂ quartiles, adjusting for age, sex, smoking, pesticide exposure, family history, SNCA genotype, BMI, and occupational solvent exposure. Kaplan-Meier survival curves will compare MSA-free survival across altitude strata.

Inclusion Criteria:

1. Self-identified Han Chinese ethnicity
2. Age between 40 and 75 years (inclusive)
3. No prior diagnosis of parkinsonism at baseline
4. Permanent residence at study site location for ≥1 year prior to enrollment
5. Ability to provide written informed consent

Exclusion Criteria:

1. Pre-existing diagnosis of Parkinson's disease, multiple system atrophy, progressive supranuclear palsy, or any other parkinsonian disorder at baseline
2. Severe chronic pulmonary disease (e.g., COPD GOLD stage ≥3) affecting baseline SpO₂ measurement
3. Severe cardiovascular disease (e.g., New York Heart Association Class III or IV heart failure)
4. Cognitive impairment precluding completion of study procedures
5. Current enrollment in any interventional clinical trial
6. Life expectancy <12 months due to any medical condition

Han Chinese adults aged 40-75 years residing at altitudes ranging from 4 m to 4,500 m across 23 sites in China, with no prior diagnosis of parkinsonism at baseline.

Data will be available beginning 12 months after study completion.

Requests should be directed to the corresponding author; a signed data access agreement will be required.