Neuroimaging evidence of glymphatic system dysfunction in possible REM sleep behavior disorder and Parkinson's disease

Interplay between the glymphatic system and neurotoxic proteins in Parkinson's disease and related disorders: current knowledge and future directions

Parkinson's disease is a common neurodegenerative disorder that is associated with abnormal aggregation and accumulation of neurotoxic proteins, including α-synuclein, amyloid-β, and tau, in addition to the impaired elimination of these neurotoxic protein. Atypical parkinsonism, which has the same clinical presentation and neuropathology as Parkinson's disease, expands the disease landscape within the continuum of Parkinson's disease and related disorders. The glymphatic system is a waste clearance system in the brain, which is responsible for eliminating the neurotoxic proteins from the interstitial fluid. Impairment of the glymphatic system has been proposed as a significant contributor to the development and progression of neurodegenerative disease, as it exacerbates the aggregation of neurotoxic proteins and deteriorates neuronal damage. Therefore, impairment of the glymphatic system could be considered as the final common pathway to neurodegeneration. Previous evidence has provided initial insights into the potential effect of the impaired glymphatic system on Parkinson's disease and related disorders; however, many unanswered questions remain. This review aims to provide a comprehensive summary of the growing literature on the glymphatic system in Parkinson's disease and related disorders. The focus of this review is on identifying the manifestations and mechanisms of interplay between the glymphatic system and neurotoxic proteins, including loss of polarization of aquaporin-4 in astrocytic endfeet, sleep and circadian rhythms, neuroinflammation, astrogliosis, and gliosis. This review further delves into the underlying pathophysiology of the glymphatic system in Parkinson's disease and related disorders, and the potential implications of targeting the glymphatic system as a novel and promising therapeutic strategy.

Early detection of dopaminergic dysfunction and glymphatic system impairment in Parkinson's disease

PURPOSE

This study aimed to assess the glymphatic function and its correlation with clinical characteristics and the loss of dopaminergic neurons in Parkinson's disease (PD) using hybrid positron emission tomography (PET)-magnetic resonance imaging (MRI) combined with diffusion tensor image analysis along the perivascular space (DTI-ALPS), choroid plexus volume (CPV), and enlarged perivascular space (EPVS) volume.

METHODS

Twenty-five PD patients and thirty matched healthy controls (HC) participated in the study. All participants underwent 18F-fluorodopa (18F-DOPA) PET-MRI scanning. The striatal standardized uptake value ratio (SUVR), DTI-ALPS index, CPV, and EPVS volume were calculated. Furthermore, we also analysed the relationship between the DTI-ALPS index, CPV, EPVS volume and striatal SUVR as well as clinical characteristics of PD patients.

RESULTS

PD patients demonstrated significantly lower values in DTI-ALPS (t = 3.053, p = 0.004) and larger CPV (t = 2.743, p = 0.008) and EPVS volume (t = 2.807, p = 0.008) compared to HC. In PD group, the ALPS-index was negatively correlated with the Unified Parkinson's Disease Rating Scale III (UPDRS-III) scores (r = -0.730, p < 0.001), and positively correlated with the mean putaminal SUVR (r = 0.560, p = 0.007) and mean caudal SUVR (r = 0.459, p = 0.032). Moreover, the mean putaminal SUVR was negatively associated with the UPDRS-III scores (r = -0.544, p = 0.009).

CONCLUSION

DTI-ALPS has the potential to uncover glymphatic dysfunction in patients with PD, with this dysfunction correlating strongly with the severity of disease, together with the mean putaminal and caudal SUVR. PET- MRI can serve as a potential multimodal imaging biomarker for early-stage PD.

Glymphatic function from diffusion-tensor MRI to predict conversion from mild cognitive impairment to dementia in Parkinson's disease

BACKGROUND

Although brain glymphatic dysfunction is a contributing factor to the cognitive deficits in Parkinson's disease (PD), its role in the longitudinal progression of cognitive dysfunction remains unknown.

OBJECTIVE

To investigate the glymphatic function in PD with mild cognitive impairment (MCI) that progresses to dementia (PDD) and to determine its predictive value in identifying individuals at high risk for developing dementia.

METHODS

We included 64 patients with PD meeting criteria for MCI and categorized them as either progressed to PDD (converters) (n = 29) or did not progress to PDD (nonconverters) (n = 35), depending on whether they developed dementia during follow-up. Meanwhile, 35 age- and gender-matched healthy controls (HC) were included. Bilateral diffusion-tensor imaging analysis along the perivascular space (DTI-ALPS) indices and enlarged perivascular spaces (EPVS) volume fraction in bilateral centrum semiovale, basal ganglia (BG), and midbrain were compared among the three groups. Correlations among the DTI-ALPS index and EPVS, as well as cognitive performance were analyzed. Additionally, we investigated the mediation effect of EPVS on DTI-ALPS and cognitive function.

RESULTS

PDD converters had lower cognitive composites scores in the executive domains than did nonconverters (P < 0.001). Besides, PDD converters had a significantly lower DTI-ALPS index in the left hemisphere (P < 0.001) and a larger volume fraction of BG-PVS (P = 0.03) compared to HC and PDD nonconverters. Lower DTI-ALPS index and increased BG-PVS volume fraction were associated with worse performance in the global cognitive performance and executive function. However, there was no significant mediating effect. Receiver operating characteristic analysis revealed that the DTI-ALPS could effectively identify PDD converters with an area under the curve (AUC) of 0.850.

CONCLUSION

The reduction of glymphatic activity, measured by the DTI-ALPS, could potentially be used as a non-invasive indicator in forecasting high risk of dementia conversion before the onset of dementia in PD patients.

Cross-Vendor Test-Retest Validation of Diffusion Tensor Image Analysis along the Perivascular Space (DTI-ALPS) for Evaluating Glymphatic System Function

The diffusion tensor image analysis along the perivascular space (DTI-ALPS) method was proposed to evaluate glymphatic system (GS) function. However, few studies have validated its reliability and reproducibility. Fifty participants' DTI data from the MarkVCID consortium were included in this study. Two pipelines by using DSI studio and FSL software were developed for data processing and ALPS index calculation. The ALPS index was obtained by the average of bilateral ALPS index and was used for testing the cross-vendor, inter-rater and test-retest reliability by using R studio software. The ALPS index demonstrated favorable inter-scanner reproducibility (ICC=0.77 to 0.95, P< 0.001), inter-rater reliability (ICC=0.96 to 1, P< 0.001) and test-retest repeatability (ICC=0.89 to 0.95, P< 0.001), offering a potential biomarker for in vivo evaluation of GS function.

Abnormal Cerebrovascular Activity, Perfusion, and Glymphatic Clearance in Lewy Body Diseases

Cerebrovascular activity is not only crucial to optimal cerebral perfusion, but also plays an important role in the glymphatic clearance of interstitial waste, including α-synuclein. This highlights a need to evaluate how cerebrovascular activity is altered in Lewy body diseases. This review begins by discussing how vascular risk factors and cardiovascular autonomic dysfunction may serve as upstream or direct influences on cerebrovascular activity. We then discuss how patients with Lewy body disease exhibit reduced and delayed cerebrovascular activity, hypoperfusion, and reductions in measures used to capture cerebrospinal fluid flow, suggestive of a reduced capacity for glymphatic clearance. Given the lack of an existing framework, we propose a model by which these processes may foster α-synuclein aggregation and neuroinflammation. Importantly, this review highlights several avenues for future research that may lead to treatments early in the disease course, prior to neurodegeneration. © 2024 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Dynamic changes in brain glymphatic function during preoperative chemotherapy in breast cancer patients

OBJECTIVE

The current study aimed to investigate the dynamic changes in brain glymphatic function during chemotherapy in breast cancer patients (BCP) and their correlation with cognitive function.

MATERIALS AND METHODS

A total of 40 healthy female participants (control group) and 80 female BCP were included. Various cognitive assessment tools were used to evaluate cognitive function. Diffusion tensor imaging along the perivascular space was employed to measure brain glymphatic function.

RESULTS

Following chemotherapy, BCP exhibited a significant decline in various cognitive scores. After chemotherapy, the along the perivascular space index, a parameter indicating brain glymphatic function, was slightly higher than that at baseline and the control group levels and was correlated with cognitive scores.

CONCLUSION

This study unveiled a close relationship between the dynamic changes in brain glymphatic function after chemotherapy and cognitive function in BCP. Our findings contribute to a deeper understanding of the brain mechanisms underlying chemotherapy-related cognitive impairment and provide a theoretical basis for future interventions and treatments. In addition, they offer a new perspective for exploring the relationship between brain function and cognitive states.

Deep cervical lymph nodes in Parkinson's disease and atypical Parkinson's disease: A potential ultrasound biomarker for differential diagnosis

Background

Parkinson's disease (PD) is a common degenerative disease caused by abnormal accumulation of α-synuclein. The glymphatic pathway is essential for removing macromolecular proteins including α-synuclein from the brain, which flows into deep cervical lymph nodes (DCLNs) through meningeal lymphatics. As a terminal station for the cerebral lymphatic system drainage, DCLNs can be easily assessed clinically.

Objectives

Although the drainage function of the cerebral lymphatic system is impaired in PD, the correlation between DCLNs and PD remains unknown.

Design

Single-center retrospective cross-sectional study.

