Intestinal permeability and orocaecal transit time in elderly patients with Parkinson's disease

Gut neuropeptide involvement in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder affecting over 10 million people. A key pathological feature of PD is the accumulation of misfolded α-synuclein (aSyn) protein in the substantia nigra pars compacta. Aggregation of aSyn can form Lewy bodies that contribute to dopaminergic neuron degeneration and motor symptoms, such as tremor, rigidity, and bradykinesia. Beyond the central nervous system, aSyn aggregates have been detected in the gastrointestinal (GI) tract, suggesting a link between peripheral aSyn and nonmotor PD symptoms. GI symptoms, often preceding motor symptoms by up to 20 years, highlight the bidirectional communication between the central nervous system and the enteric nervous system (gut-brain axis) in PD. Although microbiome alterations and intestinal inflammation have been associated with PD, functional impacts on gut-brain signaling or aSyn aggregation remain unclear. Intestinal neuropeptides are key modulators of gut-brain communication, alter immune response to pathogens and environmental toxins, and may contribute to the function of the luminal gut barrier. Dysregulation of gut neuropeptide signaling, including vasoactive intestinal peptide, neuropeptide Y, calcitonin gene-related peptide, ghrelin, cholecystokinin, glucagon-like peptide 1, and substance P, have been associated with pathologic effects of PD in animal models. Despite their potential role in pathogenesis and disease modulation, gut neuropeptide roles in PD are underexplored. This article reviews current knowledge surrounding microbial metabolite and immune influences on gut neuropeptide signaling, aSyn aggregation in the enteric nervous system, and downstream neuroimmune pathway alterations within the context of PD and its mouse models.

Paracellular permeability and tight junction regulation in gut health and disease

Epithelial tight junctions define the paracellular permeability of the intestinal barrier. Molecules can cross the tight junctions via two distinct size-selective and charge-selective paracellular pathways: the pore pathway and the leak pathway. These can be distinguished by their selectivities and differential regulation by immune cells. However, permeability increases measured in most studies are secondary to epithelial damage, which allows non-selective flux via the unrestricted pathway. Restoration of increased unrestricted pathway permeability requires mucosal healing. By contrast, tight junction barrier loss can be reversed by targeted interventions. Specific approaches are needed to restore pore pathway or leak pathway permeability increases. Recent studies have used preclinical disease models to demonstrate the potential of pore pathway or leak pathway barrier restoration in disease. In this Review, we focus on the two paracellular flux pathways that are dependent on the tight junction. We discuss the latest evidence that highlights tight junction components, structures and regulatory mechanisms, their impact on gut health and disease, and opportunities for therapeutic intervention.

Disease mechanisms as subtypes: Microbiome

Abnormalities in gut microbiota have been suggested to be involved in the pathophysiology and progression of Parkinson's disease (PD). Gastrointestinal nonmotor symptoms often precede the onset of motor features in PD, suggesting a role for gut dysbiosis in neuroinflammation and α-synuclein (α-syn) aggregation. In the first part of this chapter, we analyze critical features of healthy gut microbiota and factors (environmental and genetic) that modify its composition. In the second part, we focus on the mechanisms underlying the gut dysbiosis and how it alters anatomically and functionally the mucosal barrier, triggering neuroinflammation and subsequently α-syn aggregation. In the third part, we describe the most common alterations in the gut microbiota of PD patients, dividing the gastrointestinal system in higher and lower tract to examine the association between microbiota abnormalities and clinical features. In the final section, we report on current and future therapeutic approaches to gut dysbiosis aiming to either reduce the risk for PD, modify the disease course, or improve the pharmacokinetic profile of dopaminergic therapies. We also suggest that further studies will be needed to clarify the role of the microbiome in PD subtyping and of pharmacological and nonpharmacological interventions in modifying specific microbiota profiles in individualizing disease-modifying treatments in PD.

The Interplay between Gut Microbiota and Parkinson's Disease: Implications on Diagnosis and Treatment

The bidirectional interaction between the gut microbiota (GM) and the Central Nervous System, the so-called gut microbiota brain axis (GMBA), deeply affects brain function and has an important impact on the development of neurodegenerative diseases. In Parkinson’s disease (PD), gastrointestinal symptoms often precede the onset of motor and non-motor manifestations, and alterations in the GM composition accompany disease pathogenesis. Several studies have been conducted to unravel the role of dysbiosis and intestinal permeability in PD onset and progression, but the therapeutic and diagnostic applications of GM modifying approaches remain to be fully elucidated. After a brief introduction on the involvement of GMBA in the disease, we present evidence for GM alterations and leaky gut in PD patients. According to these data, we then review the potential of GM-based signatures to serve as disease biomarkers and we highlight the emerging role of probiotics, prebiotics, antibiotics, dietary interventions, and fecal microbiota transplantation as supportive therapeutic approaches in PD. Finally, we analyze the mutual influence between commonly prescribed PD medications and gut-microbiota, and we offer insights on the involvement also of nasal and oral microbiota in PD pathology, thus providing a comprehensive and up-to-date overview on the role of microbial features in disease diagnosis and treatment.

Gastric Emptying Is Not Delayed and Does Not Correlate With Attenuated Postprandial Blood Flow Increase in Medicated Patients With Early Parkinson's Disease

Background

We have recently used phase-contrast magnetic resonance imaging (PC-MRI) to demonstrate an attenuated postprandial blood flow response in the superior mesenteric artery (SMA) in 23 medicated patients with Parkinson's disease (PD) compared to 23 age- and sex-matched healthy controls.

Objective

To investigate in a sub-sample of the original cohort whether the observed blood flow response in SMA after oral food intake is related to a delay in gastric emptying.

Methods

We studied 15 patients with PD in an “ON-medication” state with a mean disease duration of 3.9 ± 2.2 years and 15 healthy age- and sex-matched individuals. Participants underwent dynamic gastric scintigraphy 0, 30, 60, 120, 180 and 240 minutes after the intake of a standardized radiolabeled test meal. Gastric emptying was compared between groups. 14 of the 15 PD patients and 12 of the 15 healthy control subjects had previously undergone serial postprandial PC-MRI measurements. In these individuals, we tested for a relationship between gastric emptying and postprandial blood flow response in the SMA.

