Depression in Parkinson disease--epidemiology, mechanisms and management

Low adherence to antidepressants is associated with increased mortality in Parkinson disease patients

INTRODUCTION

The purpose of this study was to evaluate the relationship between adherence to antidepressants (AD) and all-cause mortality in a population-based cohort of patients with Parkinson's Disease (PD).

METHODS

From a database of more than 4 million people, 8553 patients with PD who purchased an AD at least once between the years 2008-2011 were retrospectively followed for all-cause mortality over 4-years. Adherence was measured as a ratio between dispensed and prescribed durations and was modeled as: non-adherence (<20%, n = 1566), poor (20%-50%, n = 1184), moderate (50%-80%, n = 1584), and good (>80%, n = 4219) adherence. Multivariable survival analyses adjusted for demographic and clinical variables including physical comorbidities known to influence mortality were conducted, however there was no adjustment for other psychiatric disorders and medications.

RESULTS

Unadjusted mortality rates were 20.4%, 25.1%, 23.4% and 25.6% in those classified as non-adherent, poor, moderate and good adherence respectively (χ2 = 18.45, p < 0.0001). The non-adherent and poor adherence groups had significantly increased adjusted mortality hazard ratios (HR) of 1.43 (CI: 1.26-1.62) and 1.26 (CI: 1.1-1.44) respectively compared to the good adherence group. Using the same model, the adjusted HR for death among males was 1.49 [95% CI: 1.36-1.62] compared to females. People with PD and Charslon's Comorbidity Index score of 3-4 (HR 1.3, P < 0.001) and 5+ (HR 1.78, P < 0.001) were more likely to die than those with 0-2 comorbidities.

CONCLUSIONS

Our findings suggest that poor adherence to AD is associated with increased all-cause mortality in people with PD. Given the high prevalence of depression and AD effectiveness, efforts to promote adherence should be prioritized in clinical practice.

Implications of Circadian Rhythm in Dopamine and Mood Regulation

Mammalian physiology and behavior are regulated by an internal time-keeping system, referred to as circadian rhythm. The circadian timing system has a hierarchical organization composed of the master clock in the suprachiasmatic nucleus (SCN) and local clocks in extra-SCN brain regions and peripheral organs. The circadian clock molecular mechanism involves a network of transcription-translation feedback loops. In addition to the clinical association between circadian rhythm disruption and mood disorders, recent studies have suggested a molecular link between mood regulation and circadian rhythm. Specifically, genetic deletion of the circadian nuclear receptor Rev-erbα induces mania-like behavior caused by increased midbrain dopaminergic (DAergic) tone at dusk. The association between circadian rhythm and emotion-related behaviors can be applied to pathological conditions, including neurodegenerative diseases. In Parkinson's disease (PD), DAergic neurons in the substantia nigra pars compacta progressively degenerate leading to motor dysfunction. Patients with PD also exhibit non-motor symptoms, including sleep disorder and neuropsychiatric disorders. Thus, it is important to understand the mechanisms that link the molecular circadian clock and brain machinery in the regulation of emotional behaviors and related midbrain DAergic neuronal circuits in healthy and pathological states. This review summarizes the current literature regarding the association between circadian rhythm and mood regulation from a chronobiological perspective, and may provide insight into therapeutic approaches to target psychiatric symptoms in neurodegenerative diseases involving circadian rhythm dysfunction.

Neuropathology of Nonmotor Symptoms of Parkinson's Disease

Parkinson's disease (PD), a multiorgan neurodegenerative disorder associated with α-synuclein deposits throughout the nervous system and many organs, is clinically characterized by motor and nonmotor features, many of the latter antedating motor dysfunctions by 20 or more years. The causes of the nonmotor manifestations such as olfactory, autonomic, sensory, neuropsychiatric, visuospatial, sleep, and other disorders are unlikely to be related to single lesions. They are mediated by the involvement of both dopaminergic and nondopaminergic systems, and diverse structures outside the nigrostriatal system that is mainly responsible for the motor features of PD. The nonmotor alterations appear in early/prodromal stages of the disease and its further progression, suggesting a topographical and chronological spread of the lesions. This lends further support for the notion that PD is a multiorgan proteinopathy, although the exact relationship between presymptomatic and later developing nonmotor features of PD and neuropathology awaits further elucidation.

Folding Underlies Bidirectional Role of GPR37/Pael-R in Parkinson Disease

Since conformational flexibility, which is required for the function of a protein, comes at the expense of structural stability, many proteins, including G-protein-coupled receptors (GPCRs), are under constant risk of misfolding and aggregation. In this regard GPR37 (also named PAEL-R and ETBR-LP-1) takes a prominent role, particularly in relation to Parkinson disease (PD). GPR37 is a substrate for parkin and accumulates abnormally in autosomal recessive juvenile parkinsonism, contributing to endoplasmic reticulum stress and death of dopaminergic neurons. GPR37 also constitutes a core structure of Lewy bodies, demonstrating a more general involvement in PD pathology. However, if folded and matured properly, GPR37 seems to be neuroprotective. Moreover, GPR37 modulates functionality of the dopamine transporter and the dopamine D2 receptor and stimulates dopamine neurotransmission. Here we review the multiple roles of GPR37 with relevance to potential disease modification and symptomatic therapies of PD and highlight unsolved issues in this field.