Methods

The size of the DCLNs were measured using ultrasound. The Movement Disorder Society Sponsored Revision Unified Parkinson's Disease Rating Scale and other scales were used to assess PD motor and non-motor symptoms.

Results

Compared with the healthy control (HC) and the atypical Parkinson's disease (AP) groups, the size of the second and third DCLNs in the Parkinson's disease (PD) group was significantly smaller (P < .05). The width diameter of the third DCLN (DCLN3(y)) was significantly smaller in the PD group than in the AP group (P = .014). DCLN3(y) combined with a variety of clinical features improved the sensitivity of AP identification (sensitivity = .813).

Conclusion

DCLNs were able to distinguish HC, PD and AP and were mainly located in Robbins ΙΙA level. PD and AP were associated with different factors that influenced the size of the DCLNs. DCLN3(y) plays an important role in differentiating PD from AP, which, combined with other clinical features, has the ability to distinguish PD from AP; in particular, the sensitivity of AP diagnosis was improved.

Diffusion Tensor Imaging-Along the Perivascular-Space Index Is Associated with Disease Progression in Parkinson's Disease

BACKGROUND

The glymphatic clearance pathway is a waste clearance system that allows for removal of soluble proteins such as amyloid β (Aβ) from the brain. Higher Aβ levels are associated with cognitive dysfunction in Parkinson's disease (PD). Diffusion tensor imaging-along the perivascular space (DTI-ALPS) is an imaging measure proposed to indirectly measure glymphatic function.

OBJECTIVES

Evaluate differences in DTI-ALPS-index between PD and healthy controls (HC) and characterize relationships between this proposed measure of glymphatic clearance, cognition, and disease severity in PD.

METHODS

PD (n = 32) and HC (n = 23) participants underwent brain imaging to assess DTI-ALPS. PD participants were classified as PD-normal cognition (PD-NC; n = 20) or PD-mild cognitive impairment (PD-MCI; n = 12) based on a Level II comprehensive cognitive assessment. A subgroup of PD participants (n = 21) returned for annual assessments for up to 4 years after baseline. Longitudinal outcomes included changes in performance on the comprehensive cognitive assessment and changes in the Movement Disorders Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS).

RESULTS

PD participants had lower DTI-ALPS-index compared to HC. PD participants classified as PD-MCI had significantly lower DTI-ALPS-index compared to PD-NC. Lower DTI-ALPS-index at baseline was associated with longitudinal cognitive decline and worse longitudinal disease severity.

CONCLUSIONS

Glymphatic clearance, as measured with DTI-ALPS, has potential to serve as a marker of longitudinal disease progression. Interventions targeting glymphatic function should be explored for potential to slow cognitive decline in PD. © 2024 International Parkinson and Movement Disorder Society.

Decreased DTI-ALPS index in delirium: a preliminary MRI study

PURPOSE

Delirium is linked to brain abnormalities, yet the role of the glymphatic system is not well understood. This study aims to examine alterations in brain physiology in delirium by using diffusion-tensor imaging (DTI) to assess water diffusion along the perivascular space (ALPS) and to explore its correlation with clinical symptoms.

METHODS

We examined 15 patients with delirium and 15 healthy controls, measuring water diffusion metrics along the x-, y-, and z-axes in both projection and association fibers to determine the DTI-ALPS index. We used a general linear model, adjusted for age and sex, to compare the DTI-ALPS index between groups. We also investigated the relationship between the DTI-ALPS index and clinical symptoms using partial correlations.

RESULTS

Patients with delirium exhibited significantly lower DTI-ALPS indices compared to healthy controls (1.25 ± 0.15 vs. 1.38 ± 0.10, t = 2.903, p = 0.007; 1.27 ± 0.16 vs. 1.39 ± 0.08, 1.22 ± 0.16 vs. 1.37 ± 0.14, t = 2.617, p = 0.014; t = 2.719, p = 0.011; respectively). However, there was no significant correlation between the DTI-ALPS index and clinical symptoms.

CONCLUSION

Our findings indicate a decreased DTI-ALPS index in patients with delirium, suggesting potential alterations in brain physiology that may contribute to the pathophysiology of delirium. This study provides new insights into the mechanisms underlying delirium.

Contribution of Nutritional, Lifestyle, and Metabolic Risk Factors to Parkinson's Disease

BACKGROUND

Modifiable risk factors for Parkinson's disease (PD) are poorly known.

OBJECTIVES

The aim is to evaluate independent associations of different nutritional components, physical activity, and sedentary behavior and metabolic factors with the risk of PD.

METHODS

In this population-based prospective cohort study using the data of the United Kingdom Biobank (from 2006-2010), 502,017 men and women who were free from PD (International Classification of Diseases 10th edition; "G20") at baseline were included. We implemented a Cox proportion hazard's model to evaluate the associations of different levels of physical activity, sitting time, sleep habits, diet quality, alcohol and coffee consumption, smoking, and body mass index with PD risk, adjusting for several confounding variables.

RESULTS

During a median follow-up of 12.8 years, lifestyle factors including vigorous physical activity (hazard ration [HR] = 0.84; 95% confidence interval [CI], 0.75-0.94), low-to-moderate sitting time (HR = 0.89; 95% CI, 0.81-0.97), and high sleep quality (HR = 0.89; 95% CI, 0.80-0.99) were associated with a reduced risk of PD. Small amounts of coffee (HR = 0.88; 95% CI, 0.82-0.95), red meat (HR = 0.86; 95% CI, 0.76-0.97), and current smoking (HR = 0.65; 95% CI, 0.56-0.75) were also associated with a lower risk of PD, whereas alcohol intake (HR = 1.29; 95% CI, 1.06-1.56) with higher PD risk. Secondary analysis, including metabolic risk factors, confirmed these findings and highlighted the potential protective effect of plasma vitamin D and uric acid, but of low-density lipoprotein-cholesterol, triglycerides, and C-reactive protein as well.

CONCLUSIONS

Vigorous physical activity, reduced sitting time, good sleep quality together with small coffee intake and vitamin D supplementation are potentially neuroprotective lifestyle interventions for the prevention of PD. © 2024 International Parkinson and Movement Disorder Society.

Diffusion Tensor Image Analysis ALong the Perivascular Space (DTI-ALPS): Revisiting the Meaning and Significance of the Method

More than 5 years have passed since the Diffusion Tensor Image Analysis ALong the Perivascular Space (DTI-ALPS) method was proposed with the intention of evaluating the glymphatic system. This method is handy due to its noninvasiveness, provision of a simple index in a straightforward formula, and the possibility of retrospective analysis. Therefore, the ALPS method was adopted to evaluate the glymphatic system for many disorders in many studies. The purpose of this review is to look back and discuss the ALPS method at this moment.The ALPS-index was found to be an indicator of a number of conditions related to the glymphatic system. Thus, although this was expected in the original report, the results of the ALPS method are often interpreted as uniquely corresponding to the function of the glymphatic system. However, a number of subsequent studies have pointed out the problems on the data interpretation. As they rightly point out, a higher ALPS-index indicates predominant Brownian motion of water molecules in the radial direction at the lateral ventricular body level, no more and no less. Fortunately, the term "ALPS-index" has become common and is now known as a common term by many researchers. Therefore, the ALPS-index should simply be expressed as high or low, and whether it reflects a glymphatic system is better to be discussed carefully. In other words, when a decreased ALPS-index is observed, it should be expressed as "decreased ALPS-index" and not directly as "glymphatic dysfunction". Recently, various methods have been proposed to evaluate the glymphatic system. It has become clear that these methods also do not seem to reflect the entirety of the extremely complex glymphatic system. This means that it would be desirable to use various methods in combination to evaluate the glymphatic system in a comprehensive manner.

Imaging Interstitial Fluid With MRI: A Narrative Review on the Associations of Altered Interstitial Fluid With Vascular and Neurodegenerative Abnormalities

Interstitial fluid (ISF) refers to the fluid between the parenchymal cells and along the perivascular spaces (PVS). ISF plays a crucial role in delivering nutrients and clearing waste products from the brain. This narrative review focuses on the use of MRI techniques to measure various ISF characteristics in humans. The complementary value of contrast-enhanced and noncontrast-enhanced techniques is highlighted. While contrast-enhanced MRI methods allow measurement of ISF transport and flow, they lack quantitative assessment of ISF properties. Noninvasive MRI techniques, including multi-b-value diffusion imaging, free-water-imaging, T2-decay imaging, and DTI along the PVS, offer promising alternatives to derive ISF measures, such as ISF volume and diffusivity. The emerging role of these MRI techniques in investigating ISF alterations in neurodegenerative diseases (eg, Alzheimer's disease and Parkinson's disease) and cerebrovascular diseases (eg, cerebral small vessel disease and stroke) is discussed. This review also emphasizes current challenges of ISF imaging, such as the microscopic scale at which ISF has to be measured, and discusses potential focus points for future research to overcome these challenges, for example, the use of high-resolution imaging techniques. Noninvasive MRI methods for measuring ISF characteristics hold significant potential and may have a high clinical impact in understanding the pathophysiology of neurodegenerative and cerebrovascular disorders, as well as in evaluating the efficacy of ISF-targeted therapies in clinical trials. LEVEL OF EVIDENCE: 3 TECHNICAL EFFICACY: Stage 2.