Results

The dynamics of gastric emptying did not differ between groups (p = 0.68). There was substantial inter-subject variability of gastric emptying in PD patients and healthy participants. Only a single PD patient had delayed gastric emptying. In those participants who had undergone PC-MRI, postprandial increase in SMA blood flow was attenuated in PD compared to healthy controls as reported previously (p = 0.006). Gastric emptying did not correlate with the timing and amplitude of postprandial blood flow increase in SMA.

Conclusion

Our preliminary results, obtained in a small group of early-stage PD patients who continued their usual dopamine replacement therapy, suggest that variations in gastric emptying after solid meal intake is within the normal range in the majority of cases. There is also no evidence for a tight relationship between the attenuated postprandial blood flow response in the SMA and normal variations in gastric emptying.

Motor and non-motor circuit disturbances in early Parkinson disease: which happens first?

For the last two decades, pathogenic concepts in Parkinson disease (PD) have revolved around the toxicity and spread of α-synuclein. Thus, α-synuclein would follow caudo-rostral propagation from the periphery to the central nervous system, first producing non-motor manifestations (such as constipation, sleep disorders and hyposmia), and subsequently impinging upon the mesencephalon to account for the cardinal motor features before reaching the neocortex as the disease evolves towards dementia. This model is the prevailing theory of the principal neurobiological mechanism of disease. Here, we scrutinize the temporal evolution of motor and non-motor manifestations in PD and suggest that, even though the postulated bottom-up mechanisms are likely to be involved, early involvement of the nigrostriatal system is a key and prominent pathophysiological mechanism. Upcoming studies of detailed clinical manifestations with newer neuroimaging techniques will allow us to more closely define, in vivo, the role of α-synuclein aggregates with respect to neuronal loss during the onset and progression of PD.

Safety and Tolerability, Dose-Escalating, Double-Blind Trial of Oral Mannitol in Parkinson's Disease

Mannitol, a natural alcoholic-sugar, was recently suggested as a potential disease-modifying agent in Parkinson's disease. In animal models of the disease, mannitol interferes with the formation of α-synuclein fibrils, inhibits the formation of α-synuclein oligomers and leads to phenotypic recovery of impaired motor functions. Parkinson's patients who consume mannitol report improvements of both motor and non-motor symptoms. Safety of long-term use of oral mannitol, tolerable dose and possible benefit, however, were never clinically studied. We studied the safety of oral mannitol in Parkinson's disease and assessed the maximal tolerable oral dose by conducting a phase IIa, randomized, double-blind, placebo-controlled, single-center, dose-escalating study (ClinicalTrials.gov Identifier: NCT03823638). The study lasted 36 weeks and included four dose escalations of oral mannitol or dextrose to a maximal dose of 18 g per day. The primary outcome was the safety of oral mannitol, as assessed by the number of adverse events and abnormal laboratory results. Clinical and biochemical efficacy measures were collected but were not statistically-powered. Fourteen patients receiving mannitol completed the trial (in addition to eight patients on placebo). Mannitol-related severe adverse events were not observed. Gastrointestinal symptoms limited dose escalation in 6/14 participants on mannitol. None of the clinical or biochemical efficacy secondary outcome measures significantly differed between groups. We concluded that long-term use of 18 g per day of oral mannitol is safe in Parkinson's disease patients but only two third of patients tolerate this maximal dose. These findings should be considered in the design of future efficacy trials.

T cells, α-synuclein and Parkinson disease

The notion that autoimmune responses to α-synuclein may be involved in the pathogenesis of this disorder stems from reports that mutations in α-synuclein or certain alleles of the major histocompatibility complex (MHC) are associated with the disease and that dopaminergic and norepinephrinergic neurons in the midbrain can present antigenic epitopes. Here, we discuss recent evidence that a defined set of peptides derived from α-synuclein act as antigenic epitopes displayed by specific MHC alleles and drive helper and cytotoxic T cell responses in patients with PD. Moreover, phosphorylated α-synuclein may activate T cell responses in a less restricted manner in PD. While the roles for the acquired immune system in disease pathogenesis remain unknown, preclinical animal models and in vitro studies indicate that T cells may interact with neurons and exert effects related to neuronal death and neuroprotection. These findings suggest that therapeutics that target T cells and ameliorate the incidence or disease severity of inflammatory bowel disorders or CNS autoimmune diseases such as multiple sclerosis may be useful in PD.

Mediterranean Diet Adherence in People With Parkinson's Disease Reduces Constipation Symptoms and Changes Fecal Microbiota After a 5-Week Single-Arm Pilot Study

Introduction: Non-motor symptoms of Parkinson's disease (PD) such as gastrointestinal (GI) dysfunction are common, yet little is known about how modifying dietary intake impacts PD symptoms. The aim of this study in individuals with PD was to determine whether a Mediterranean diet intervention is feasible and affects GI function, intestinal permeability and fecal microbial communities.

Methods: A single-arm, 5-week Mediterranean diet intervention study was conducted in eight people with PD. Daily and weekly questionnaires were administered to determine changes in GI symptoms. Urine and stool samples were collected at baseline and after 5 weeks to assess intestinal permeability and fecal microbial communities. Additionally, live-in partners of the participants with PD were matched as controls (n = 8) for baseline urine and stool samples.

Results: Participants with PD increased intake of Mediterranean diet based on adherence scores from baseline to week 5 (4.4 ± 0.6 vs. 11.9 ± 0.7; P < 0.01 with >10 representing good adherence), which was linked with weight loss (77.4 kg vs. 74.9 kg, P = 0.01). Constipation syndrome scores decreased after 5 weeks (2.3 ± 0.5 vs. 1.5 ± 0.3; P = 0.04). Bilophila, was higher at baseline in PD (0.6 ± 0.1% vs. 0.2 ± 0.1% P = 0.02) and slightly decreased after the diet intervention (0.5 ± 0.1%; P = 0.01). Interestingly, the proportion of Roseburia was significantly lower in PD compared to controls (0.6 ± 0.2% vs. 1.6 ± 0.3%; P = 0.02) and increased at week 5 (0.9 ± 0.2%; P < 0.01). No differences were observed for markers of intestinal permeability between the control and PD groups or post-intervention.

Conclusions: Short-term Mediterranean diet adherence is feasible in participants with PD; correlated with weight loss, improved constipation, and modified gut microbiota.

Clinical Trial Registration:ClinicalTrials.gov, identifier: NCT03851861.