The bidirectional longitudinal relationship between insomnia, depression and anxiety in patients with early-stage, medication-naïve Parkinson's disease

INTRODUCTION

While anxiety, depression and insomnia frequently (co-)occur in Parkinson's disease (PD) patients, little is known about their temporal relationship. In this study, we tested two hypotheses: i) insomnia predicts an increase in symptoms of depression or anxiety and ii) anxiety or depression at baseline predicts insomnia in PD patients six months later.

METHODS

We used longitudinal data from a prospective cohort study of early-stage, medication-naïve PD patients. Primary outcome measures were: anxiety symptoms, measured with the State-Trait Anxiety Inventory (STAI); depressive symptoms, measured with the 15-item Geriatric Depression Scale (GDS-15); and insomnia, defined as a score ≥ 2 on item 1.7 of the Movement Disorder Society - Unified Parkinson's Disease Rating Scale. We performed linear and logistic regression analyses, correcting for baseline value of the respective outcome variable.

RESULTS

Baseline insomnia was not associated with GDS-15 or STAI total score at follow-up. In a post hoc analysis, we found that insomnia predicted a higher STAI State score (B(SE) = 2.50 (1.07), p < 0.05), while the association with the STAI Trait score was not significant. Baseline STAI scores (B(SE) = 0.02 (0.01), p = 0.001) and GDS-15 score (B(SE) = 0.15 (0.05), p < 0.001) were significantly associated with insomnia at follow-up.

CONCLUSION

Symptoms of anxiety and depression may constitute a risk factor for insomnia in PD. The relationship between insomnia and anxiety is bidirectional, which suggests that both anxiety and sleep disorders can start a negative spiral in PD patients, where one enhances the other. Independent clinical attention for these symptoms in PD patients is therefore warranted.

Aberrant Intra- and Internetwork Functional Connectivity in Depressed Parkinson's Disease

Much is known concerning the underlying mechanisms of Parkinson’s disease (PD) with depression, but our understanding of this disease at the neural-system level remains incomplete. This study used resting-state functional MRI (rs-fMRI) and independent component analysis (ICA) to investigate intrinsic functional connectivity (FC) within and between large-scale neural networks in 20 depressed PD (dPD) patients, 35 non-depressed PD (ndPD) patients, and 34 healthy controls (HC). To alleviate the influence caused by ICA model order selection, this work reported results from analyses at 2 levels (low and high model order). Within these two analyses, similar results were obtained: 1) dPD and ndPD patients relative to HC had reduced FC in basal ganglia network (BGN); 2) dPD compared with ndPD patients exhibited increased FC in left frontoparietal network (LFPN) and salience network (SN), and decreased FC in default-mode network (DMN); 3) dPD patients compared to HC showed increased FC between DMN and LFPN. Additionally, connectivity anomalies in the DMN, LFPN and SN correlated with the depression severity in patients with PD. Our findings confirm the involvement of BGN, DMN, LFPN and SN in depression in PD, facilitating the development of more detailed and integrative neural models of PD with depression.

Dissecting the Molecular Mechanisms of Neurodegenerative Diseases through Network Biology

Neurodegenerative diseases are rarely caused by a mutation in a single gene but rather influenced by a combination of genetic, epigenetic and environmental factors. Emerging high-throughput technologies such as RNA sequencing have been instrumental in deciphering the molecular landscape of neurodegenerative diseases, however, the interpretation of such large amounts of data remains a challenge. Network biology has become a powerful platform to integrate multiple omics data to comprehensively explore the molecular networks in the context of health and disease. In this review article, we highlight recent advances in network biology approaches with an emphasis in brain-networks that have provided insights into the molecular mechanisms leading to the most prevalent neurodegenerative diseases including Alzheimer’s (AD), Parkinson’s (PD) and Huntington’s diseases (HD). We discuss how integrative approaches using multi-omics data from different tissues have been valuable for identifying biomarkers and therapeutic targets. In addition, we discuss the challenges the field of network medicine faces toward the translation of network-based findings into clinically actionable tools for personalized medicine applications.

Diagnosing mild cognitive impairment in Parkinson's disease: which tests perform best in the Italian population?

Mild cognitive impairment (MCI) is common in patients with Parkinson's disease (PD) and should be recognized early because it represents a predictor of PD-related dementia and worse disease course. Diagnostic criteria for PD-related MCI (PD-MCI) have recently been defined by a Movement Disorders Society (MDS) task force. The present study explored which neuropsychological tests perform best for a level II (i.e., comprehensive neuropsychological assessment) diagnosis of PD-MCI according to the MDS task force criteria in Italian-speaking PD patients. To this aim, we assessed a comprehensive 23-item neuropsychological battery, derived the best-performing 10-test battery (i.e., two tests per domain for each of the five cognitive domains), and explored its accuracy for diagnosing PD-MCI in comparison to the full battery in a group of PD patients. A secondary aim was to explore the role of this battery for subtyping PD-MCI according to single-domain vs. multiple-domain involvement. The 10-test battery showed 73% sensitivity and 100% specificity for diagnosing PD-MCI, and 69% sensitivity and 100% specificity for PD-MCI subtyping. In patients older than 70 years, we derived a slightly different 10-test battery with 84% sensitivity and 100% specificity for PD-MCI diagnosis, and 86% sensitivity and 100% specificity for PD-MCI subtyping. These 10-item neuropsychological batteries might represent a good trade-off between diagnostic accuracy and time of application, and their role in PD-MCI diagnosis and subtyping should be further explored in future prospective studies.