Altered Brain Glymphatic Function at Diffusion-Tensor MRI in Pre-cirrhotic Metabolic Dysfunction-Associated Fatty Liver Disease

RATIONALE AND OBJECTIVES

To evaluate glymphatic function changes and their relationships with clinical features in patients with metabolic dysfunction-associated fatty liver disease (MAFLD), thereby facilitating early intervention before this disease progresses to cirrhosis.

MATERIALS AND METHODS

A cross-sectional cohort of 46 pre-cirrhotic MAFLD patients and 30 age-, sex-, and education-matched controls was enrolled, with diffusion-tensor imaging (DTI) data, laboratory and neurocognitive scores collected. The DTI analysis along the perivascular space (DTI-ALPS) index was computed for qualifying glymphatic function. Generalized linear model and partial correlation analyses were applied to evaluate relationships between the ALPS index and clinical variables.

RESULTS

MAFLD group exhibited a decreased ALPS index and increased diffusivity along the y-axis in the projection fiber compared to the controls. The altered ALPS index was associated with clock drawing test (CDT) score (3.931 [0.914, 6.947], P = 0.011) and was correlated with diastolic pressure level (r = -0.315, P = 0.033) in MAFLD group. The relationships of ALPS index with CDT score (6.263 [2.069, 10.458], P = 0.003) and diastolic pressure level (r = -0.518, P = 0.014) remained in the MAFLD with metabolic syndrome (MetS) group. Furthermore, the ALPS index was even associated with Auditory Verbal Learning Test-Immediate recall score (-23.853 [-45.417, -2.289], P = 0.030) in MAFLD with MetS group.

CONCLUSION

MAFLD patients may have a glymphatic dysfunction prior to cirrhosis, and this alteration may be related to cognition and diastolic pressure. Glymphatic dysfunction has a more severe impact on cognition when MAFLD patient is accompanied by MetS.

Astrocyte regulation of extracellular space parameters across the sleep-wake cycle

Multiple subfields of neuroscience research are beginning to incorporate astrocytes into current frameworks of understanding overall brain physiology, neuronal circuitry, and disease etiology that underlie sleep and sleep-related disorders. Astrocytes have emerged as a dynamic regulator of neuronal activity through control of extracellular space (ECS) volume and composition, both of which can vary dramatically during different levels of sleep and arousal. Astrocytes are also an attractive target of sleep research due to their prominent role in the glymphatic system, a method by which toxic metabolites generated during wakefulness are cleared away. In this review we assess the literature surrounding glial influences on fluctuations in ECS volume and composition across the sleep-wake cycle. We also examine mechanisms of astrocyte volume regulation in glymphatic solute clearance and their role in sleep and wake states. Overall, findings highlight the importance of astrocytes in sleep and sleep research.

Multimodal assessment of brain fluid clearance is associated with amyloid-beta deposition in humans

PURPOSE

The present study investigates a multimodal imaging assessment of glymphatic function and its association with brain amyloid-beta deposition.

METHODS

Two brain CSF clearance measures (vCSF and DTI-ALPS) were derived from dynamic PET and MR diffusion tensor imaging (DTI) for 50 subjects, 24/50 were Aβ positive (Aβ+). T1W, T2W, DTI, T2FLAIR, and 11C-PiB and 18F-MK-6240 PET were acquired. Multivariate linear regression models were assessed with both vCSF and DTI-ALPS as independent variables and brain Aβ as the dependent variable. Three types of models were evaluated, including the vCSF-only model, the ALPS-only model and the vCSF+ALPS combined model. Models were applied to the whole group, and Aβ subgroups. All analyses were controlled for age, gender, and intracranial volume.

RESULTS

Sample demographics (N=50) include 20 males and 30 females with a mean age of 69.30 (sd=8.55). Our results show that the combination of vCSF and ALPS associates with Aβ deposition (p < 0.05, R2 = 0.575) better than either vCSF (p < 0.05, R2 = 0.431) or ALPS (p < 0.05, R2 = 0.372) alone in the Aβ+ group. We observed similar results in whole-group analyses (combined model: p < 0.05, R2 = 0.287; vCSF model: p <0.05, R2 = 0.175; ALPS model: p < 0.05, R2 = 0.196) with less significance. Our data also showed that vCSF has higher correlation (r = -0.548) in subjects with mild Aβ deposition and DTI-ALPS has higher correlation (r=-0.451) with severe Aβ deposition subjects.

CONCLUSION

The regression model with both vCSF and DTI-ALPS is better associated with brain Aβ deposition. These two independent brain clearance measures may better explain the variation in Aβ deposition than either term individually. Our results suggest that vCSF and DTI-ALPS reflect complementary aspects of brain clearance functions.

Evaluating the glymphatic system via magnetic resonance diffusion tensor imaging along the perivascular spaces in brain tumor patients

PURPOSE

To investigate glymphatic system function in patients with brain tumors, including both primary and secondary tumors, using diffusion tensor imaging along perivascular spaces (DTI-ALPS).

METHODS

We retrospectively analyzed the MR DTI of 24 patients with unilateral brain tumors and compared them with age and sex-matched controls. We compared the DTI-ALPS index of the ipsi- and contralateral brain hemispheres. The region of interest was placed in the periventricular vessels adjacent to the lateral ventricles. Differences between sex, age, and kind of tumor (primary or brain metastasis) were evaluated. Correlations between DTI-ALPS index and age and the tumor's apparent diffusion coefficient (ADC) were also investigated.

RESULTS

The DTI-ALPS index was significantly lower (p < 0.05) in the tumor-affected hemisphere (mean = 1.26 ± 0.24) than contralateral (mean = 1.43 ± 0.28). A comparison with healthy controls revealed no significant difference on the matched ipsilateral side. However, the DTI-ALPS index of the contralateral side of the patients was larger than the HC. Additionally, no statistically significant differences were found when analyzing the DTI-ALPS index vs. age, sex, and tumor entity. Additionally, we did not find a correlation between the DTI-ALPS index and patient age or tumor ADC.

CONCLUSION

The decreased DTI-ALPS index in the tumor-affected hemisphere may be related to impaired glymphatic system function. However, cancer is often a systemic disease; thus, the DTI-ALPS index from the contralateral brain hemisphere may not generally be considered as a normal control. Nonetheless, the DTI-ALPS index does not only reflect diffusion in the perivascular spaces but it can also be influenced by factors such as axonal degeneration. Therefore, it does not directly reflect brain waste clearance and changes in the index should be interpreted carefully.

Enlarged choroid plexus in relapsing-remitting multiple sclerosis may lead to brain structural changes through the glymphatic impairment

OBJECTIVES

To investigate the potential link among choroid plexus (CP) volume, glymphatic clearance and brain structural change in relapsing-remitting multiple sclerosis (RRMS) patients.

MATERIALS AND METHODS

Sixty-five RRMS patients and 48 healthy controls (HC) underwent MRI examination. The diffusion tensor image analysis along the perivascular space (DTI-ALPS) was calculated to reflect glymphatic system function. The brain structure volume and DTI-ALPS index were compared between RRMS and HC. The mediating effect of the DTI-ALPS index between CP volume and brain structural changes was further investigated. The longitudinal changes of brain structure and DTI-ALPS index were compared in 20 RRMS patients.

RESULTS

Compared to HC, CP volume in RRMS was significantly increased (P < 0.001), and DTI-ALPS index was significantly decreased (P = 0.001). The volumes of white matter, thalamus, putamen and pallidum were significantly decreased in RRMS, and the volumes of lateral ventricle and third ventricle were increased. Mediation analysis showed DTI-ALPS index partially mediated the association between CP enlargement and deep gray matter (DGM) atrophy in RRMS, and between CP enlargement and ventricle enlargement. CP volume and DTI-ALPS index were also significantly correlated with Expanded Disability Status Scale (EDSS) (P = 0.006, P = 0.043). Notably, the variation of DTI_ALPS index during the follow-up period were significantly and negatively correlated with the variation of EDSS (P = 0.045).

CONCLUSION

Enlarged CP volume and decreased DTI_ALPS index may be closely related to DGM atrophy and ventricular enlargement in RRMS, and may be potential imaging markers of clinical disability.

Glymphatic system dysfunction predicts amyloid deposition, neurodegeneration, and clinical progression in Alzheimer's disease

INTRODUCTION

Although glymphatic function is involved in Alzheimer's disease (AD), its potential for predicting the pathological and clinical progression of AD and its sequential association with core AD biomarkers is poorly understood.

METHODS

Whole-brain glymphatic activity was measured by diffusion tensor image analysis along the perivascular space (DTI-ALPS) in participants with AD dementia (n = 47), mild cognitive impairment (MCI; n = 137), and normal controls (n = 235) from the Alzheimer's Disease Neuroimaging Initiative.

RESULTS

ALPS index was significantly lower in AD dementia than in MCI or controls. Lower ALPS index was significantly associated with faster changes in amyloid positron emission tomography (PET) burden and AD signature region of interest volume, higher risk of amyloid-positive transition and clinical progression, and faster rates of amyloid- and neurodegeneration-related cognitive decline. Furthermore, the associations of the ALPS index with cognitive decline were fully mediated by amyloid PET and brain atrophy.

DISCUSSION

Glymphatic failure may precede amyloid pathology, and predicts amyloid deposition, neurodegeneration, and clinical progression in AD.

