Leave-one-out sensitivity analysis demonstrated the robust stability of the pooled SMD (Cohen's d range: -0.55 to -0.59) when the individual studies were sequentially excluded (Fig. 23). The overall effect of glymphatic dysfunction (DTI-ALPS index reduction in PD vs. HCs) remained statistically significant across all iterations, with minimal variability in magnitude (Δ Cohen's d = 0.04). The narrow confidence intervals and consistent significance confirmed that no single study disproportionately influenced the meta-analysis results.
Today, several dMRI techniques, including low b-value diffusion-weighted imaging, optimized b-value sampling for dMRI, IVIM, and DCE-MRI for tracking tracer movement, phase-contrast MRI, and ultrafast encephalography measure pulsatile CSF as well as advanced dMRI models, such as diffusion analysis, neurite orientation dispersion and density imaging for disentangling free-water contributions (NODDI), diffusion kurtosis imaging (DKI), and FW indices, have also been developed. Beyond all of these, recent developments have introduced DTI-ALPS to measure diffusion anisotropy, while intrathecal MRI demonstrates only minor CSF influx into the deep white matter, opening new windows into CSF fluid dynamics.
Our DTI-ALPS findings align with and extend those of numerous individual studies reporting glymphatic dysfunction in PD and Parkinsonism. Critically, glymphatic dysfunction progresses with clinical severity and disease duration, independent of age or cognitive status. These results underscore the potential role of the glymphatic system in PD pathophysiology and its relevance as a biomarker for staging and subtyping neurodegenerative processes.
The glymphatic system has been conceptualized as a brain-wide perivascular waste clearance pathway, and multiple lines of evidence have implicated its failure in the pathogenesis of proteinopathies. Notably, animal experiments have shown that glymphatic flux contributes to α-synuclein clearance: suppression of perivascular flow (e.g., via AQP4 knockout) retards α-syn removal and accelerates synucleinopathy in PD models. Thus, our meta-analytic observation of reduced ALPS in PD is consistent with the hypothesis that impaired ISF clearance promotes pathological α-synuclein accumulation.
While a prior meta-analysis established initial evidence for glymphatic impairment in Parkinson's disease by pooling 11 studies, the present work offers a more comprehensive and updated synthesis. By including 28 studies, our analysis provides enhanced statistical power and, more importantly, delves deeper by (1) evaluating glymphatic function across distinct clinical and cognitive phenotypes (e.g., PD with dementia, mild cognitive impairment, and normal cognition), (2) assessing atypical Parkinsonian syndromes, and (3) performing detailed meta-regressions to investigate the association between glymphatic impairment and moderators, such as disease severity and duration. These expanded analyses are crucial for elucidating the role of the glymphatic system in the pathophysiology and progression of Parkinson's disease.
The observed glymphatic impairment may reflect multiple interacting mechanisms, including PVS enlargement due to impaired ISF drainage, astrocytic dysfunction disrupting AQP4 polarization, and neurovascular decoupling reducing pulsatile arterial drives for glymphatic flow. Furthermore, the noradrenergic system, which is known to be compromised in PD, plays a critical role in modulating glymphatic clearance through its control of neurovascular coupling and arousal states, adding another layer to the potential pathophysiology.
The glymphatic pathway normally supports the exchange of CSF-ISF along the PVSs, helping to wash out soluble metabolites (including amyloid-β, tau, and α-synuclein). When this exchange diminishes, neurotoxic proteins may accumulate. In PD, a failure to clear extracellular α-synuclein could exacerbate Lewy-body formation and dopaminergic cell loss. Indeed, glymphatic blockade has been reported to exacerbate motor and memory deficits in PD models. Conversely, protein aggregates may obstruct perivascular channels. This bidirectional link suggests that PD may represent a form of "interstitial fluidopathy", wherein abnormal solute dynamics contribute to disease progression. Our finding that glymphatic dysfunction was most pronounced in cognitively impaired PD patients also contradicts the notion that clearance deficits may underlie cortical neurodegeneration, in line with evidence that sleep disturbances impair glymphatic flow and promoting dementia. This parallels Alzheimer's disease, in which glymphatic transport is thought to influence amyloid-β and tau burdens. The gradient of effect sizes (PD-D > PD-MCI > PD-NC) suggests that glymphatic impairment may worsen with cognitive decline, possibly reflecting spreading pathology or advancing AQP4 mislocalization. Early detection of glymphatic impairment may inform therapeutic interventions aimed at enhancing clearance, such as sleep optimization or targeted CSF circulation therapies. Additionally, the absence of significant impairment in the PD-NC group indicates a potential window for intervention before the manifestation of cognitive deficits. Although causality cannot be inferred here, these results raise the possibility that reduced interstitial flow contributes to the risk of developing dementia in patients with PD.