Comorbidity burden of patients with Parkinson's disease and Parkinsonism between 2003 and 2012: A multicentre, nationwide, retrospective study in China

Parkinson’s disease (PD) and Parkinsonism are common neurodegenerative disorders with continuously increasing prevalence, causing high global burdens. However, data concerning the comorbidity burden of patients with PD or Parkinsonism in China are lacking. To investigate the health condition and comorbidity burden, a total of 3367 PD and 823 Parkinsonism patients were included from seven tertiary hospitals in seven cities across China from 2003 to 2012. Their comorbidity burden was collected and quantified by the Elixhauser Comorbidity Index (ECI) and Charlson Comorbidity Index (CCI). The comorbidity spectra differed between PD and Parkinsonism patients. Compared with PD patients, Parkinsonism patients were older (69.8 ± 11.5 vs. 67.9 ± 11.4, P < 0.001); had a higher comorbidity burden, including ECI (1.1 ± 1.2 vs. 1.0 ± 1.2, P < 0.001) and CCI (1.3 ± 1.6 vs. 1.1 ± 1.5, P < 0.001); and had higher hospitalization expenses. The ECI (1.1 ± 1.3 vs. 0.9 ± 1.1, P < 0.001) and CCI (1.3 ± 1.6 vs. 0.9 ± 1.2, P < 0.001) were higher in males than in females. The average length of stay and daily hospitalization expenses increased with age, as did ECI and CCI. This is the first study to report the disease burden of Chinese PD and Parkinsonism patients. It provides useful information to better understand their health status, and to raise the awareness of clinicians for providing better health care.

The effect of unilateral subthalamic nucleus deep brain stimulation on depression in Parkinson's disease

BACKGROUND

Depression is common in Parkinson's disease (PD) and adversely affects quality of life. Both unilateral and bilateral subthalamic (STN) deep brain stimulation (DBS) effectively treat the motor symptoms of PD, but questions remain regarding the impact of unilateral STN DBS on non-motor symptoms, such as depression.

METHODS

We report changes in depression, as measured by the Hamilton Depression Rating Scale (HAMD-17), in 50 consecutive PD patients who underwent unilateral STN DBS. Participants were also evaluated with UPDRS part III, Parkinson's Disease Questionnaire-39, and Pittsburgh Sleep Quality Index. The primary outcome was change in HAMD-17 at 6 months versus pre-operative baseline, using repeated measures analysis of variance (ANOVA). Secondary outcomes included the change in HAMD-17 at 3, 12, 18, and 24 months post-operatively and correlations amongst outcome variables using Pearson correlation coefficients. As a control, we also evaluated changes in HAMD-17 in 25 advanced PD patients who did not undergo DBS.

RESULTS

Participants with unilateral STN DBS experienced significant improvement in depression 6 months post-operatively (4.94 ± 4.02) compared to preoperative baseline (7.90 ± 4.44) (mean ± SD) (p = <0.0001). HAMD-17 scores did not correlate with UPDRS part III at any time-point. Interestingly, the HAMD-17 was significantly correlated with sleep quality and quality of life at baseline, 3 months, and 6 months post-operatively. Participants without DBS experienced no significant change in HAMD-17 over the same interval.

CONCLUSION

Unilateral STN DBS improves depression 6 months post-operatively in patients with PD. Improvement in depression is maintained over time and correlates with improvement in sleep quality and quality of life.

Treadmill exercise alleviates depressive symptoms in rotenone-induced Parkinson disease rats

Parkinson disease (PD) is characterized by selective loss of the dopaminergic neurons. The symptoms of depression following PD are closely associated with reduced activity of the serotonergic system in the dorsal raphe. We explored the antidepressive effect of exercise and its possible mechanism using the rotenone-induced PD rats. PD rats were induced by subcutaneously injection with rotenone for 14 days. The rats in the exercise groups were made to run on a treadmill for 30 min once a day during 14 consecutive days. Forced swimming test, immunohistochemistry for serotonin (5-hydroxytryptamine, 5-HT), tryptophan hydroxylase (TPH), and western blot for serotonin 1A (5-HT1A) receptor were conducted. Injection of rotenone induced PD rats. PD rats showed depressive state and treadmill exercise ameliorated this depressive state. 5-HT, TPH, and 5-HT1A receptor expressions in the dorsal raphe were suppressed by rotenone injection and treadmill exercise increased the expressions of 5-HT, TPH, and 5-HT1A receptor in the rotenone-injected rats. The present results show that treadmill exercise ameliorated depressive symptoms in the rotenone-induced PD rats. The antidepressive effect of treadmill exercise might be ascribed to the enhancement of serotonergic function through upregulation of 5-HT1A expression in the dorsal raphe.