HIGHLIGHTS

The analysis along the perivascular space (ALPS) index is reduced in patients with Alzheimer's disease (AD) dementia, prodromal AD, and preclinical AD. Lower ALPS index predicted accelerated amyloid beta (Aβ) positron emission tomography (PET) burden and Aβ-positive transition. The decrease in the ALPS index occurs before cerebrospinal fluid Aβ42 reaches the positive threshold. ALPS index predicted brain atrophy, clinical progression, and cognitive decline. Aβ PET and brain atrophy mediated the link of ALPS index with cognitive decline.

Exploring neurodegenerative disorders using advanced magnetic resonance imaging of the glymphatic system

The glymphatic system, a macroscopic waste clearance system in the brain, is crucial for maintaining neural health. It facilitates the exchange of cerebrospinal and interstitial fluid, aiding the clearance of soluble proteins and metabolites and distributing essential nutrients and signaling molecules. Emerging evidence suggests a link between glymphatic dysfunction and the pathogenesis of neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's disease. These disorders are characterized by the accumulation and propagation of misfolded or mutant proteins, a process in which the glymphatic system is likely involved. Impaired glymphatic clearance could lead to the buildup of these toxic proteins, contributing to neurodegeneration. Understanding the glymphatic system's role in these disorders could provide insights into their pathophysiology and pave the way for new therapeutic strategies. Pharmacological enhancement of glymphatic clearance could reduce the burden of toxic proteins and slow disease progression. Neuroimaging techniques, particularly MRI-based methods, have emerged as promising tools for studying the glymphatic system in vivo. These techniques allow for the visualization of glymphatic flow, providing insights into its function under healthy and pathological conditions. This narrative review highlights current MRI-based methodologies, such as motion-sensitizing pulsed field gradient (PFG) based methods, as well as dynamic gadolinium-based and glucose-enhanced methodologies currently used in the study of neurodegenerative disorders.

Contribution of Direct Cerebral Vascular Transport in Brain Substance Clearance

The accumulation of harmful substances has long been recognized as a likely cause of many neurodegenerative diseases. The two classic brain clearance pathways are cerebrospinal fluid (CSF) and vascular circulation systems. Since the discovery of the glymphatic system, research on the CSF pathway has gained momentum, and impaired CSF clearance has been implicated in virtually all neurodegenerative animal models. However, the contribution of the direct participation of vascular transport across the blood-brain barrier in clearing substances is often ignored in glymphatic papers. Supportive evidence for the direct involvement of parenchymal vasculature in substance clearance is accumulated. First, multiple mechanisms have been proposed for the vascular drainage of exogenous and endogenous substances across the blood-brain barriers. Second, the "traditional" role of arachnoid villi and granulations as the main site for CSF draining into the vasculature system has been questioned. Third, MRI studies using different CSF tracers indicate that parenchymal vasculature directly participates in tracer efflux, consistent with immunohistochemical findings. Here we will review evidence in the literature that supports the direct participation of the parenchymal vascular system in substance clearance, in addition to the CSF clearance pathways.

Relationship of Perivascular Space Markers With Incident Dementia in Cerebral Small Vessel Disease

BACKGROUND

Recent studies, using diffusion tensor image analysis along the perivascular space (DTI-ALPS), suggest impaired perivascular space (PVS) function in cerebral small vessel disease, but they were cross-sectional, making inferences on causality difficult. We determined associations between impaired PVS, measured using DTI-ALPS and PVS volume, and cognition and incident dementia.

METHODS

In patients with lacunar stroke and confluent white matter hyperintensities, without dementia at baseline, recruited prospectively in a single center, magnetic resonance imaging was performed annually for 3 years, and cognitive assessments, including global, memory, executive function, and processing speed, were performed annually for 5 years. We determined associations between DTI-ALPS and PVS volume with cerebral small vessel disease imaging markers (white matter hyperintensity volume, lacunes, and microbleeds) at baseline and with changes in imaging markers. We determined whether DTI-ALPS and PVS volume at baseline and change over 3 years predicted incident dementia. Analyses were controlled for conventional diffusion tensor image metrics using 2 markers (median mean diffusivity [MD] and peak width of skeletonized MD) and adjusted for age, sex, and vascular risk factors.

RESULTS

A total of 120 patients, mean age 70.0 years and 65.0% male, were included. DTI-ALPS declined over 3 years, while no change in PVS volume was found. Neither DTI-ALPS nor PVS volume was associated with cerebral small vessel disease imaging marker progression. Baseline DTI-ALPS was associated with changes in global cognition (β=0.142, P=0.032), executive function (β=0.287, P=0.027), and long-term memory (β=0.228, P=0.027). Higher DTI-ALPS at baseline predicted a lower risk of dementia (hazard ratio, 0.328 [0.183-0.588]; P<0.001), and this remained significant after including median MD as a covariate (hazard ratio, 0.290 [0.139-0.602]; P<0.001). Change in DTI-ALPS predicted dementia conversion (hazard ratio, 0.630 [0.428-0.964]; P=0.048), but when peak width of skeletonized MD and median MD were entered as covariates, the association was not significant. There was no association between baseline PVS volume, or PVS change over 3 years, and conversion to dementia.

CONCLUSIONS

DTI-ALPS predicts future dementia risk in patients with lacunar strokes and confluent white matter hyperintensities. However, the weakening of the association between change in DTI-ALPS and incident dementia after controlling for peak width of skeletonized MD and median MD suggests part of the signal may represent conventional diffusion tensor image metrics. PVS volume is not a predictor of future dementia risk.

The relationship between inflammation, impaired glymphatic system, and neurodegenerative disorders: A vicious cycle

The term "glymphatic" emerged roughly a decade ago, marking a pivotal point in neuroscience research. The glymphatic system, a glial-dependent perivascular network distributed throughout the brain, has since become a focal point of investigation. There is increasing evidence suggesting that impairment of the glymphatic system appears to be a common feature of neurodegenerative disorders, and this impairment exacerbates as disease progression. Nevertheless, the common factors contributing to glymphatic system dysfunction across most neurodegenerative disorders remain unclear. Inflammation, however, is suspected to play a pivotal role. Dysfunction of the glymphatic system can lead to a significant accumulation of protein and waste products, which can trigger inflammation. The interaction between the glymphatic system and inflammation appears to be cyclical and potentially synergistic. Yet, current research is limited, and there is a lack of comprehensive models explaining this association. In this perspective review, we propose a novel model suggesting that inflammation, impaired glymphatic function, and neurodegenerative disorders interconnected in a vicious cycle. By presenting experimental evidence from the existing literature, we aim to demonstrate that: (1) inflammation aggravates glymphatic system dysfunction, (2) the impaired glymphatic system exacerbated neurodegenerative disorders progression, (3) neurodegenerative disorders progression promotes inflammation. Finally, the implication of proposed model is discussed.

The glymphatic system and multiple sclerosis: An evolving connection

Multiple sclerosis (MS) is a complex autoimmune disorder that affects the central nervous system, resulting in demyelination and an array of neurological manifestations. Recently, there has been significant scientific interest in the glymphatic system, which operates as a waste-clearance system for the brain. This article reviews the existing literature, and explores potential links between the glymphatic system and MS, shedding light on its evolving significance in the context of MS pathogenesis. The authors consider the pathophysiological implications of glymphatic dysfunction in MS, the impact of disrupted sleep on glymphatic function, and the bidirectional relationship between MS and sleep disturbances. By offering an understanding of the intricate interplay between the glymphatic system and MS, this review provides valuable insights which may lead to improved diagnostic techniques and more effective therapeutic interventions.

Sleep disorders cause Parkinson's disease or the reverse is true: Good GABA good night

BACKGROUND

Parkinson's disease (PD) is a progressive neurodegenerative brain disease due to degeneration of dopaminergic neurons (DNs) presented with motor and non-motor symptoms. PD symptoms are developed in response to the disturbance of diverse neurotransmitters including γ-aminobutyric acid (GABA). GABA has a neuroprotective effect against PD neuropathology by protecting DNs in the substantia nigra pars compacta (SNpc). It has been shown that the degeneration of GABAergic neurons is linked with the degeneration of DNs and the progression of motor and non-motor PD symptoms. GABA neurotransmission is a necessary pathway for normal sleep patterns, thus deregulation of GABAergic neurotransmission in PD could be the potential cause of sleep disorders in PD.

AIM

Sleep disorders affect GABA neurotransmission leading to memory and cognitive dysfunction in PD. For example, insomnia and short sleep duration are associated with a reduction of brain GABA levels. Moreover, PD-related disorders including rigidity and nocturia influence sleep patterns leading to fragmented sleep which may also affect PD neuropathology. However, the mechanistic role of GABA in PD neuropathology regarding motor and non-motor symptoms is not fully elucidated. Therefore, this narrative review aims to clarify the mechanistic role of GABA in PD neuropathology mainly in sleep disorders, and how good GABA improves PD. In addition, this review of published articles tries to elucidate how sleep disorders such as insomnia and REM sleep behavior disorder (RBD) affect PD neuropathology and severity. The present review has many limitations including the paucity of prospective studies and most findings are taken from observational and preclinical studies. GABA involvement in the pathogenesis of PD has been recently discussed by recent studies. Therefore, future prospective studies regarding the use of GABA agonists in the management of PD are suggested to observe their distinct effects on motor and non-motor symptoms.

CONCLUSION

There is a bidirectional relationship between the pathogenesis of PD and sleep disorders which might be due to GABA deregulation.