Intestinal Permeability, Dysbiosis, Inflammation and Enteric Glia Cells: The Intestinal Etiology of Parkinson’s Disease

The scientific and medical communities are becoming more aware of the substantial relationship between the function of the central nervous system (CNS) and the state of the gut environment. Parkinson's disease (PD) is a neurodegenerative disorder that affects the nigrostriatal pathway in the midbrain, presenting not only motor symptoms but also various non-motor manifestations, including neuropsychiatric symptoms and gastrointestinal (GI) symptoms. Over time, our knowledge of PD has progressed from the detection of midbrain dopaminergic deficits to the identification of a multifaceted disease with a variety of central and peripheral manifestations, with increased attention to the intestinal tract. Accumulating evidence has revealed that intestinal disorders are not only the peripheral consequence of PD pathogenesis, but also the possible pathological initiator decades before it progresses to the CNS. Here, we summarized recent research findings on the involvement of the intestinal environment in PD, with an emphasis on the involvement of the intestinal barrier, microbiome and its metabolites, inflammation, and enteric glial cells

Gastrointestinal mucosal biopsies in Parkinson's disease: beyond alpha-synuclein detection

Alpha-synuclein deposits, the pathological hallmarks of Parkinson's disease, are consistently found in the gastrointestinal tract of parkinsonian subjects. These observations have raised the potential that endoscopically obtainable mucosal biopsies can aid to a molecular diagnosis of the disease. The possible usefulness of mucosal biopsies is, however, not limited to the detection of alpha-synuclein, but also extends to other essential aspects underlying pathophysiological mechanisms of gastrointestinal manifestations in Parkinson's disease. The aim of the current review is to provide an appraisal of the existing studies showing that gastrointestinal biopsies can be used for the analysis of enteric neuronal and glial cell morphology, intestinal epithelial barrier function, and gastrointestinal inflammation in Parkinson's disease. A perspective on the generation of organoids with GI biopsies and the potential use of single-cell and spatial transcriptomic technologies will be also addressed.

Small Intestinal Bacterial Overgrowth as Potential Therapeutic Target in Parkinson's Disease

Increasing evidence suggests that the gut microbiota and the brain are closely connected via the so-called gut–brain axis. Small intestinal bacterial overgrowth (SIBO) is a gut dysbiosis in which the small intestine is abundantly colonized by bacteria that are typically found in the colon. Though not a disease, it may result in intestinal symptoms caused by the accumulation of microbial gases in the intestine. Intestinal inflammation, malabsorption and vitamin imbalances may also develop. SIBO can be eradicated by one or several courses of antibiotics but reappears if the predisposing condition persists. Parkinson’s disease (PD) is a common neurodegenerative proteinopathy for which disease modifying interventions are not available. Sporadic forms may start in the gut years before the development of clinical features. Increased gastrointestinal transit time is present in most people with PD early during the course of the disease, predisposing to gut dysbiosis, including SIBO. The role that gut dysbiosis may play in the etiopathogenesis of PD is not fully understood yet. Here, we discuss the possibility that SIBO could contribute to the progression of PD, by promoting or preventing neurodegeneration, thus being a potential target for treatments aiming at slowing down the progression of PD. The direct symptomatic impact of SIBO and its impact on symptomatic medication are also briefly discussed.

What Is Our Understanding of the Influence of Gut Microbiota on the Pathophysiology of Parkinson's Disease?

Microbiota have increasingly become implicated in predisposition to human diseases, including neurodegenerative disorders such as Parkinson's disease (PD). Traditionally, a central nervous system (CNS)-centric approach to understanding PD has predominated; however, an association of the gut with PD has existed since Parkinson himself reported the disease. The gut-brain axis refers to the bidirectional communication between the gastrointestinal tract (GIT) and the brain. Gut microbiota dysbiosis, reported in PD patients, may extend this to a microbiota-gut-brain axis. To date, mainly the bacteriome has been investigated. The change in abundance of bacterial products which accompanies dysbiosis is hypothesised to influence PD pathophysiology via multiple mechanisms which broadly centre on inflammation, a cause of alpha-synuclein (a-syn) misfolding. Two main routes are hypothesised by which gut microbiota can influence PD pathophysiology, the neural and humoral routes. The neural route involves a-syn misfolding peripherally in the enteric nerves which can then be transported to the brain via the vagus nerve. The humoral route involves transportation of bacterial products and proinflammatory cytokines from the gut via the circulation which can cause central a-syn misfolding by inducing neuroinflammation. This article will assess whether the current literature supports gut bacteria influencing PD pathophysiology via both routes.

The Role of The Gut Microbiome in Parkinson's Disease

The gut microbiota is known to play a role in various disease states through inflammatory, immune and endocrinologic response. Parkinson's Disease is of particular interest as gastrointestinal involvement is one of the earlier features seen in this disease. This paper examines the relationship between gut microbiota and Parkinson's Disease, which has a growing body of literature. Inflammation caused by gut dysbiosis is thought to increase a-synuclein aggregation and worsen motor and neurologic symptoms of Parkinson's disease. We discuss potential treatment and supplementation to modify the microbiota. Some of these treatments require further research before recommendations can be made, such as cord blood transplant, antibiotic use, immunomodulation and fecal microbiota transplant. Other interventions, such as increasing dietary fiber, polyphenol and fermented food intake, can be made with few risks and may have some benefit for symptom relief and speed of disease progression.

Association of small intestinal bacterial overgrowth with Parkinson's disease: a systematic review and meta-analysis

Objective

Parkinson’s disease (PD) is the second most prevalent neurodegenerative disease after Alzheimer's disease (AD) worldwide. The prevalence of small intestinal bacterial overgrowth (SIBO) in PD patients is high. We conducted this comprehensive systematic review and meta-analysis to determine the association between SIBO and PD.

Methods

A comprehensive literature search of the PubMed, Cochrane Library and EMBASE databases was performed to identify studies correlating SIBO with PD. Studies were screened, and relevant data were extracted and analysed. We calculated the pooled prevalence of SIBO in all individuals with PD and compared the prevalence of SIBO between the two groups to calculate an odds ratio (OR) and 95% confidence interval (CI). Egger’s test was performed to assess publication bias.