The role of high-field magnetic resonance imaging in parkinsonian disorders: Pushing the boundaries forward

Historically, magnetic resonance imaging (MRI) has contributed little to the study of Parkinson's disease (PD), but modern MRI approaches have unveiled several complementary markers that are useful for research and clinical applications. Iron- and neuromelanin-sensitive MRI detect qualitative changes in the substantia nigra. Quantitative MRI markers can be derived from diffusion weighted and iron-sensitive imaging or volumetry. Functional brain alterations at rest or during task performance have been captured with functional and arterial spin labeling perfusion MRI. These markers are useful for the diagnosis of PD and atypical parkinsonism, to track disease progression from the premotor stages of these diseases and to better understand the neurobiological basis of clinical deficits. A current research goal using MRI is to generate time-dependent models of the evolution of PD biomarkers that can help understand neurodegeneration and provide reliable markers for therapeutic trials. This article reviews recent advances in MRI biomarker research at high-field (3T) and ultra high field-imaging (7T) in PD and atypical parkinsonism. © 2017 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Type A monoamine oxidase and serotonin are coordinately involved in depressive disorders: from neurotransmitter imbalance to impaired neurogenesis

Type A monoamine oxidase (MAOA) catabolizes monoamine transmitters, serotonin, norepinephrine and dopamine, and plays a major role in the onset, progression and therapy of neuropsychiatric disorders. In depressive disorders, increase in MAOA expression and decrease in brain levels of serotonin and norepinephrine are proposed as the major pathogenic factors. The functional polymorphism of MAOA gene and genes in serotonin signal pathway are associated with depression. This review presents recent advance in studies on the role of MAOA in major depressive disorder and related emotional disorders. MAOA and serotonin regulate the prenatal development and postnatal maintenance of brain architecture and neurocircuit, as shown by MAOA-deficient humans and MAO knockout animal models. Impaired neurogenesis in the mature hippocampus has been proposed as "adult neurogenesis" hypothesis of depression. MAOA modulates the sensitivity to stress in the stages of brain development and maturation, and the interaction of gene-environmental factors in the early stage regulates the onset of depressive behaviors in adulthood. Vice versa environmental factors affect MAOA expression by epigenetic regulation. MAO inhibitors not only restore compromised neurotransmitters, but also protect neurons from cell death in depression through induction of anti-apoptotic Bcl-2 and prosurvival neurotrophic factors, especially brain-derived neurotrophic factor, the deficiency of which is detected in depression. This review discusses novel role of MAOA and serotonin in the pathogenesis and therapy of depressive disorders.

Association between gene polymorphism and depression in Parkinson's disease: A case-control study

OBJECTIVE

To investigate possible associations of Parkinson's disease (PD) with polymorphism in depression-related genes and in the alpha-synuclein (SNCA) gene.

METHODS

A consecutive series of patients with PD were divided into those with depression and those without it. Patients (330) were genotyped at four single-nucleotide polymorphisms (SNPs) in four genes previously associated with depression, as well as four SNPs in the PD-associated SNCA gene.

RESULTS

Of 330 patients, 125 (37.9%) had depression and 205 (62.1%) did not. Univariate analysis revealed significant differences between the two groups in minor allele frequency at the SNP rs1545843 in the SLC6A15 gene (p<0.05), as well as in frequencies of genotypes and minor alleles at rs78162420 in the TPH2 gene (all p<0.05). Logistic regression identified the following risk factors for depression among patients with PD: Hoehn and Yahr stage>2 (OR 1.759, 95%CI 1.035-2.989, p=0.037), AA genotype at rs1545843 (OR 1.866, 95%CI 1.017-3.426, p=0.044), and AC genotype at rs78162420 (OR 5.036, 95%CI 1.451-17.484, p=0.011).

CONCLUSIONS

Among patients with PD, depression is associated with polymorphism at rs78162420 and rs1545843, both previously linked with depression. Our results may help clarify the pathogenesis of depression in PD.

Alterations of p11 in brain tissue and peripheral blood leukocytes in Parkinson's disease

Individuals with Parkinson's disease (PD) often suffer from comorbid depression. P11 (S100A10), a member of the S100 family of proteins, is expressed widely throughout the body and is involved in major depressive disorder and antidepressant response. Central p11 levels are reduced in postmortem tissue from depressed individuals; however, p11 has not yet been investigated in PD patients with depression or those without depression. We investigated p11 levels in postmortem PD brains and assessed whether peripheral p11 levels correlate with disease severity. Substantia nigra, putamen, and cortical p11 protein levels were assessed in postmortem brain samples from PD patients and matched controls. In a different set of postmortem brains, p11 mRNA expression was measured in dopaminergic cells from the substantia nigra. Both p11 protein and mRNA levels were decreased in PD patients. Peripheral p11 protein levels were investigated in distinct leukocyte populations from PD patients with depression and those without depression. Monocyte, natural killer (NK) cell, and cytotoxic T-cell p11 levels were positively associated with the severity of PD, and NK cell p11 levels were positively associated with depression scores. Given that inflammation plays a role in both PD and depression, it is intriguing that peripheral p11 levels are altered in immune cells in both conditions. Our data provide insight into the pathological alterations occurring centrally and peripherally in PD. Moreover, if replicated in other cohorts, p11 could be an easily accessible biomarker for monitoring the severity of PD, especially in the context of comorbid depression.