The relationship between amyloid pathology, cerebral small vessel disease, glymphatic dysfunction, and cognition: a study based on Alzheimer's disease continuum participants

BACKGROUND

Glymphatic dysfunction is a crucial pathway for dementia. Alzheimer's disease (AD) pathologies co-existing with cerebral small vessel disease (CSVD) is the most common pathogenesis for dementia. We hypothesize that AD pathologies and CSVD could be associated with glymphatic dysfunction, contributing to cognitive impairment.

METHOD

Participants completed with amyloid PET, diffusion tensor imaging (DTI), and T2 fluid-attenuated inversion-recovery (FLAIR) sequences were included from the Alzheimer's Disease Neuroimaging Initiative (ADNI). White matter hyperintensities (WMH), the most common CSVD marker, was evaluated from T2FLAIR images and represented the burden of CSVD. Amyloid PET was used to assess Aβ aggregation in the brain. We used diffusion tensor image analysis along the perivascular space (DTI-ALPS) index, the burden of enlarged perivascular spaces (PVS), and choroid plexus volume to reflect glymphatic function. The relationships between WMH burden/Aβ aggregation and these glymphatic markers as well as the correlations between glymphatic markers and cognitive function were investigated. Furthermore, we conducted mediation analyses to explore the potential mediating effects of glymphatic markers in the relationship between WMH burden/Aβ aggregation and cognition.

RESULTS

One hundred and thirty-three participants along the AD continuum were included, consisting of 40 CN - , 48 CN + , 26 MCI + , and 19 AD + participants. Our findings revealed that there were negative associations between whole-brain Aβ aggregation (r =  - 0.249, p = 0.022) and WMH burden (r =  - 0.458, p < 0.001) with DTI-ALPS. Additionally, Aβ aggregation (r = 0.223, p = 0.041) and WMH burden (r = 0.294, p = 0.006) were both positively associated with choroid plexus volume. However, we did not observe significant correlations with PVS enlargement severity. DTI-ALPS was positively associated with memory (r = 0.470, FDR-p < 0.001), executive function (r = 0.358, FDR-p = 0.001), visual-spatial (r = 0.223, FDR-p < 0.040), and language (r = 0.419, FDR-p < 0.001). Conversely, choroid plexus volume showed negative correlations with memory (r =  - 0.315, FDR-p = 0.007), executive function (r =  - 0.321, FDR-p = 0.007), visual-spatial (r =  - 0.233, FDR-p = 0.031), and language (r =  - 0.261, FDR-p = 0.021). There were no significant correlations between PVS enlargement severity and cognitive performance. In the mediation analysis, we found that DTI-ALPS acted as a mediator in the relationship between WMH burden/Aβ accumulation and memory and language performances.

CONCLUSION

Our study provided evidence that both AD pathology (Aβ) and CSVD were associated with glymphatic dysfunction, which is further related to cognitive impairment. These results may provide a theoretical basis for new targets for treating AD.

Matrix metalloproteinase-9 inhibition prevents aquaporin-4 depolarization-mediated glymphatic dysfunction in Parkinson's disease

INTRODUCTION

The glymphatic system offers a perivascular pathway for the clearance of pathological proteins and metabolites to optimize neurological functions. Glymphatic dysfunction plays a pathogenic role in Parkinson's disease (PD); however, the molecular mechanism of glymphatic dysfunction in PD remains elusive.

OBJECTIVE

To explore whether matrix metalloproteinase-9 (MMP-9)-mediated β-dystroglycan (β-DG) cleavage is involved in the regulation of aquaporin-4 (AQP4) polarity-mediated glymphatic system in PD.

METHODS

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD and A53T mice were used in this study. The glymphatic function was evaluated using ex vivo imaging. TGN-020, an AQP4 antagonist, was administered to investigate the role of AQP4 in glymphatic dysfunction in PD. GM6001, an MMP-9 antagonist, was administered to investigate the role of the MMP-9/β-DG pathway in regulating AQP4. The expression and distribution of AQP4, MMP-9, and β-DG were assessed using western blotting, immunofluorescence, and co-immunoprecipitation. The ultrastructure of basement membrane (BM)-astrocyte endfeet was detected using transmission electron microscopy. Rotarod and open-field tests were performed to evaluate motor behavior.

RESULTS

Perivascular influx and efflux of cerebral spinal fluid tracers were reduced in MPTP-induced PD mice with impaired AQP4 polarization. AQP4 inhibition aggravated reactive astrogliosis, glymphatic drainage restriction, and dopaminergic neuronal loss in MPTP-induced PD mice. MMP-9 and cleaved β-DG were upregulated in both MPTP-induced PD and A53T mice, with reduced polarized localization of β-DG and AQP4 to astrocyte endfeet. MMP-9 inhibition restored BM-astrocyte endfeet-AQP4 integrity and attenuated MPTP-induced metabolic perturbations and dopaminergic neuronal loss.

CONCLUSION

AQP4 depolarization contributes to glymphatic dysfunction and aggravates PD pathologies, and MMP-9-mediated β-DG cleavage regulates glymphatic function through AQP4 polarization in PD, which may provide novel insights into the pathogenesis of PD.

Phosphorylation of AQP4 by LRRK2 R1441G impairs glymphatic clearance of IFNγ and aggravates dopaminergic neurodegeneration

Aquaporin-4 (AQP4) is essential for normal functioning of the brain's glymphatic system. Impaired glymphatic function is associated with neuroinflammation. Recent clinical evidence suggests the involvement of glymphatic dysfunction in LRRK2-associated Parkinson's disease (PD); however, the precise mechanism remains unclear. The pro-inflammatory cytokine interferon (IFN) γ interacts with LRRK2 to induce neuroinflammation. Therefore, we examined the AQP4-dependent glymphatic system's role in IFNγ-mediated neuroinflammation in LRRK2-associated PD. We found that LRRK2 interacts with and phosphorylates AQP4 in vitro and in vivo. AQP4 phosphorylation by LRRK2 R1441G induced AQP4 depolarization and disrupted glymphatic IFNγ clearance. Exogeneous IFNγ significantly increased astrocyte expression of IFNγ receptor, amplified AQP4 depolarization, and exacerbated neuroinflammation in R1441G transgenic mice. Conversely, inhibiting LRRK2 restored AQP4 polarity, improved glymphatic function, and reduced IFNγ-mediated neuroinflammation and dopaminergic neurodegeneration. Our findings establish a link between LRRK2-mediated AQP4 phosphorylation and IFNγ-mediated neuroinflammation in LRRK2-associated PD, guiding the development of LRRK2 targeting therapy.

Aquaporin-4 and Parkinson's Disease

The water-selective channel aquaporin-4 (AQP4) is implicated in water homeostasis and the functioning of the glymphatic system, which eliminates various metabolites from the brain tissue, including amyloidogenic proteins. Misfolding of the α-synuclein protein and its post-translational modifications play a crucial role in the development of Parkinson's disease (PD) and other synucleopathies, leading to the formation of cytotoxic oligomers and aggregates that cause neurodegeneration. Human and animal studies have shown an interconnection between AQP4 dysfunction and α-synuclein accumulation; however, the specific role of AQP4 in these mechanisms remains unclear. This review summarizes the current knowledge on the role of AQP4 dysfunction in the progression of α-synuclein pathology, considering the possible effects of AQP4 dysregulation on brain molecular mechanisms that can impact α-synuclein modification, accumulation and aggregation. It also highlights future directions that can help study the role of AQP4 in the functioning of the protective mechanisms of the brain during the development of PD and other neurodegenerative diseases.

Imaging of brain barrier inflammation and brain fluid drainage in human neurological diseases

The intricate relationship between the central nervous system (CNS) and the immune system plays a crucial role in the pathogenesis of various neurological diseases. Understanding the interactions among the immunopathological processes at the brain borders is essential for advancing our knowledge of disease mechanisms and developing novel diagnostic and therapeutic approaches. In this review, we explore the emerging role of neuroimaging in providing valuable insights into brain barrier inflammation and brain fluid drainage in human neurological diseases. Neuroimaging techniques have enabled us not only to visualize and assess brain structures, but also to study the dynamics of the CNS in health and disease in vivo. By analyzing imaging findings, we can gain a deeper understanding of the immunopathology observed at the brain-immune interface barriers, which serve as critical gatekeepers that regulate immune cell trafficking, cytokine release, and clearance of waste products from the brain. This review explores the integration of neuroimaging data with immunopathological findings, providing valuable insights into brain barrier integrity and immune responses in neurological diseases. Such integration may lead to the development of novel diagnostic markers and targeted therapeutic approaches that can benefit patients with neurological disorders.

Correlation of glymphatic system abnormalities with Parkinson's disease progression: a clinical study based on non-invasive fMRI

BACKGROUND

The glymphatic system is reportedly involved in Parkinson's disease (PD). Based on previous studies, we aimed to confirm the correlation between the glymphatic system and PD progression by combining two imaging parameters, diffusion tensor image analysis along the perivascular space (DTI-ALPS), and enlarged perivascular spaces (EPVS).

METHODS

Fifty-one PD patients and fifty healthy control (HC) were included. Based on the Hoehn-Yahr scale, the PD group was divided into early-stage and medium-to late-stage. All PD patients were scored using the Unified PD Rating Scale (UPDRS). We assessed the DTI-ALPS indices in the bilateral hemispheres and EPVS numbers in bilateral centrum semiovale (CSO), basal ganglia (BG), and midbrain.