Results

Eleven studies with 973 participants met the inclusion criteria. The pooled prevalence of SIBO in patients with PD was 46% (95% CI 36–56). A random-effects model was applied given the heterogeneity (I² = 83%) detected among the studies. Egger’s test indicated no publication bias (p = 0.0657). Subgroup analyses showed that the prevalence of SIBO was greater in studies including patients diagnosed using the lactulose hydrogen breath test (LBT) (51%, 95% CI 37–65) than in those including patients diagnosed using the glucose hydrogen breath test (GBT) (35%, 95% CI 20–50), and the prevalence of SIBO in PD was highest (55%, 95% CI 38–72) in patients diagnosed by the LBT and GBT. The prevalence of SIBO was 52% (95% CI 40–64) among patients from Western countries and 33% (95% CI 22–43) among patients from Eastern countries. The pooled OR of SIBO in PD patients compared with healthy controls was 5.22 (95% CI 3.33–8.19, p < 0.00001). We did not identify an obvious predictor of SIBO in PD patients.

Conclusion

In conclusion, our meta-analysis found a strong association between SIBO and PD with approximately half of PD patients testing positive for SIBO. These relationships significantly differed based on diagnostic test and geographic area.

Assessment of Gastrointestinal Autonomic Dysfunction: Present and Future Perspectives

The autonomic nervous system delicately regulates the function of several target organs, including the gastrointestinal tract. Thus, nerve lesions or other nerve pathologies may cause autonomic dysfunction (AD). Some of the most common causes of AD are diabetes mellitus and α-synucleinopathies such as Parkinson’s disease. Widespread dysmotility throughout the gastrointestinal tract is a common finding in AD, but no commercially available method exists for direct verification of enteric dysfunction. Thus, assessing segmental enteric physiological function is recommended to aid diagnostics and guide treatment. Several established assessment methods exist, but disadvantages such as lack of standardization, exposure to radiation, advanced data interpretation, or high cost, limit their utility. Emerging methods, including high-resolution colonic manometry, 3D-transit, advanced imaging methods, analysis of gut biopsies, and microbiota, may all assist in the evaluation of gastroenteropathy related to AD. This review provides an overview of established and emerging assessment methods of physiological function within the gut and assessment methods of autonomic neuropathy outside the gut, especially in regards to clinical performance, strengths, and limitations for each method.

Gastrointestinal and metabolic function in the MPTP-treated macaque model of Parkinson's disease

Background

Gastrointestinal (GI) and metabolic function are frequently altered in Parkinson's disease (PD). Although enteric nervous system anatomopathological alterations have previously been reported in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkey model of PD, the resulting gastric emptying and intestinal permeability functional parameters are unknown. The current exploratory study was, thus, designed to investigate these GI functional factors and insulin resistance in the MPTP-treated monkey.

Methods

Eight rhesus macaque monkeys (4 controls and 4 MPTP-treated) received the oral acetaminophen absorption test to measure gastric emptying, the oral FITC-dextran absorption test to investigate intestinal permeability, and the intravenous glucose tolerance test to assess insulin resistance. Constipation was evaluated using the Bristol stool scale.

Results

None of the tests, acetaminophen absorption, FITC-dextran absorption or glucose tolerance, showed a difference between control and MPTP-treated monkeys. MPTP-treated monkeys did present signs of transit acceleration.

Conclusion

While the MPTP monkey model reliably displays motor and certain non-motor symptoms of PD, the current study did not demonstrate the GI symptoms associated with PD.

Gastrointestinal dysfunction in the synucleinopathies

Interest in gastrointestinal dysfunction in Parkinson's disease has blossomed over the past 30 years and has generated a wealth of investigation into this non-motor aspect of the disorder, research that has encompassed its pathophysiology, its clinical features, and its impact on quality of life. The question of gastrointestinal dysfunction in the other synucleinopathies has not received nearly as much attention, but information and knowledge are growing. In this review, the current knowledge, controversies, and gaps in our understanding of the pathophysiology of gastrointestinal dysfunction in Parkinson's disease and the other synucleinopathies will be addressed, and extended focus will be directed toward the clinical problems involving saliva management, swallowing, gastric emptying, small intestinal function, and bowel function that are so problematic in these disorders.

Autonomic Dysfunction in Parkinson's Disease

Recognition of the importance of nonmotor dysfunction as a component of Parkinson's disease has exploded over the past three decades. Autonomic dysfunction is a frequent and particularly important nonmotor feature because of the broad clinical spectrum it covers. Cardiovascular, gastrointestinal, urinary, sexual, and thermoregulatory abnormalities all can appear in the setting of Parkinson's disease. Cardiovascular dysfunction is characterized most prominently by orthostatic hypotension. Gastrointestinal dysfunction can involve virtually all levels of the gastrointestinal tract. Urinary dysfunction can entail either too frequent voiding or difficulty voiding. Sexual dysfunction is frequent and frustrating for both patient and partner. Alterations in sweating and body temperature are not widely recognized but often are present. Autonomic dysfunction can significantly and deleteriously impact quality of life for individuals with Parkinson's disease. Because effective treatment for many aspects of autonomic dysfunction is available, it is vitally important that assessment of autonomic dysfunction be a regular component of the neurologic history and exam and that appropriate treatment be initiated and maintained.

The Intestinal Barrier in Parkinson's Disease: Current State of Knowledge

The intestinal barrier, which primarily consists of epithelial cells stitched together with connecting proteins called tight junctions, plays a critical role in health and disease. It is in close contact with the gut microbiota on its luminal side and with the enteric neurons on the tissue side. Both microbiota and the enteric nervous system are regulatory housekeepers of the intestinal barrier. Therefore, the recently observed enteric neuropathology along with gut dysbiosis in Parkinson's disease have prompted research on intestinal permeability in this neurodegenerative disorder. In this mini-review we attempt to concisely summarize the current knowledge on intestinal barrier in Parkinson's disease. We envision future direction research that should be pursued in order to demonstrate its possible role in disease development and progression.