Neuropeptide Y and Calcitonin Gene-Related Peptide in Cerebrospinal Fluid in Parkinson's Disease with Comorbid Depression versus Patients with Major Depressive Disorder

Parkinson's disease (PD) is the second most common neurodegenerative disease in the world. The diagnosis of PD is based on movement dysfunctions. Many patients also suffer from comorbid depression in spite of adequate treatment with dopamine replacement, indicating that also other non-dopaminergic mechanisms are involved. Indeed, neuropeptides are critically implicated in the pathophysiology of major depressive disorder (MDD). To increase our understanding of the biochemical basis of depression in PD patients, we examined the levels of neuropeptide Y (NPY) and calcitonin gene-related peptide (CGRP) in cerebrospinal fluid (CSF) from PD patients, with or without comorbid depression, and compared them to the levels in patients with MDD. We also compared the levels of NPY and CGRP with 5-hydroxyindoleacetic acid (5-HIAA), the major serotonin metabolite. Both NPY and CGRP were higher in PD patients with comorbid depression compared to MDD patients. No similar difference was found in 5-HIAA levels. Accordingly, there were no correlations between NPY and 5-HIAA or CGRP and 5-HIAA levels. The finding of higher NPY and CGRP CSF levels in PD patients with MDD raises the possibility that different pathophysiological processes may underlie depression in PD and MDD.

Neuropsychiatric symptoms in untreated Parkinson's disease

Neuropsychiatric and cognitive symptoms are common in Parkinson's disease (PD) and may precede and exceed motor symptoms as major factors impacting disease course and quality of life. Neuropsychiatric symptoms (NPS) in PD are various and are attributed to pathologic changes within multiple brain regions, to psychological stress, and to adverse effects of dopamine replacement therapy. Sleep disorders and mood symptoms such as apathy, depression, and anxiety may antedate the development of motor symptoms by years, while other NPS such as impulse control disorders, psychosis, and cognitive impairment are more common in later stages of the disease. Few studies report on NPS in the early, untreated phase of PD. We reviewed the current literature on NPS in PD with a focus on the early, drug-naive stages of PD. Among these early disease stages, premotor and early motor phases were separately addressed in our review, highlighting the underlying pathophysiological mechanisms as well as epidemiological characteristics, clinical features, risk factors, and available techniques of clinical assessment.

Call to Caution with the Use of Atypical Antipsychotics for Treatment of Depression in Older Adults

Atypical antipsychotics are increasingly being used to manage depression in older adults where these symptoms can often be refractory to first-line treatment with selective serotonin reuptake inhibitors (SSRIs) or serotonin norepinephrine reuptake inhibitors (SNRIs). Unfortunately, atypical antipsychotics can be associated with the development of extrapyramidal symptoms (EPS), with drug-induced parkinsonism (DIP) being the most common movement disorder induced by this class of medication. The management of treatment-resistant depression in older adults is of particular concern as depression is a common feature of idiopathic Parkinson’s disease (IPD) and can manifest prior to the development of motor symptoms. Herein, we discuss the use of atypical antipsychotics for the management of depression in older adults including the risk of DIP and propose that antipsychotics may potentially unmask IPD.

A guide to management of neuropsychiatric manifestations of Parkinson's disease

OBJECTIVE

This clinical update review focuses on treatment approaches of neuropsychiatric manifestations in Parkinson's disease.

METHODS

We conducted a systematic search of the literature using Pubmed and selected recent and relevant papers for this review.

RESULTS

Neuropsychiatric symptoms in Parkinson's disease usually require optimization of levodopa therapy as a first step. Most psychotropic drugs can be used in Parkinson's disease, however there is still lack of an evidence base due to limited studies and difficulties in diagnosis of neuropsychiatric disorders. Non-pharmacological treatments have also proved effective in Parkinson's disease. Cognitive impairment requires special consideration.

CONCLUSIONS

Management of neuropsychiatric manifestations in Parkinson's disease is complicated by the lack of evidence. Treatment should be individualized and benefits and risks must be balanced.

A double-blind randomized controlled trial to assess the effect of bright light therapy on depression in patients with Parkinson's disease

BACKGROUND

A disturbed circadian rhythm seems to be a causal factor in the occurrence of depressive disorders in patients with Parkinson's disease (PD). The circadian rhythm can be restored with light. Therefore, Bright Light Therapy (BLT) might be a new treatment option for depression in PD patients.

METHODS/DESIGN

In this double-blind controlled trial, 84 subjects with idiopathic PD are randomized to either BLT or a control light condition. The BLT condition emits white light with an intensity of 10,000 Lux, while the control device emits dim white light of 200 Lux, which is presumed to be too low to influence the circadian rhythm. Subjects receive 30 min of home treatment twice daily for three months. Timing of treatment is based on the individual chronotype. After finishing treatment, subjects enter a follow-up period of six months. The primary outcome of the study is the severity of depressive symptoms, as measured with the Hamilton Depression Rating Scale. Secondary outcomes are alternative depression measures, objective and subjective sleep measures, and salivary melatonin and cortisol concentrations. For exploratory purposes, we also assess the effects on motor symptoms, global cognitive function, comorbid psychiatric disorders, quality of life and caregiver burden. Data will be analyzed using a linear mixed models analysis.