RESULTS

The DTI-ALPS indices were significantly lower bilaterally in PD patients than in the HC group, and EPVS numbers in any of the bilateral CSO, BG, and midbrain were significantly higher, especially for the medium- to late-stage group and the BG region. In PD patients, the DTI-ALPS index was significantly negatively correlated with age, while the BG-EPVS numbers were significantly positively correlated with age. Furthermore, the DTI-ALPS index was negatively correlated with UPDRS II and III scores, while the BG-EPVS numbers were positively correlated with UPDRS II and III scores. Similarly, the correlation was more pronounced in the medium- to late-stage group.

CONCLUSION

The DTI-ALPS index and EPVS numbers (especially in the BG region) are closely related to age and PD progression and can serve as non-invasive assessments for glymphatic dysfunction and its interventions in clinical studies.

Predicting neurodegeneration from sleep related biofluid changes

Sleep-wake disturbances are common in neurodegenerative diseases and may occur years before the clinical diagnosis, potentially either representing an early stage of the disease itself or acting as a pathophysiological driver. Therefore, discovering biomarkers that identify individuals with sleep-wake disturbances who are at risk of developing neurodegenerative diseases will allow early diagnosis and intervention. Given the association between sleep and neurodegeneration, the most frequently analyzed fluid biomarkers in people with sleep-wake disturbances to date include those directly associated with neurodegeneration itself, such as neurofilament light chain, phosphorylated tau, amyloid-beta and alpha-synuclein. Abnormalities in these biomarkers in patients with sleep-wake disturbances are considered as evidence of an underlying neurodegenerative process. Levels of hormonal sleep-related biomarkers such as melatonin, cortisol and orexin are often abnormal in patients with clinical neurodegenerative diseases, but their relationships with the more standard neurodegenerative biomarkers remain unclear. Similarly, it is unclear whether other chronobiological/circadian biomarkers, such as disrupted clock gene expression, are causal factors or a consequence of neurodegeneration. Current data would suggest that a combination of fluid biomarkers may identify sleep-wake disturbances that are most predictive for the risk of developing neurodegenerative disease with more optimal sensitivity and specificity.

Interstitial Fluidopathy of the Central Nervous System: An Umbrella Term for Disorders with Impaired Neurofluid Dynamics

Interest in interstitial fluid dynamics has increased since the proposal of the glymphatic system hypothesis. Abnormal dynamics of the interstitial fluid have been pointed out to be an important factor in various pathological statuses. In this article, we propose the concept of central nervous system interstitial fluidopathy as a disease or condition in which abnormal interstitial fluid dynamics is one of the important factors for the development of a pathological condition. We discuss the aspects of interstitial fluidopathy in various diseases, including Alzheimer's disease, Parkinson's disease, normal pressure hydrocephalus, and cerebral small vessel disease. We also discuss a method called "diffusion tensor image analysis along the perivascular space" using MR diffusion images, which is used to evaluate the degree of interstitial fluidopathy or the activity of the glymphatic system.

Association of choroid plexus volume with motor symptoms and dopaminergic degeneration in Parkinson's disease

BACKGROUND

The choroid plexus (CP) is involved in the clearance of harmful metabolites from the brain, as a part of the glymphatic system. This study aimed to investigate the association between CP volume (CPV), nigrostriatal dopaminergic degeneration and motor outcomes in Parkinson's disease (PD).

METHODS

We retrospectively searched drug-naïve patients with early-stage PD who underwent dopamine transporter (DAT) scanning and MRI. Automatic CP segmentation was performed, and the CPV was calculated. The relationship between CPV, DAT availability and Unified PD Rating Scale Part III (UPDRS-III) scores was assessed using multivariate linear regression. We performed longitudinal analyses to assess motor outcomes according to CPV.

RESULTS

CPV was negatively associated with DAT availability in each striatal subregion (anterior caudate, β=-0.134, p=0.012; posterior caudate, β=-0.162, p=0.002; anterior putamen, β=-0.133, p=0.024; posterior putamen, β=-0.125, p=0.039; ventral putamen, β=-0.125, p=0.035), except for the ventral striatum. CPV was positively associated with the UPDRS-III score even after adjusting for DAT availability in the posterior putamen (β=0.121; p=0.035). A larger CPV was associated with the future development of freezing of gait in the Cox regression model (HR 1.539, p=0.027) and a more rapid increase in dopaminergic medication in the linear mixed model (CPV×time, p=0.037), but was not associated with the risk of developing levodopa-induced dyskinesia or wearing off.

CONCLUSION

These findings suggest that CPV has the potential to serve as a biomarker for baseline and longitudinal motor disabilities in PD.

The Association of the Glymphatic Function with Parkinson's Disease Symptoms: Neuroimaging Evidence from Longitudinal and Cross-Sectional Studies

OBJECTIVE

Emerging pathological evidence suggests that there is an association between glymphatic dysfunction and the progression of Parkinson's disease (PD). However, the clinical evidence of this association remains lacking.

METHODS

In this study, the index for diffusion tensor image analysis along the perivascular space (ALPS index) was calculated to evaluate glymphatic function.

RESULTS

Overall, 289 patients with PD were enrolled in the cross-sectional study. The ALPS index was found to be negatively correlated with age, disease severity, and dyskinesia. In the longitudinal study, the information on a total of 95 PD patients with 5-year follow-up examinations was collected from the Parkinson's Progression Marker Initiative, 33 of which were classified into the low ALPS index group, and all others were classified into the mid-high ALPS index group based on the first tertile of the baseline ALPS index. The results of longitudinal regression indicated that there was a significant main group effect on autonomic dysfunction, as well as on activities of daily living. In addition, the low ALPS index group had faster deterioration in MDS-UPDRS part III and part II, Symbol Digit Modalities Test and Hopkins Verbal Learning Test. Path analysis showed that ALPS index acted as a significant mediator between tTau/ Aβ1-42 and cognitive change in the Symbol Digit Modalities Test score at year 4 and year 5.

INTERPRETATION

The ALPS index, an neuroimaging marker of glymphatic function, is correlated with PD disease severity, motor symptoms, and autonomic function, and is predictive of faster deterioration in motor symptoms and cognitive function. Additionally, glymphatic function may mediate the pathological role of toxic protein in cognitive decline. ANN NEUROL 2023;94:672-683.

Association between choroid plexus volume and cognition in Parkinson disease

BACKGROUND AND PURPOSE

The choroid plexus (CP) clears harmful metabolites from the central nervous system as part of the glymphatic system. We investigated the association of CP volume (CPV) with baseline and longitudinal cognitive decline in patients with Parkinson disease (PD).

METHODS

We retrospectively reviewed the medical records of 240 patients with newly diagnosed PD who had undergone detailed neuropsychological tests and high-resolution T1-weighted structural magnetic resonance imaging during the initial assessment. The CPV of each patient was automatically segmented, and the intracranial volume ratio was used in subsequent analyses. The relationship between CPV and baseline composite scores of each cognitive domain was assessed using multivariate linear regression analyses. A Cox proportional hazards model was used to compare the risk of dementia conversion with CPV.

RESULTS

CPV negatively correlated with composite scores of the frontal/executive function domain (β = -0.375, p = 0.002) after adjusting for age, sex, years of education, and parkinsonian symptom duration. The Cox regression model revealed that a larger CPV was associated with a higher risk of dementia conversion (hazard ratio [HR] = 1.509, p = 0.038), which was no longer significant after adjusting for the composite scores of the frontal/executive function domain. A mediation analysis demonstrated that the effect of CPV on the risk of dementia conversion was completely mediated by frontal/executive function (direct effect: HR = 1.203, p = 0.396; indirect effect: HR = 1.400, p = 0.015).

CONCLUSIONS

Baseline CPV is associated with baseline frontal/executive function, which subsequently influences dementia conversion risk in patients with PD.

Brain MRI Biomarkers in Isolated Rapid Eye Movement Sleep Behavior Disorder: Where Are We? A Systematic Review

The increasing number of MRI studies focused on prodromal Parkinson's Disease (PD) demonstrates a strong interest in identifying early biomarkers capable of monitoring neurodegeneration. In this systematic review, we present the latest information regarding the most promising MRI markers of neurodegeneration in relation to the most specific prodromal symptoms of PD, namely isolated rapid eye movement (REM) sleep behavior disorder (iRBD). We reviewed structural, diffusion, functional, iron-sensitive, neuro-melanin-sensitive MRI, and proton magnetic resonance spectroscopy studies conducted between 2000 and 2023, which yielded a total of 77 relevant papers. Among these markers, iron and neuromelanin emerged as the most robust and promising indicators for early neurodegenerative processes in iRBD. Atrophy was observed in several regions, including the frontal and temporal cortices, limbic cortices, and basal ganglia, suggesting that neurodegenerative processes had been underway for some time. Diffusion and functional MRI produced heterogeneous yet intriguing results. Additionally, reduced glymphatic clearance function was reported. Technological advancements, such as the development of ultra-high field MRI, have enabled the exploration of minute anatomical structures and the detection of previously undetectable anomalies. The race to achieve early detection of neurodegeneration is well underway.

Motor progression phenotypes in early-stage Parkinson's Disease: A clinical prediction model and the role of glymphatic system imaging biomarkers

BACKGROUND

Substantial heterogeneity of motor symptoms in Parkinson's disease (PD) poses a challenge to disease prediction.