Distinctive Pathophysiology Underlying Constipation in Parkinson's Disease: Implications for Cognitive Inefficiency

Depression is associated with constipation within and outside Parkinson’s disease (PD). Since inefficient cognitive-processing (bradyphrenia) features in PD and an enterokinetic agent improved cognitive performance in healthy individuals, bradyphrenia may be associated with constipation. We aim to define the archetypical bowel function of PD, and its association with cognition, mood, and motor features within and outside PD. We assessed colonic transit time (oral radio-opaque markers over 6 days), bowel function and psychometric questionnaires and measures of PD facets, including bradyphrenia, in 58 participants with diagnosed PD, and 71 without (controls). The best abdominal X-ray (day 7) predictors of PD status were total retained marker count and transverse colon segmental delay. However, Rome functional constipation status complemented segmental delay better, giving good specificity (85%) but low sensitivity (56%). Transverse colon marker count appeared to be age-associated only in PD. In PD, those correctly classified by bowel dysfunction had higher depression scores (p = 0.02) and longer cognitive-processing times than the misclassified (p = 0.05). Controls misclassified as PD by bowel dysfunction had higher depression and anxiety scores than the correctly classified (p = 0.002 and 0.003, respectively), but not slower cognitive processing. Measures of motor features were independent of sub-classification by bowel function in PD and in controls. In conclusion, constipation in PD has distinct localized pathophysiology, and is associated with bradyphrenia.

The enteric nervous system: “A little brain in the gut”

The gut’s own autonomous nervous system, the enteric nervous system (ENS), has fascinated scientists for more than 100 years. It functions, in the true sense of the word, autonomously, by performing complex tasks and controlling vital functions independently of extrinsic inputs. At the same time, the ENS is bombarded with signals from other cells in the gut wall and lumen and has to integrate all of these inputs. We describe the main functions of the ENS under physiological conditions and give a few examples of its role in gut diseases. The ENS has received increasing attention recently as scientists outside the field of Neurogastroenterology realize its important role in the pathogenesis of Parkinson’s, autism and multiple sclerosis.

Parkinson's disease and the gastrointestinal microbiome

Recently, there has been a surge in awareness of the gastrointestinal microbiome (GM) and its role in health and disease. Of particular note is an association between the GM and Parkinson's disease (PD) and the realisation that the GM can act via a complex bidirectional communication between the gut and the brain. Compelling evidence suggests that a shift in GM composition may play an important role in the pathogenesis of PD by facilitating the characteristic ascending neurodegenerative spread of α-synuclein aggregates from the enteric nervous system to the brain. Here, we review evidence linking GM changes with PD, highlighting mechanisms supportive of pathological α-synuclein spread and intestinal inflammation in PD. We summarise existing patterns and correlations seen in clinical studies of the GM in PD, together with the impacts of non-motor symptoms, medications, lifestyle, diet and ageing on the GM. Roles of GM modulating therapies including probiotics and faecal microbiota transplantation are discussed. Encouragingly, alterations in the GM have repeatedly been observed in PD, supporting a biological link and highlighting it as a potential therapeutic target.

Autoimmunity in Parkinson's Disease: The Role of α-Synuclein-Specific T Cells

Evidence from a variety of studies implicates a role for the adaptive immune system in Parkinson's disease (PD). Similar to multiple sclerosis (MS) patients who display a high number of T cells in the brain attacking oligodendrocytes, PD patients show higher numbers of T cells in the ventral midbrain than healthy, age-matched controls. Mouse models of the disease also show the presence of T cells in the brain. The role of these infiltrating T cells in the propagation of disease is controversial; however, recent studies indicate that they may be autoreactive in nature, recognizing disease-altered self-proteins as foreign antigens. T cells of PD patients can generate an autoimmune response to α-synuclein, a protein that is aggregated in PD. α-Synuclein and other proteins are post-translationally modified in an environment in which protein processing is altered, possibly leading to the generation of neo-epitopes, or self-peptides that have not been identified by the host immune system as non-foreign. Infiltrating T cells may also be responding to such modified proteins. Genome-wide association studies (GWAS) have shown associations of PD with haplotypes of major histocompatibility complex (MHC) class II genes, and a polymorphism in a non-coding region that may increase MHC class II in PD patients. We speculate that the inflammation observed in PD may play both pathogenic and protective roles. Future studies on the adaptive immune system in neurodegenerative disorders may elucidate steps in disease pathogenesis and assist with the development of both biomarkers and treatments.

Gut reaction: impact of systemic diseases on gastrointestinal physiology and drug absorption

It was in 400 BC that Hippocrates reportedly stated that "death sits in the colon". The growth in our knowledge of the intestinal microbiome, the gut-brain axis and their function and imbalance has distinctly uncovered the complex relationship between the gut to disease predisposition and development, heralding the problem and the solution to disease pathology. Human studies of new drug molecules are typically performed in healthy volunteers and their specific disease indication. Approved drugs, however, are used by patients with diverse disease backgrounds. Here, we review the current literature of the gastrointestinal tract reacting to systemic disease pathology that elicits physiological and functional changes that consequently affect oral drug product performance.

Can the gut be the missing piece in uncovering PD pathogenesis?

It is now well established that Parkinson's disease (PD) is not only a movement disorder of the CNS but also a gastrointestinal disorder affecting the enteric nervous system (ENS). The gut-brain axis is a bidirectional communication between the brain and the gastrointestinal tract, which comprises besides the CNS and the ENS, the intestinal epithelial barrier, the intestinal microbiota and the enteroendocrine systems. In this review, we present the clinical and pathological evidence suggesting that the gut-brain axis is dysfunctional in PD by discussing the possible role of gut microbiota, inflammation and permeability in the development of the disease.

Imaging the Autonomic Nervous System in Parkinson's Disease

PURPOSE OF REVIEW

Patients with Parkinson's disease (PD) often display gastrointestinal and genitourinary autonomic symptoms years or even decades prior to diagnosis. These symptoms are thought to be caused in part by pathological α-synuclein inclusions in the peripheral autonomic and enteric nervous systems. It has been proposed that the initial α-synuclein aggregation may in some PD patients originate in peripheral nerve terminals and then spread centripetally to the spinal cord and brainstem. In vivo imaging methods can directly quantify the degeneration of the autonomic nervous system as well as the functional consequences such as perturbed motility. Here, we review the methodological principles of these imaging techniques and the major findings in patients with PD and atypical parkinsonism.

RECENT FINDINGS

Loss of sympathetic and parasympathetic nerve terminals in PD can be visualized using radiotracer imaging, including ¹²³I-MIBG scintigraphy, and ¹⁸F-dopamine and ¹¹C-donepezil PET. Recently, ultrasonographical studies disclosed reduced diameter of the vagal nerves in PD patients. Radiological and radioisotope techniques have demonstrated dysmotility and prolonged transit time throughout all subdivisions of the gastrointestinal tract in PD. The prevalence of objective dysfunction as measured with these imaging methods is often considerably higher compared to the prevalence of subjective symptoms experienced by the patients. Degeneration of the autonomic nervous system may play a key role in the pathogenesis of PD. In vivo imaging techniques provide powerful and noninvasive tools to quantify the degree and extent of this degeneration and its functional consequences.