DISCUSSION

Performing a placebo-controlled trial on the effects of BLT in PD patients is challenging, as the appearance of the light may provide clues on the treatment condition. Moreover, fixed treatment times lead to an improved sleep-wake rhythm, which also influences the circadian system. With our study design, we do not compare BLT to placebo treatment, i.e. an ineffective control treatment. Rather, we compare structuring of the sleep-wake cycle in both conditions with additional BLT in the experimental condition, and additional dim light in the control condition. Participants are not informed about the exact details of the two light devices and the expected therapeutic effect, and expectancies are rated prior to the start of treatment. Ideally, the design of a future study on BLT should include two extra treatment arms where BLT and control light are administered at random times.

TRIAL REGISTRATION

This trial was registered on ClinicalTrials.gov on May 17th 2012 (ClinicalTrials.gov Identifier: NCT01604876 ).

Melatoninergic System in Parkinson's Disease: From Neuroprotection to the Management of Motor and Nonmotor Symptoms

Melatonin is synthesized by several tissues besides the pineal gland, and beyond its regulatory effects in light-dark cycle, melatonin is a hormone with neuroprotective, anti-inflammatory, and antioxidant properties. Melatonin acts as a free-radical scavenger, reducing reactive species and improving mitochondrial homeostasis. Melatonin also regulates the expression of neurotrophins that are involved in the survival of dopaminergic neurons and reduces α-synuclein aggregation, thus protecting the dopaminergic system against damage. The unbalance of pineal melatonin synthesis can predispose the organism to inflammatory and neurodegenerative diseases such as Parkinson's disease (PD). The aim of this review is to summarize the knowledge about the potential role of the melatoninergic system in the pathogenesis and treatment of PD. The literature reviewed here indicates that PD is associated with impaired brain expression of melatonin and its receptors MT₁ and MT₂. Exogenous melatonin treatment presented an outstanding neuroprotective effect in animal models of PD induced by different toxins, such as 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), rotenone, paraquat, and maneb. Despite the neuroprotective effects and the improvement of motor impairments, melatonin also presents the potential to improve nonmotor symptoms commonly experienced by PD patients such as sleep and anxiety disorders, depression, and memory dysfunction.

Changes in Verbal Fluency in Parkinson's Disease

Background

The test for semantic verbal fluency is quick and easy to administer. Decreases in semantic verbal fluency would suggest executive dysfunction among individuals with Parkinson's disease (PD).

Methods

The National Parkinson Foundation's Outcomes Project is a multicenter study that seeks to determine best practices in PD management. We analyzed data from the baseline and two annual follow-up visits to determine the annual rate of verbal fluency change and determinants of that change. Linear mixed modeling was used to assess relationships between verbal fluency, clinical characteristics, quality of life, and caregiver burden.

Results

There were 1,322 participants with an average age of 67.3 years, of whom 37% were women. Mean baseline verbal fluency scores at baseline were 18.81 (standard deviation = 6.25). Verbal fluency scores did not change among patients who were at our cohort's average age and average PD duration (8.4 years) and who had no other associated conditions (beta = -0.02; P = 0.80). Verbal fluency, however, did decrease for individuals with PD duration greater than the average (beta = -0.25; P = 0.03), age greater than the average (beta = -0.022; P < 0.01), a Hoehn and Yahr >=3 (beta = -0.31; P = 0.04), and in those with cardiovascular disease (beta = -0.32; P = 0.01) or psychiatric symptomatology (beta = -0.34; P = 0.01). Individuals with higher verbal fluency scores had better quality of life (P < 0.01) and decreased caregiver burden (P < 0.01).

Conclusions

Clinicians should monitor verbal fluency scores to evaluate cognitive decline among individuals with PD. Modifiable risk factors for verbal fluency changes include psychiatric symptomatology and cardiovascular disease. Clinicians may use verbal fluency testing to identify individuals at risk for decreased quality of life and increased caregiver burden, allowing for focused interventions.

Affective Disorders, Psychosis and Dementia in a Community Sample of Older Men with and without Parkinson's Disease

Background

Dementia and affective and psychotic symptoms are commonly associated with Parkinson’s disease, but information about their prevalence and incidence in community representative samples remains sparse.

Methods

We recruited a community-representative sample 38173 older men aged 65–85 years in 1996 and used data linkage to ascertain the presence of PD, affective disorders, psychotic disorders and dementia. Diagnoses followed the International Classification of Disease coding system. Age was recorded in years. Follow up data were available until December 2011.

Results

The mean age of participants was 72.5 years and 333 men (0.9%) had PD at study entry. Affective and psychotic disorders and dementia were more frequent in men with than without PD (respective odds ratios: 6.3 [95%CI = 4.7, 8.4]; 14.2 [95%CI = 8.4, 24.0] and 18.2 [95%CI = 13.4, 24.6]). Incidence rate ratios of affective and psychotic disorders were higher among men with than without PD, although ratios decreased with increasing age. The age-adjusted hazard ratio (HR) of an affective episode associated with PD was 5.0 (95%CI = 4.2, 5.9). PD was associated with an age-adjusted HR of 8.6 (95%CI = 6.1, 12.0) for psychotic disorders and 6.1 (95%CI = 5.5, 6.8) for dementia. PD and dementia increased the HR of depressive and psychotic disorders.

