Specificity of Non-Motor Symptoms in Parkinson's Disease: A Systematic Review and Meta-Analysis

This is a systematic review and meta-analysis on the diagnostic specificity of non-motor symptoms in Parkinson's disease, aiming to address the clinical challenge of delayed early diagnosis of Parkinson's disease. By integrating data from 28 studies involving 7393 participants, the study found that non-motor symptoms such as hyposmia and rapid eye movement sleep behavior disorder (RBD) have high specificity for Parkinson's disease, providing evidence-based support for early identification and significant implications for the development of screening tools, risk stratification, and differential diagnosis.

Parkinson's disease is the second most common neurodegenerative disease globally. Traditional diagnosis relies on motor symptoms such as tremors, but by then, patients have already lost 60-80% of dopaminergic neurons. Non-motor symptoms (e.g., hyposmia, RBD) can appear 10-20 years earlier than motor symptoms, are easy to assess, and non-invasive, but their specificity is controversial. Individual studies have limited samples, so meta-analysis is needed to integrate data for reliable conclusions.

Literature Search: Covered MEDLINE, Embase, BIOSIS, and SciSearch databases up to January 2025. Inclusion and Exclusion Criteria: Included observational studies (with PD patients and control groups, reporting NMS incidence/prevalence); excluded case reports, studies without control groups, and those with incomplete data. Data Extraction and Quality Assessment: Extracted study characteristics, participant information, NMS types, and result data; assessed quality using the Newcastle-Ottawa Scale. Statistical Analysis: Random-effects model was used to calculate pooled risk ratios (RR) and 95% confidence intervals (CI); I² was used to assess heterogeneity; sensitivity and subgroup analyses were performed.

Study Overview: Included 28 studies with 7393 participants. Key Results:

• Hyposmia: RR=7.3 (95% CI: 5.05-10.57)
• RBD: RR=7.3 (95% CI:3.61-11.64)
• Constipation: RR=4.67 (95% CI:3.07-7.10)
• Sialorrhea: RR=6.48 Subgroup Analysis: Higher specificity in people over 55 years old; symptom profiles differ between early and late PD; effect size of prospective cohort studies is larger than that of cross-sectional studies.

Early Screening: Provides a basis for scoring systems such as the MDS prodromal criteria; integrating multiple NMS improves identification accuracy. Risk Stratification: Close follow-up can be conducted for high-risk individuals (with multiple NMS) to explore the neuroprotective effects of lifestyle interventions (exercise, diet). Differential Diagnosis: The NMS profile can help distinguish PD from other parkinsonian syndromes (e.g., progressive supranuclear palsy).

Limitations: High heterogeneity among included studies (differences in population and symptom definitions); presence of publication bias; observational studies cannot establish causality; predictive value not assessed. Future Directions:

• Integration of multimodal biomarkers (imaging + body fluids)
• Large-sample prospective cohort to track conversion status
• Development of personalized risk models using AI algorithms
• Neuroprotective intervention trials in prodromal populations

Medical Consultation Indications: Persistent hyposmia, RBD (vivid dreams + limb movements), long-term constipation (unresponsive to conventional treatment), unexplained depression/anxiety, mild cognitive decline (multiple symptoms require attention). Lifestyle: Regular aerobic exercise, Mediterranean diet, adequate sleep, maintaining social activities (beneficial for overall health).

This study provides important evidence-based support for the early identification of PD; non-motor symptoms are the 'window' to the early stage of the disease. In the future, multidisciplinary collaboration (basic science, clinical practice, epidemiology, etc.) is needed to shift from descriptive to predictive research, integrate multimodal data, and ultimately achieve early diagnosis and treatment for the benefit of patients.