Impaired tissue barriers as potential therapeutic targets for Parkinson's disease and amyotrophic lateral sclerosis

The blood-brain barrier and the intestinal barrier show signs of disruption in patients with idiopathic Parkinson's disease (PD) and animal models of nigrostriatal degeneration, and likewise in amyotrophic lateral sclerosis (ALS) models. A substantial body of evidence shows that defects in epithelial membrane barriers, both in the gut and within the cerebral vasculature, can result in increased vulnerability of tissues to external factors potentially participating in the pathogenesis of PD and ALS. As such, restoration of tissue barriers may prove to be a novel therapeutic target in neurodegenerative disease. In this review, we focus on the potential of new intervention strategies for rescuing and maintaining barrier functions in PD and ALS.

Gastric emptying in Parkinson's disease - A mini-review

Patients with Parkinson's disease (PD) experience a range of non-motor symptoms, including constipation and other gastrointestinal problems. These symptoms are sometimes present in the prodromal disease phase. An improved understanding of the underlying pathophysiology is needed considering that PD has been hypothesized to originate in the gut. Delayed gastric emptying time (GET) is often listed as a prevalent gastrointestinal symptom in PD, but the true prevalence is controversial. The aim of this short review was to investigate if GET in PD is dependent on the applied measuring methodology. A systemic search of Pubmed identified 15 relevant studies, including six using gold standard method gastric scintigraphy and nine using ¹³C-octanoate breath tests. Overall, gastric scintigraphy studies showed a non-significant GET delay (standardized mean difference (SMD) 0.42) in PD patients. After exclusion of one outlier study, GET was significantly increased (SMD 0.59). In contrast, highly significant GET delay (SMD 1.70) was seen in breath test studies. A limitation of the meta-analyses was reuse of the same control group in some studies. In summary, the marked GET delay observed in breath test studies is not confirmed by gold standard gastric scintigraphy studies. This discrepancy can perhaps be explained by breath test being an indirect GET measure, depending not only on mechanic stomach emptying but also intestinal absorption and liver metabolism. Thus, multi-modality studies under standardized conditions are needed to elucidate the prevalence and severity of gastric dysmotility in PD, along with contributions from other factors including intestinal absorption and permeability.

Does Parkinson's disease start in the gut?

Parkinson's disease (PD) is pathologically characterized by the presence of intraneuronal inclusions, termed Lewy bodies and Lewy neurites, whose main component is alpha-synuclein. Based on the topographic distribution of Lewy bodies and neurites established after autopsy from PD patients, Braak and coworkers hypothesized that PD pathology may start in the gastrointestinal tract then spread through the vagus nerve to the brain. This hypothesis has been reinforced by the discovery that alpha-synuclein may be capable of spreading transcellularly, thereby providing a mechanistic basis for Braak's hypothesis. This 'gut to brain' scenario has ignited heated debates within the movement disorders community and prompted a large number of studies in both humans and animals. Here, we review the arguments for and against the gut as the origin of PD. We conclude that the human autopsy evidence does not support the hypothesis and that it is too early to draw any definitive conclusions. We discuss how this issue might be further addressed in future research.

REM sleep behavior disorder is related to enteric neuropathology in Parkinson disease

OBJECTIVE

To determine whether REM sleep behavior disorder (RBD) in Parkinson disease (PD) is associated with lesions and dysfunctions of the autonomic nervous system by evaluating enteric phosphorylated α-synuclein histopathology (PASH) and permeability.

METHODS

A total of 45 patients with PD were included in this cross-sectional study. RBD was diagnosed on the basis of a standardized clinical interview and confirmed by polysomnography. For each patient, 5 biopsies were taken at the junction between the sigmoid and descending colon during the course of a rectosigmoidoscopy. For the detection of enteric PASH, 2 colonic biopsies were analyzed by immunohistochemistry with antibodies against phosphorylated α-synuclein and PGP9.5 in 43 patients (2 patients were excluded because only 1 biopsy was available). The paracellular permeability and transcellular permeability were evaluated by measuring sulfonic acid and horseradish peroxidase flux, respectively, in the 3 remaining biopsies mounted in Ussing chambers.

RESULTS

Enteric PASH was more frequent in the subgroup of patients with PD with RBD compared to patients without RBD (18 of 28, 64.3%, vs 2 of 15, 13.3%, respectively, p < 0.01). No differences were observed in intestinal permeability between patients with PD with and without RBD.

CONCLUSIONS

Patients with PD and RBD have a greater frequency of synuclein pathology in the enteric nervous system, suggesting that RBD is associated with widespread synuclein neuropathology.

Imaging Parkinson's disease below the neck

Parkinson's disease is a systemic disorder with widespread and early α-synuclein pathology in the autonomic and enteric nervous systems, which is present throughout the gastrointestinal canal prior to diagnosis. Gastrointestinal and genitourinary autonomic symptoms often predate clinical diagnosis by several years. It has been hypothesized that progressive α-synuclein aggregation is initiated in hyperbranched, non-myelinated neuron terminals, and may subsequently spread via retrograde axonal transport. This would explain why autonomic nerves are so prone to formation of α-synuclein pathology. However, the hypothesis remains unproven and in vivo imaging methods of peripheral organs may be essential to study this important research field. The loss of sympathetic and parasympathetic nerve terminal function in Parkinson's disease has been demonstrated using radiotracers such as 123I-meta-iodobenzylguanidin, 18F-dopamine, and 11C-donepezil. Other radiotracer and radiological imaging methods have shown highly prevalent dysfunction of pharyngeal and esophageal motility, gastric emptying, colonic transit time, and anorectal function. Here, we summarize the methodology and main findings of radio-isotope and radiological modalities for imaging peripheral pathology in Parkinson's disease.

What a gastrointestinal biopsy can tell us about Parkinson's disease?

BACKGROUND

The intraneuronal inclusions called Lewy bodies and neurites, which represent the characteristic pathological changes in Parkinson's disease, are found in the enteric neurons in the great majority of parkinsonian patients. This observation led to a substantial amount of research over the last few years in order to develop a minimally invasive diagnostic procedure in living patients based on gastrointestinal (GI) biopsies.