Patients with PSP, CBS, and MSA-P exhibit profound dysfunction of the glymphatic system. However, significant heterogeneity across these studies likely reflects the diverse neuropathologies underlying atypical Parkinsonism tauopathy in PSP/CBS versus α-synucleinopathy in MSA-P and their distinct effects on perivascular clearance pathways. These results underscore the need for subtype-specific analyses in future studies, as glymphatic impairment may arise through different mechanisms in various Parkinsonian disorders.
Meta-regression analyses identified the H-Y stage and disease duration as independent predictors of glymphatic impairment, with a higher H-Y stage and longer disease duration correlating with reduced DTI-ALPS indices. In early stage PD (H-Y < 2.5), the association between the H-Y stage and glymphatic dysfunction was particularly pronounced, suggesting that glymphatic compromise accelerates during the initial disease phases. These results point towards a progressive decline in glymphatic clearance capacity as PD advances and align with neuropathological models positing that α-synuclein aggregation and neuroinflammation processes, potentially modulated by glymphatic clearance, are already active in early PD. Glymphatic impairment may exacerbate α-synuclein aggregation owing to reduced ISF clearance, creating a vicious cycle of neurodegeneration. Therapeutic strategies that enhance glymphatic function, such as sleep optimization or AQP4 modulation, could mitigate this cascade, as suggested by preclinical models. The persistence of this association after adjusting for age and cognitive status (MoCA scores) further implicates glymphatic dysfunction as a core component of PD progression rather than a secondary epiphenomenon.
We tested whether demographic or technical factors moderated the glymphatic deficits. The PD versus HC effect remained significant across the subgroups without any significant group differences. Stratification by age, MRI coil configuration, diffusion direction, and sex distribution revealed no statistically significant subgroup differences, indicating that glymphatic dysfunction in PD is a generalized phenomenon that transcends these variables. However, nuanced trends have emerged: studies utilizing high-channel MRI coils ( ≥ 32 channels) reported larger SMDs than those with lower-resolution systems. This pattern suggests that advanced imaging protocols may enhance the detection sensitivity for glymphatic alterations, which is a critical consideration in future biomarker studies. Similarly, while age stratification showed comparable SMDs between younger ( ≤ 64 years) and older ( > 64 years) PD cohorts, the former exhibited greater heterogeneity, potentially reflecting earlier and more variable glymphatic decline in patients with younger-onset PD.
To further explore the robustness of our findings, we conducted several subgroup analyses based on other key technical and methodological variables. These analyses revealed that the observed glymphatic dysfunction in Parkinson's disease was largely consistent across different DTI acquisition schemes (multi-shell vs. single-shell), study settings (multi-center vs. single-center), and ROI definition strategies (atlas-based vs. manual; cube/rectangular vs. spherical/circular), as no significant group differences were found in these comparisons. This consistency strengthens the conclusion that the DTI-ALPS index is a reliable marker of glymphatic impairment in PD, independent of these specific methodological choices. However, we did identify a significant moderating effect for ROI size, with studies utilizing a 5 mm ROI reporting a larger effect size than those using smaller ROIs. This suggests that ROI parameterization can influence the magnitude of the detected effect, highlighting the critical need for methodological standardization, particularly regarding ROI dimensions, to improve cross-study comparability and the clinical translation of the DTI-ALPS index.
This study had several limitations that warrant caution. Most studies have been conducted in East Asian populations; therefore, the generalizability of these findings to other ethnicities or milder (earlier) stages of PD remains to be tested. Finally, although publication bias appeared to be minimal, unpublished negative findings may still exist.
