The objective of this observational study is to investigate whether non-invasive fundus retinal detection technology can be used for the early diagnosis of parkinson's disease (PD). It aims to answer the following primary questions: the sensitivity and specificity of non-invasive fundus retinal detection technology in the early diagnosis of PD; and whether this technology offers advantages over dopamine transporter positron emission tomography (DAT-PET), a conventional screening method for PD. The researchers will analyze the diagnostic performance of this technology for early-stage PD patients among cohorts including early parkinson's disease, parkinson's syndromes, essential tremor patients, and healthy individuals. Furthermore, in PD patients who concurrently undergo DAT-PET imaging, the study will compare the diagnostic value of non-invasive retinal imaging against that of DAT-PET.

Inclusion Criteria:

• 1. Aged 40-80 years; 2. Parkinson's Disease Group: Patients who meet the 2015 International Movement Disorder Society clinical diagnostic criteria for Parkinson's disease, with a Hoehn and Yahr stage of 1 to 2.5 and a disease duration of ≤5 years; 3. Parkinson's Syndromes Group: Patients who meet the International Movement Disorder Society diagnostic criteria for atypical parkinson's syndromes (including progressive supranuclear palsy, dementia with Lewy bodies, and corticobasal degeneration), multiple system atrophy (P subtype), or secondary parkinson's syndrome (drug-induced: having taken antipsychotic medications for ≥3 months; vascular: confirmed by brain MRI showing leukoaraiosis/lacunar infarction), with a disease duration of ≤5 years; 4. Essential Tremor Group: Patients who meet the 2018 International Movement Disorder Society clinical diagnostic criteria for essential tremor, with no parkinsonian symptoms, and a disease duration of ≥1 year; 5. Healthy Control Group: Individuals with no parkinsonian symptoms (confirmed by neurological examination) and no use of antipsychotic/dopaminergic medications in the past 3 months; 6. All subjects have provided written informed consent and are willing to comply with the study procedures.

Exclusion Criteria:

• 1. Presence of severe ocular fundus diseases, such as glaucoma, cataract, retinal detachment, macular degeneration, etc; 2. Inability to tolerate the non-invasive fundus retinal detection; 3. Known allergy or investigator-suspected high risk of allergy to anti-PD drugs; 4. Presence of severe cardiovascular or cerebrovascular diseases (e.g., coronary heart disease, myocardial infarction, stroke, etc.), hepatic or renal dysfunction, cancer, or other conditions that may affect the prognosis; 5. Pregnancy or lactation; 6. Presence of active infectious diseases (e.g., tuberculosis, AIDS) or systemic inflammatory diseases (e.g., rheumatoid arthritis); 7. Presence of psychiatric disorders.

This study will enroll four distinct groups: (1)patients who meet the 2015 International Movement Disorder Society clinical diagnostic criteria for Parkinson's disease, with a Hoehn and Yahr stage of 1 to 2.5 and a disease duration of ≤5 years; (2)patients with Parkinson's syndromes meeting respective MDS criteria; (3) patients with essential tremor meeting the 2018 MDS clinical diagnostic criteria; and (4) healthy controls. Detailed inclusion and exclusion criteria are listed in Eligibility Criteria.