PURPOSE

In this review, we will begin by discussing the studies that focused on the detection of Lewy bodies and neurites in GI biopsies, then broaden the discussion to the pathological changes that also occur in the enteric glial cells and intestinal epithelial cells. We conclude by proposing that a GI biopsy could represent a unique window to assess the whole pathological process of the brain in Parkinson's disease.

Structural alterations of the intestinal epithelial barrier in Parkinson's disease

Functional and morphological alterations of the intestinal epithelial barrier (IEB) have been consistently reported in digestive disorders such as irritable bowel syndrome and inflammatory bowel disease. There is mounting evidence that Parkinson's disease (PD) is not only a brain disease but also a digestive disorder. Gastrointestinal involvement is a frequent and early event in the course of PD, and it may be critically involved in the early development of the disease. We therefore undertook the present survey to investigate whether changes in the IEB function and/or morphology occur in PD. Colonic biopsies were performed in 31 PD patients and 11 age-matched healthy controls. The para- and transcellular permeability were evaluated by measuring sulfonic acid and horseradish peroxidase flux respectively, in colonic biopsies mounted in Ussing chambers. The expression and localization of the two tight junctions proteins ZO-1 and occludin were analyzed by Western blot and immunofluorescence, respectively. The para- and transcellular permeability were not different between PD patients and controls. The expression of occludin, but not ZO-1, was significantly lower in colonic samples from PD patients as compared to controls and the cellular distribution of both proteins was altered in colonic mucosal specimens from PD patients. Our findings provide evidence that the IEB is morphologically altered in PD and further reinforce the potential role of the gastrointestinal tract in the initiation and/or the progression of the disease.

Causes and consequences of degeneration of the dorsal motor nucleus of the vagus nerve in Parkinson's disease

SIGNIFICANCE

Parkinson's disease (PD) is no longer considered merely a movement disorder caused by degeneration of dopamine neurons in the midbrain. It is now recognized as a widespread neuropathological syndrome accompanied by a variety of motor and nonmotor clinical symptoms. As such, any hypothesis concerning PD pathogenesis and pathophysiology must account for the entire spectrum of disease and not solely focus on the dopamine system.

RECENT ADVANCES

Based on its anatomy and the intrinsic properties of its neurons, the dorsal motor nucleus of the vagus nerve (DMV) is uniquely vulnerable to damage from PD. Fibers in the vagus nerve course throughout the gastrointestinal (GI) tract to and from the brainstem forming a close link between the peripheral and central nervous systems and a point of proximal contact between the environment and areas where PD pathology is believed to start. In addition, DMV neurons are under high levels of oxidative stress due to their high level of α-synuclein expression, fragile axons, and specific neuronal physiology. Moreover, several consequences of DMV damage, namely, GI dysfunction and unrestrained inflammation, may propagate a vicious cycle of injury affecting vulnerable brain regions.

CRITICAL ISSUES

Current evidence to suggest the vagal system plays a pivotal role in PD pathogenesis is circumstantial, but given the current state of the field, the time is ripe to obtain direct experimental evidence to better delineate it.

FUTURE DIRECTIONS

Better understanding of the DMV and vagus nerve may provide insight into PD pathogenesis and a neural highway with direct brain access that could be harnessed for novel therapeutic interventions.

Small intestinal bacterial overgrowth in Parkinson's disease

BACKGROUND

Recent studies reported a high prevalence of small intestinal bacterial overgrowth (SIBO) in Parkinson's disease (PD), and a possible association with gastrointestinal symptoms and worse motor function. We aimed to study the prevalence and the potential impact of SIBO on gastrointestinal symptoms, motor function, and quality of life in a large cohort of PD patients.

METHODS

103 Consecutive PD patients were assessed using the lactulose-hydrogen breath test; questionnaires of gastrointestinal symptoms and quality of life (PDQ-39); the Unified PD Rating Scale (UPDRS) including "on"-medication Part III (motor severity) score; and objective and quantitative measures of bradykinesia (Purdue Pegboard and timed test of gait). Patients and evaluating investigators were blind to SIBO status.

RESULTS

25.3% of PD patients were SIBO-positive. SIBO-positive patients had a shorter mean duration of PD (5.2 ± 4.1 vs. 8.1 ± 5.5 years, P = 0.007). After adjusting for disease duration, SIBO was significantly associated with lower constipation and tenesmus severity scores, but worse scores across a range of "on"-medication motor assessments (accounting for 4.2-9.0% of the variance in motor scores). There was no association between SIBO and motor fluctuations or PDQ-39 Summary Index scores.

CONCLUSIONS

This is the largest study to date on SIBO in PD. SIBO was detected in one quarter of patients, including patients recently diagnosed with the disease. SIBO was not associated with worse gastrointestinal symptoms, but independently predicted worse motor function. Properly designed treatment trials are needed to confirm a causal link between SIBO and worse motor function in PD.

The nutritional status of Dutch elderly patients with Parkinson's disease

OBJECTIVES

To assess the prevalence of (risk of) undernutrition in Dutch elder Parkinson's disease patients as well as it's risk factors.

DESIGN

Observational cross-sectional study.

SETTING

An outpatient clinic at the department Neurology of Medical Centre Leeuwarden, a large teaching hospital.

PARTICIPANTS

102 outpatients with Parkinson's disease aged 65 years and older were recruited.

MEASUREMENTS

Data regarding various aspects of undernutrition including socio-demographic aspect, disease characterisitics, nutritional status, appetite and overall-physical and psychological functioning were collected.

RESULTS

Undernutrition was diagnosed in 2.0% and 20.5% of the patients were categorized as being at risk of undernutrition. Care dependency and appetite were the two risk factors with the highest predictive value for an unfavorable nutritional status.

CONCLUSION

Of Dutch elderly patients with Parkinson's Disease 22.5% had an unfavourable nutritional status. Dependency and appetite were the two risk factors with the highest predictive value fort his outcome. Because undernutrition can be regarded as a geriatric syndrome a comprehensive nutritional assessment should be done followed by nutritional interventions next to interventions focused on the risk factors. Further studies are needed to evaluate these interventions.

Intestinal permeability defects: is it time to treat?