The DTI-ALPS index is an indirect surrogate marker of glymphatic flow. This technique assumes that the diffusivity along the medullary veins reflects CSF-ISF exchange; however, this assumption has been questioned. For instance, contrast-enhanced MRI studies suggest that parenchymal CSF penetration into the deep white matter is limited, implying that ALPS may not directly measure true glymphatic flux. Thus, our outcome measure should be viewed as a marker of perivascular microstructure or clearance potential, rather than as definitive proof of altered bulk flow.
The minimal impact of MRI parameters (coil configuration and diffusion direction) on the pooled SMDs supports the reliability of DTI-ALPS as a glymphatic biomarker across imaging protocols. However, the trend toward larger SMDs with high-channel coils suggests that technological advancements could enhance the detection of subtle glymphatic changes, particularly in the pre-symptomatic or prodromal PD stages. Standardization of DTI-ALPS acquisition protocols-including diffusion direction number, echo time, b-values, and automatic region-of-interest placement remains crucial for cross-study comparability and clinical translation. Furthermore, our analysis was constrained by the level of technical detail reported in the primary studies. Factors such as the DTI acquisition scheme (e.g., single-shell vs. multi-shell) and the specific methodology for region-of-interest (ROI) placement (e.g., manual vs. automated, choice of software pipeline) can influence the DTI-ALPS index but were not consistently documented across all included articles. This reporting gap prevented a dedicated subgroup analysis. Future studies should adhere to standardized reporting guidelines to facilitate more precise meta-analyses and clarify the impact of these technical variables on glymphatic assessment.
Future longitudinal and multiparametric studies of DTI-ALPS (or other glymphatic metrics) changes over time in PD and whether baseline ALPS predicts subsequent motor or cognitive decline are needed. Studies on prodromal conditions (e.g., REM sleep behavior disorder) could clarify whether glymphatic impairment precedes clinical PD. Combining DTI-ALPS with other imaging markers, such as the quantitative PVS burden, AQP4 positron emission tomography, or intrathecal contrast MRI, would help validate its meaning. Interventional studies are also of interest; for example, improving sleep quality or targeting AQP4 polarization may enhance glymphatic clearance and slow the progression of the disease. Moreover, future research should aim to integrate DTI-ALPS with fluid biomarkers. An important next step would be to investigate the relationship between the DTI-ALPS index and CSF concentrations of pathological proteins, such as α-synuclein. Such studies would help validate whether a lower ALPS index is functionally linked to impaired clearance of neurotoxic solutes from the brain interstitium.
Finally, another significant limitation is the potential for unmeasured confounding factors. Glymphatic function is influenced by a wide range of variables, including vascular risk factors (e.g., lipid profile, glucose intolerance), cerebral small vessel disease, sleep quality, and common PD co-morbidities such as depression, anxiety, and autonomic dysfunction. However, the majority of the studies included in our meta-analysis did not systematically report or control for these potential confounders, primarily adjusting only for age and sex. This reporting gap prevented us from performing a comprehensive meta-regression to evaluate the impact of these variables on the DTI-ALPS index. Therefore, while our findings are robust, the potential influence of these unaddressed factors cannot be dismissed and highlights the need for future studies with more rigorous control of such variables.
To conclude, this meta-analysis provides robust evidence that glymphatic dysfunction is a significant and consistent feature of Parkinsonian disorders. Our findings strongly support the role of glymphatic impairment in PD pathophysiology, progressing alongside clinical severity (H-Y stage) and disease duration, with an accelerated decline observed in early-stage PD. The analysis highlights distinct patterns across phenotypes, with severe dysfunction in PD-Dementia and atypical Parkinsonian syndromes, in contrast to the relatively preserved glymphatic activity in cognitively intact patients. Furthermore, these findings remained robust across most demographic, technical, and methodological subgroups, emphasizing the core role of clinical progression. However, the significant moderating effect of ROI size highlights the need for standardized imaging protocols to enhance cross-study comparability. While acknowledging the inherent limitations of the DTI-ALPS index as an indirect measure, these findings underscore its potential as a valuable biomarker for disease staging and phenotypic differentiation, offering important insights for future therapeutic strategies targeting glymphatic pathways in PD.