Conclusions

PD increases the risk of affective and psychotic disorders, as well as dementia, among community dwelling older men. The risk of a recorded diagnosis of affective and psychotic disorders decreases with increasing age.

Integrative transcriptomic meta-analysis of Parkinson's disease and depression identifies NAMPT as a potential blood biomarker for de novo Parkinson's disease

Emerging research indicates that depression could be one of the earliest prodromal symptoms or risk factors associated with the pathogenesis of Parkinson’s disease (PD), the second most common neurodegenerative disorder worldwide, but the mechanisms underlying the association between both diseases remains unknown. Understanding the molecular networks linking these diseases could facilitate the discovery of novel diagnostic and therapeutics. Transcriptomic meta-analysis and network analysis of blood microarrays from untreated patients with PD and depression identified genes enriched in pathways related to the immune system, metabolism of lipids, glucose, fatty acids, nicotinamide, lysosome, insulin signaling and type 1 diabetes. Nicotinamide phosphoribosyltransferase (NAMPT), an adipokine that plays a role in lipid and glucose metabolism, was identified as the most significant dysregulated gene. Relative abundance of NAMPT was upregulated in blood of 99 early stage and drug-naïve PD patients compared to 101 healthy controls (HC) nested in the cross-sectional Parkinson’s Progression Markers Initiative (PPMI). Thus, here we demonstrate that shared molecular networks between PD and depression provide an additional source of biologically relevant biomarkers. Evaluation of NAMPT in a larger prospective longitudinal study including samples from other neurodegenerative diseases, and patients at risk of PD is warranted.

Freezing of Gait and its Associations in the Early and Advanced Clinical Motor Stages of Parkinson's Disease: A Cross-Sectional Study

BACKGROUND

Freezing of gait is a common disabling symptom of Parkinson's disease (PD) with limited treatment options. The pathophysiological mechanisms of freezing behaviour are still contentious.

OBJECTIVE

To investigate the prevalence of freezing of gait and its associations with increasing disease severity to gain a better understanding of the underlying pathophysiology.

METHODS

This exploratory study included 389 idiopathic PD patients, divided into four groups; early and advanced PD with freezing of gait, and early and advanced PD without freezing of gait. Motor, cognitive and affective symptoms, REM sleep behaviour disorder and autonomic function were assessed.

RESULTS

Regardless of disease stage, patients with freezing of gait had more severe motor symptoms and a predominant non-tremor phenotype. In the early stages, freezers had a selective impairment in executive function and had more marked REM sleep behaviour disorder. Autonomic disturbances were not associated with freezing of gait across early or advanced disease stages.

CONCLUSION

These findings support the notion that impairments across the frontostriatal pathways are intricately linked to the pathophysiology underlying freezing of gait across all stages of PD. Features of REM sleep behaviour disorder suggest a contribution to freezing from brainstem pathology but this does not extend to more general autonomic dysfunction.

Fragile X-associated tremor/ataxia syndrome: phenotypic comparisons with other movement disorders

OBJECTIVE

The purpose of this paper is to review the typical cognitive and motor impairments seen in fragile X-associated tremor/ataxia syndrome (FXTAS), essential tremor (ET), Parkinson disease (PD), spinocerebellar ataxias (SCAs), multiple system atrophy (MSA), and progressive supranuclear palsy (PSP) in order to enhance diagnosis of FXTAS patients.

METHODS

We compared the cognitive and motor phenotypes of FXTAS with each of these other movement disorders. Relevant neuropathological and neuroimaging findings are also reviewed. Finally, we describe the differences in age of onset, disease severity, progression rates, and average lifespan in FXTAS compared to ET, PD, SCAs, MSA, and PSP. We conclude with a flow chart algorithm to guide the clinician in the differential diagnosis of FXTAS.

RESULTS

By comparing the cognitive and motor phenotypes of FXTAS with the phenotypes of ET, PD, SCAs, MSA, and PSP we have clarified potential symptom overlap while elucidating factors that make these disorders unique from one another. In summary, the clinician should consider a FXTAS diagnosis and testing for the Fragile X mental retardation 1 (FMR1) gene premutation if a patient over the age of 50 (1) presents with cerebellar ataxia and/or intention tremor with mild parkinsonism, (2) has the middle cerebellar peduncle (MCP) sign, global cerebellar and cerebral atrophy, and/or subcortical white matter lesions on MRI, or (3) has a family history of fragile X related disorders, intellectual disability, autism, premature ovarian failure and has neurological signs consistent with FXTAS. Peripheral neuropathy, executive function deficits, anxiety, or depression are supportive of the diagnosis.

CONCLUSIONS

Distinct profiles in the cognitive and motor domains between these movement disorders may guide practitioners in the differential diagnosis process and ultimately lead to better medical management of FXTAS patients.