An essential role of the intestinal epithelium is to separate luminal contents from the interstitium, a function primarily determined by the integrity of the epithelium and the tight junction that seals the paracellular space. Intestinal tight junctions are selectively permeable, and intestinal permeability can be increased physiologically in response to luminal nutrients or pathologically by mucosal immune cells and cytokines, the enteric nervous system, and pathogens. Compromised intestinal barrier function is associated with an array of clinical conditions, both intestinal and systemic. Although most available data are correlative, some studies support a model where cycles of increased intestinal permeability, intestinal immune activation, and subsequent immune-mediated barrier loss contribute to disease progression. This model is applicable to intestinal and systemic diseases. However, it has not been proven, and both mechanistic and therapeutic studies are ongoing. Nevertheless, the correlation between increased intestinal permeability and disease has caught the attention of the public, leading to a rise in popularity of the diagnosis of "leaky gut syndrome," which encompasses a range of systemic disorders. Proponents claim that barrier restoration will cure underlying disease, but this has not been demonstrated in clinical trials. Moreover, human and mouse studies show that intestinal barrier loss alone is insufficient to initiate disease. It is therefore uncertain whether increased permeability in these patients is a cause or effect of the underlying disorder. Although drug targets that may mediate barrier restoration have been proposed, none have been proven effective. As such, current treatments for barrier dysfunction should target the underlying disease.

The beneficial role of thiamine in Parkinson disease

Parkinson disease (PD) is the second most common form of neurodegeneration among elderly individuals. PD is clinically characterized by tremors, rigidity, slowness of movement, and postural imbalance. In this paper, we review the evidence for an association between PD and thiamine. Interestingly, a significant association has been demonstrated between PD and low levels of serum thiamine, and thiamine supplements appear to have beneficial clinical effects against PD. Multiple studies have evaluated the connection between thiamine and PD pathology, and candidate pathways involve the transcription factor Sp1, p53, Bcl-2, caspase-3, tyrosine hydroxylase, glycogen synthase kinase-3β, vascular endothelial growth factor, advanced glycation end products, nuclear factor kappa B, mitogen-activated protein kinase, and the reduced form of nicotinamide adenine dinucleotide phosphate. Thus, a review of the literature suggests that thiamine plays a role in PD, although further investigation into the effects of thiamine in PD is needed.

Increased intestinal permeability and Parkinson disease patients: chicken or egg?

Gastrointestinal involvement is a frequent and early event in the course of Parkinson Disease (PD), and may have a prominent role in the early pathophysiology of the disease. On the other hand, derangement in intestinal permeability could also result from the involvement of the gastrointestinal tract over the course of the disease.

PATIENTS AND METHODS

The intestinal permeability of 12 non-selected PD patients was studied using a validated, non-invasive test; these results were compared to predefined age-adjusted reference values.

RESULTS

4/12 PD patients had abnormal gastrointestinal permeability; two had both an abnormal lactulose/mannitol ratio and an abnormal sucrose concentration, and two an isolated abnormal result. An increased lactulose/mannitol ratio is consistent with defect of either the enterocytes or the tight junctions between them.

CONCLUSION

Intestinal permeability is increased in a significant proportion of unselected PD patients with minimal gastrointestinal symptoms. The significance of this finding needs to be further evaluated.

Increased intestinal permeability correlates with sigmoid mucosa alpha-synuclein staining and endotoxin exposure markers in early Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder of aging. The pathological hallmark of PD is neuronal inclusions termed Lewy bodies whose main component is alpha-synuclein protein. The finding of these Lewy bodies in the intestinal enteric nerves led to the hypothesis that the intestine might be an early site of PD disease in response to an environmental toxin or pathogen. One potential mechanism for environmental toxin(s) and proinflammatory luminal products to gain access to mucosal neuronal tissue and promote oxidative stress is compromised intestinal barrier integrity. However, the role of intestinal permeability in PD has never been tested. We hypothesized that PD subjects might exhibit increased intestinal permeability to proinflammatory bacterial products in the intestine. To test our hypothesis we evaluated intestinal permeability in subjects newly diagnosed with PD and compared their values to healthy subjects. In addition, we obtained intestinal biopsies from both groups and used immunohistochemistry to assess bacterial translocation, nitrotyrosine (oxidative stress), and alpha-synuclein. We also evaluated serum markers of endotoxin exposure including LPS binding protein (LBP). Our data show that our PD subjects exhibit significantly greater intestinal permeability (gut leakiness) than controls. In addition, this intestinal hyperpermeability significantly correlated with increased intestinal mucosa staining for E. coli bacteria, nitrotyrosine, and alpha-synuclein as well as serum LBP levels in PD subjects. These data represent not only the first demonstration of abnormal intestinal permeability in PD subjects but also the first correlation of increased intestinal permeability in PD with intestinal alpha–synuclein (the hallmark of PD), as well as staining for gram negative bacteria and tissue oxidative stress. Our study may thus shed new light on PD pathogenesis as well as provide a new method for earlier diagnosis of PD and suggests potential therapeutic targets in PD subjects.

Trial Registration

Clinicaltrials.govNCT01155492

Bladder, bowel, and sexual dysfunction in Parkinson's disease

Bladder dysfunction (urinary urgency/frequency), bowel dysfunction (constipation), and sexual dysfunction (erectile dysfunction) (also called “pelvic organ” dysfunctions) are common nonmotor disorders in Parkinson's disease (PD). In contrast to motor disorders, pelvic organ autonomic dysfunctions are often nonresponsive to levodopa treatment. The brain pathology causing the bladder dysfunction (appearance of overactivity) involves an altered dopamine-basal ganglia circuit, which normally suppresses the micturition reflex. By contrast, peripheral myenteric pathology causing slowed colonic transit (loss of rectal contractions) and central pathology causing weak strain and paradoxical anal sphincter contraction on defecation (PSD, also called as anismus) are responsible for the bowel dysfunction. In addition, hypothalamic dysfunction is mostly responsible for the sexual dysfunction (decrease in libido and erection) in PD, via altered dopamine-oxytocin pathways, which normally promote libido and erection. The pathophysiology of the pelvic organ dysfunction in PD differs from that in multiple system atrophy; therefore, it might aid in differential diagnosis. Anticholinergic agents are used to treat bladder dysfunction in PD, although these drugs should be used with caution particularly in elderly patients who have cognitive decline. Dietary fibers, laxatives, and “prokinetic” drugs such as serotonergic agonists are used to treat bowel dysfunction in PD. Phosphodiesterase inhibitors are used to treat sexual dysfunction in PD. These treatments might be beneficial in maximizing the patients' quality of life.