OBJECTIVES

The aim of this study was to construct a nomogram model that can distinguish different longitudinal trajectories of motor symptom changes in early-stage PD patients.

METHODS

Data on 90 patients with 5-years of follow-up were collected from the Parkinson's Progression Marker Initiative (PPMI) cohort. We used a latent class mixed modeling (LCMM) to identify distinct progression patterns of motor symptoms, and backward stepwise logistic regression with baseline information was conducted to identify the potential predictors for motor trajectory and to develop a nomogram. The performance of the nomogram model was then evaluated using the optimism-corrected C-index for internal validation, the area under the curve (AUC) of the receiver operating characteristic (ROC) curve for discrimination, the calibration curve for predictive accuracy, and decision curve analysis (DCA) for its clinical value.

RESULTS

We identified two trajectories for motor progression patterns. The first, Class 1 (Motor deteriorated group), was characterized by sustained, continuously worsening motor symptoms, and the second, Class 2 (Motor stable group), had stable motor symptoms throughout the follow-up period. The best combination of 7 baseline variables was identified and assembled into the nomogram: Scopa-AUT [odds ratio (OR), 1.11; p = 0.091], Letter number sequencing (LNS) (OR, 0.76; p = 0.068), the asymmetry index of putamen (OR, 0.95; p = 0.034), mean caudate uptake (OR, 0.14; p = 0.086), CSF pTau/α-synuclein (OR, 0.00; p = 0.011), CSF tTau/Aβ (OR, 25434806; p = 0.025), and the index for diffusion tensor image analysis along the perivascular space (ALPS-index) (OR, 0.02; p = 0.030). The nomogram achieved good discrimination, with an original AUC of 0.901 (95% CI, 0.813-0.989), and the bias-corrected concordance index (C-index) with 1,000 bootstraps was 0.834. The calibration curve and DCA also suggested both the high accuracy and clinical usefulness of the nomogram, respectively.

CONCLUSIONS

This study proposes an effective nomogram to predict different motor progression patterns in early-stage PD. Furthermore, the imaging biomarker indicating glymphatic function could be an independent predictive factor for PD motor progression.

Screening and Targeting Risk Factors for Prodromal Synucleinopathy: Taking Steps toward a Prescriptive Multi-modal Framework

As the prevalence of Parkinson's disease (PD) grows, so too does the population at-risk of developing PD, those in the so-called prodromal period. This period can span from those experiencing subtle motor deficits yet not meeting full diagnostic criteria or those with physiologic markers of disease alone. Several disease-modifying therapies have failed to show a neuroprotective effect. A common criticism is that neurodegeneration, even in the early motor stages, has advanced too far for neuro-restoration-based interventions to be effective. Therefore, identifying this early population is essential. Once identified, these patients could then potentially benefit from sweeping lifestyle modifications to alter their disease trajectory. Herein, we review the literature on risk factors for, and prodromal symptoms of, PD with an emphasis on ones which may be modifiable in the earliest possible stages. We propose a process for identifying this population and speculate on some strategies which may modulate disease trajectory. Ultimately, this proposal warrants prospective studies.

Altered Brain Glymphatic Flow at Diffusion-Tensor MRI in Rapid Eye Movement Sleep Behavior Disorder

Background Brain glymphatic dysfunction may contribute to the development of α-synucleinopathies. Yet, noninvasive imaging and quantification remain lacking. Purpose To examine glymphatic function of the brain in isolated rapid eye movement sleep behavior disorder (RBD) and its relevance to phenoconversion with use of diffusion-tensor imaging (DTI) analysis along the perivascular space (ALPS). Materials and Methods This prospective study included consecutive participants diagnosed with RBD, age- and sex-matched control participants, and participants with Parkinson disease (PD) who were enrolled and examined between May 2017 and April 2020. All study participants underwent 3.0-T brain MRI including DTI, susceptibility-weighted and susceptibility map-weighted imaging, and/or dopamine transporter imaging using iodine 123-2β-carbomethoxy-3β-(4-iodophenyl)-N-(3-fluoropropyl)-nortropane SPECT at the time of participation. Phenoconversion status to α-synucleinopathies was unknown at the time of MRI. Participants were regularly followed up and monitored for any signs of α-synucleinopathies. The ALPS index reflecting glymphatic activity was calculated by a ratio of the diffusivities along the x-axis in the projection and association neural fibers to the diffusivities perpendicular to them and compared according to the groups with use of the Kruskal-Wallis and Mann-Whitney U tests. The phenoconversion risk in participants with RBD was evaluated according to the ALPS index with use of a Cox proportional hazards model. Results Twenty participants diagnosed with RBD (12 men; median age, 73 years [IQR, 66-76 years]), 20 control participants, and 20 participants with PD were included. The median ALPS index was lower in the group with RBD versus controls (1.53 vs 1.72; P = .001) but showed no evidence of a difference compared with the group with PD (1.49; P = .68). The conversion risk decreased with an increasing ALPS index (hazard ratio, 0.57 per 0.1 increase in the ALPS index [95% CI: 0.35, 0.93]; P = .03). Conclusion DTI-ALPS in RBD demonstrated a more severe reduction of glymphatic activity in individuals with phenoconversion to α-synucleinopathies. © RSNA, 2023 Supplemental material is available for this article. See also the editorial by Filippi and Balestrino in this issue.

Emerging Perspectives on MRI Application in Multiple Sclerosis: Moving from Pathophysiology to Clinical Practice

MRI plays a central role in the diagnosis of multiple sclerosis (MS) and in the monitoring of disease course and treatment response. Advanced MRI techniques have shed light on MS biology and facilitated the search for neuroimaging markers that may be applicable in clinical practice. MRI has led to improvements in the accuracy of MS diagnosis and a deeper understanding of disease progression. This has also resulted in a plethora of potential MRI markers, the importance and validity of which remain to be proven. Here, five recent emerging perspectives arising from the use of MRI in MS, from pathophysiology to clinical application, will be discussed. These are the feasibility of noninvasive MRI-based approaches to measure glymphatic function and its impairment; T1-weighted to T2-weighted intensity ratio to quantify myelin content; classification of MS phenotypes based on their MRI features rather than on their clinical features; clinical relevance of gray matter atrophy versus white matter atrophy; and time-varying versus static resting-state functional connectivity in evaluating brain functional organization. These topics are critically discussed, which may guide future applications in the field.

Melatonin as a Chronobiotic/Cytoprotective Agent in REM Sleep Behavior Disorder

Dream-enactment behavior that emerges during episodes of rapid eye movement (REM) sleep without muscle atonia is a parasomnia known as REM sleep behavior disorder (RBD). RBD constitutes a prodromal marker of α-synucleinopathies and serves as one of the best biomarkers available to predict diseases such as Parkinson disease, multiple system atrophy and dementia with Lewy bodies. Most patients showing RBD will convert to an α-synucleinopathy about 10 years after diagnosis. The diagnostic advantage of RBD relies on the prolonged prodromal time, its predictive power and the absence of disease-related treatments that could act as confounders. Therefore, patients with RBD are candidates for neuroprotection trials that delay or prevent conversion to a pathology with abnormal α-synuclein metabolism. The administration of melatonin in doses exhibiting a chronobiotic/hypnotic effect (less than 10 mg daily) is commonly used as a first line treatment (together with clonazepam) of RBD. At a higher dose, melatonin may also be an effective cytoprotector to halt α-synucleinopathy progression. However, allometric conversion doses derived from animal studies (in the 100 mg/day range) are rarely employed clinically regardless of the demonstrated absence of toxicity of melatonin in phase 1 pharmacological studies with doses up to 100 mg in normal volunteers. This review discusses the application of melatonin in RBD: (a) as a symptomatic treatment in RBD; (b) as a possible disease-modifying treatment in α-synucleinopathies. To what degree melatonin has therapeutic efficacy in the prevention of α-synucleinopathies awaits further investigation, in particular multicenter double-blind trials.

Orexin pathway in Parkinson's disease: a review

Parkinson's disease (PD) is a progressive neurodegenerative disease (NDD) caused by dopaminergic neuron degeneration in the substantia nigra (SN). Orexin is a neuropeptide that plays a role in the pathogenesis of PD. Orexin has neuroprotective properties in dopaminergic neurons. In PD neuropathology, there is also degeneration of orexinergic neurons in the hypothalamus, in addition to dopaminergic neurons. However, the loss of orexinergic neurons in PD began after the degeneration of dopaminergic neurons. Reduced activity of orexinergic neurons has been linked to developing and progressing motor and non-motor symptoms in PD. In addition, the dysregulation of the orexin pathway is linked to the development of sleep disorders. The hypothalamic orexin pathway regulates various aspects of PD neuropathology at the cellular, subcellular, and molecular levels. Finally, non-motor symptoms, particularly insomnia and disturbed sleep, promote neuroinflammation and the accumulation of neurotoxic proteins as a result of defects in autophagy, endoplasmic reticulum (ER) stress, and the glymphatic system. As a result, this review aimed to highlight the potential role of orexin in PD neuropathology.

Glymphatic pathway in sporadic cerebral small vessel diseases: From bench to bedside

Cerebral small vessel diseases (CSVD) consist of a group of diseases with high heterogeneity induced by pathologies of intracranial small blood vessels. Endothelium dysfunction, bloodbrain barrier leakage and the inflammatory response are traditionally considered to participate in the pathogenesis of CSVD. However, these features cannot fully explain the complex syndrome and related neuroimaging characteristics. In recent years, the glymphatic pathway has been discovered to play a pivotal role in clearing perivascular fluid and metabolic solutes, which has provided novel insights into neurological disorders. Researchers have also explored the potential role of perivascular clearance dysfunction in CSVD. In this review, we presented a brief overview of CSVD and the glymphatic pathway. In addition, we elucidated CSVD pathogenesis from the perspective of glymphatic failure, including basic animal models and clinical neuroimaging markers. Finally, we proposed forthcoming clinical applications targeting the glymphatic pathway, hoping to provide novel ideas on promising therapies and preventions of CSVD.

Aquaporin-4 in glymphatic system, and its implication for central nervous system disorders

The clearance function is essential for maintaining brain tissue homeostasis, and the glymphatic system is the main pathway for removing brain interstitial solutes. Aquaporin-4 (AQP4) is the most abundantly expressed aquaporin in the central nervous system (CNS) and is an integral component of the glymphatic system. In recent years, many studies have shown that AQP4 affects the morbidity and recovery process of CNS disorders through the glymphatic system, and AQP4 shows notable variability in CNS disorders and is part of the pathogenesis of these diseases. Therefore, there has been considerable interest in AQP4 as a potential and promising target for regulating and improving neurological impairment. This review aims to summarize the pathophysiological role that AQP4 plays in several CNS disorders by affecting the clearance function of the glymphatic system. The findings can contribute to a better understanding of the self-regulatory functions in CNS disorders that AQP4 were involved in and provide new therapeutic alternatives for incurable debilitating neurodegenerative disorders of CNS in the future.

DTI-ALPS: An MR biomarker for motor dysfunction in patients with subacute ischemic stroke

Purpose

Brain glymphatic dysfunction is involved in the pathologic process of acute ischemic stroke (IS). The relationship between brain glymphatic activity and dysfunction in subacute IS has not been fully elucidated. Diffusion tensor image analysis along the perivascular space (DTI-ALPS) index was used in this study to explore whether glymphatic activity was related to motor dysfunction in subacute IS patients.

Methods

Twenty-six subacute IS patients with a single lesion in the left subcortical region and 32 healthy controls (HCs) were recruited in this study. The DTI-ALPS index and DTI metrics (fractional anisotropy, FA, and mean diffusivity, MD) were compared within and between groups. Spearman's and Pearson's partial correlation analyses were performed to analyze the relationships of the DTI-ALPS index with Fugl-Meyer assessment (FMA) scores and with corticospinal tract (CST) integrity in the IS group, respectively.

Results

Six IS patients and two HCs were excluded. The left DTI-ALPS index of the IS group was significantly lower than that of the HC group (t = −3.02, p = 0.004). In the IS group, a positive correlation between the left DTI-ALPS index and the simple Fugl-Meyer motor function score (ρ = 0.52, p = 0.019) and a significant negative correlation between the left DTI-ALPS index and the FA (R = −0.55, p = 0.023) and MD (R = −0.48, p = 0.032) values of the right CST were found.

Conclusions

Glymphatic dysfunction is involved in subacute IS. DTI-ALPS could be a potential magnetic resonance (MR) biomarker of motor dysfunction in subacute IS patients. These findings contribute to a better understanding of the pathophysiological mechanisms of IS and provide a new target for alternative treatments for IS.

Neuroimaging uncovers distinct relationships of glymphatic dysfunction and motor symptoms in Parkinson's disease

BACKGROUND

Studies of glymphatic dysfunction in Parkinson's disease (PD) patients have attracted much attention in recent years. However, the relationships between glymphatic dysfunction and clinical symptoms remains unclear.

OBJECTIVES

To determine whether the diffusion tensor image analysis along the perivascular space (DTI-ALPS) affect the severity and types of motor and non-motor symptoms in PD patients.

METHODS

De novo PD patients and controls who performed both DTI and ¹²³I-DaTscan single photon emission computed tomography (SPECT) scanning were retrieved from the international multicenter Parkinson's Progression Marker Initiative (PPMI) cohort. Glymphatic system was evaluated by the DTI-ALPS. Motor symptoms were assessed by Movement Disorders Society Unified Parkinson's Disease Rating Scale III (MDS-UPDRS-III). The influence of glymphatic activity on motor and non-motor symptoms was explored by multivariate linear regression models.

RESULTS

A total of 153 PD patients (mean age 60.97 ± 9.47 years; 99 male) and 67 normal controls (mean age 60.10 ± 10.562 years; 43 male) were included. The DTI-ALPS index of PD patients was significantly lower than normal controls (Z = - 2.160, p = 0.031). MDS-UPDRS III score (r = - 0.213, p = 0.008) and subscore for rigidity (r = - 0.177, p = 0.029) were negatively correlated with DTI-ALPS index. The DTI-ALPS index was significantly associated with MDS-UPDRS-III score (β = - 0.160, p = 0.048) and subscore for rigidity (β = - 0.170, p = 0.041) after adjusting for putamen dopamine transporter availability and clinical factors.

CONCLUSIONS

Our results showed distinct relationships between glymphatic dysfunction and the severity and types of PD motor symptoms, suggesting the potential of DTI-ALPS index as a biomarker for PD motor symptoms.

Noninvasive neuroimaging provides evidence for deterioration of the glymphatic system in Parkinson's disease relative to essential tremor

INTRODUCTION

Growing evidence has demonstrated dysfunction of the glymphatic system in α-synucleinopathy and related diseases. In this study, we aimed to use diffusion tensor image analysis along the perivascular space (DTI-ALPS) and MRI-visible enlarged perivascular spaces (EPVS) to evaluate glymphatic system function quantitatively and qualitatively and its relationship to clinical scores of disease severity in Parkinson's disease (PD) and essential tremor (ET).

METHODS

Overall, 124 patients with PD, 74 with ET, and 106 healthy controls (HC) were enrolled. Two trained neurologists performed quantitative calculations of ALPS on DTI and visual ratings of EPVS on T2-weighted images in the centrum semiovale (CSO), basal ganglia (BG), midbrain, and cerebellum.

RESULTS

The ALPS index was lower in patients with PD than in patients with ET (p < 0.001) and HC (p < 0.001). Similarly, patients with PD showed a more severe EPVS burden in the CSO, BG, and midbrain compared to ET and HC. Moreover, the ALPS index was negatively correlated with disease severity in the PD subgroups; however, it did not differ within the ET subgroup. No differences in ALPS or EPVS were observed between the ET and HC groups.

CONCLUSION

In conclusion, DTI-ALPS and EPVS both provide neuroimaging evidence of glymphatic system dysfunction in PD, which further supports that PD is an α-synucleinopathy disease, while ET is a cerebellar dysfunction-related disease.

Assessment of glymphatic function in narcolepsy using DTI-ALPS index

INTRODUCTION

Sleep is a modulator of glymphatic activity which is altered in various sleep disorders. Narcolepsy is a sleep disorder characterized by excessive daytime sleepiness (EDS), rapid onset of rapid eye movement (REM) sleep, cataplexy, disturbed night sleep with fragmentation. It is categorized into two types, type 1 (NT1) and type 2 (NT2) depending on the presence of cataplexy and/or absence of orexin. We sought for alterations in glymphatic activity in narcoleptic patients using diffusion tensor imaging (DTI) along perivascular space (ALPS) index on magnetic resonance imaging (MRI).

MATERIAL AND METHODS

Adult patients diagnosed with NT1 or NT2 who had polysomnography (PSG) and MRI with DTI were included in the study. Sleep recording included Epworth Sleepiness Scale (ESS) score, sleep latency during multiple sleep latency test (MSLT), sleep efficiency during night PSG, wake after sleep onset (WASO), REM sleep latency during PSG, percentage of non-REM (NREM), REM sleep and wakefulness during night PSG. DTI-ALPS index was calculated for each patient and age-sex matched healthy control(HC)s.

RESULTS

The study group was composed of 25 patients [F/M = 15/10, median age = 34 (29.5-44.5)], 14 with NT1 and 11 with NT2 disease. ESS, WASO and percentage of wakefulness were significantly higher in NT1 patients (p < 0.05). Mean DTI-ALPS was not significantly different neither between narcoleptic patients and HCs, nor between NT1 and NT2 patients (all, p > 0.05). However, DTI-ALPS was negatively correlated with WASO (r = -0.745, p = 0.013) and percentage of wakefulness (r = -0.837, p = 0.005) in NT1 patients. DTI-ALPS correlated negatively with percentage of N1 sleep (r = -0.781, p = 0.005) but positively with REM percentage (r = 0.618, p = 0.043) in NT2 patients.

CONCLUSION

In this study, DTI-ALPS was not significantly different in narcoleptic patients than the HCs. However, the glymphatic index as assessed by DTI-ALPS correlated with PSG parameters; negatively with WASO, percentage of wakefulness in NT1, percentage of N1 sleep in NT2, and positively with REM sleep in NT2. A tendency for a reduction in DTI-ALPS in NT1 patients compared to both NT2 patients and HCs was also found. These findings might show the first evidence of an alteration of glymphatic activity, especially in NT1 patients, thus warrant further prospective studies in larger size of narcoleptic patient cohorts.