Modulatory effects of α7 nAChRs on the immune system and its relevance for CNS disorders

The clinical development of selective alpha-7 nicotinic acetylcholine receptor (α7 nAChR) agonists has hitherto been focused on disorders characterized by cognitive deficits (e.g., Alzheimer's disease, schizophrenia). However, α7 nAChRs are also widely expressed by cells of the immune system and by cells with a secondary role in pathogen defense. Activation of α7 nAChRs leads to an anti-inflammatory effect. Since sterile inflammation is a frequently observed phenomenon in both psychiatric disorders (e.g., schizophrenia, melancholic and bipolar depression) and neurological disorders (e.g., Alzheimer's disease, Parkinson's disease, and multiple sclerosis), α7 nAChR agonists might show beneficial effects in these central nervous system disorders. In the current review, we summarize information on receptor expression, the intracellular signaling pathways they modulate and reasons for receptor dysfunction. Information from tobacco smoking, vagus nerve stimulation, and cholinesterase inhibition is used to evaluate the therapeutic potential of selective α7 nAChR agonists in these inflammation-related disorders.

Changes in the sympathetic innervation of the gut in rotenone treated mice as possible early biomarker for Parkinson's disease

Introduction

Involvement of the peripheral nervous system (PNS) is relatively common in Parkinson’s disease (PD) patients. PNS alterations appear early in the course of the disease and are responsible for some of the non-motor symptoms observed in PD patients. In previous studies, we have shown that environmental toxins can trigger the disease by acting on the enteric nervous system.

Material and methods

Here, we analyzed the effect of mitochondrial Complex I inhibition on sympathetic neuritis in vivo and sympathetic neurons in vitro. Combining in vivo imaging and protein expression profiling.

Results

we found that rotenone, a widely used mitochondrial Complex I inhibitor decreases the density of sympathetic neurites innervating the gut in vivo, while in vitro, it induces the redistribution of intracellular alpha-synuclein and neurite degeneration. Interestingly, sympathetic neurons are much more resistant to rotenone exposure than mesencephalic dopaminergic neurons.

Conclusion

Altogether, these results suggest that enteric sympathetic denervation could be an initial pre-motor alteration in PD progression that could be used as an early biomarker of the disease.

Electronic supplementary material

The online version of this article (doi:10.1007/s10286-016-0358-6) contains supplementary material, which is available to authorized users.

Cortical abnormalities in Parkinson's disease patients and relationship to depression: A surface-based morphometry study

Depression is a common occurrence in patients with Parkinson's disease (PD). Brain deficits may be the underlying cause of depression in PD. In the present study, we investigated whether morphometric alterations contribute to depression in PD. Seventeen depressed PD patients, 17 non-depressed PD patients and 45 normal controls were enrolled in the study. All subjects went through neurological and psychiatric clinical assessments. T1 weighted magnetic resonance imaging and surface-based morphometric analyses were performed to examine morphometric abnormalities in PD patients and their relationship to depression. We found that compared with normal controls, PD patients exhibited significantly decreased cortical thickness in the left precentral gyrus and the right postcentral gyrus extending to the middle frontal gyrus. Compared with non-depressed PD patients, depressed patients showed significantly increased cortical areas in the orbitofrontal regions and insula, which may imply white matter atrophy in these areas. The results of orbitofrontal and insula white matter atrophy are consistent with our previous finding that white matter integrity and functional connectivity are damaged in these regions in depressed PD patients, confirming their contribution to depression in PD.

Preliminary evidence for human globus pallidus pars interna neurons signaling reward and sensory stimuli

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Non-motor responses of human GPi neurons are described.

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Cells were identified that showed increased firing to reward-stimuli.

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Visual-sensory responses unrelated to reward also observed.

The globus pallidus pars interna (GPi) is a component of the basal ganglia, a network of subcortical nuclei that process motor, associative, and limbic information. While non-human primate studies have suggested a role for the GPi in non-motor functions, there have been no single-unit studies of non-motor electrophysiological behavior of human GPi neurons. We therefore sought to extend these findings by collecting single-unit recordings from awake patients during functional stereotactic neurosurgery targeting the GPi for deep brain stimulation. To assess cellular responses to non-motor information, patients performed a reward task where virtual money could be won, lost, or neither, depending on their performance while cellular activity was monitored. Changes in the firing rates of isolated GPi neurons after the presentation of reward-related stimuli were compared between different reward contingencies (win, loss, null). We observed neurons that modulated their firing rate significantly to the presentation of reward-related stimuli. We furthermore found neurons that responded to visual-stimuli more broadly. This is the first single-unit evidence of human GPi neurons carrying non-motor information. These results are broadly consistent with previous findings in the animal literature and suggest non-motor information may be represented in the single-unit activity of human GPi neurons.

Translational research on cognitive and behavioural disorders in neurological and psychiatric diseases

The important medical and social burden of nervous system diseases contrasts with the currently limited therapeutic armamentarium and with the difficulty encountered in developing new therapeutic options. These failures can be explained by the conjunction of various phenomena related to the limitations of animal models, the narrow focus of research on precise pathophysiological mechanisms, and methodological issues in clinical trials. It is perhaps the paradigm itself of the way research is conducted that may be the real reason for our incapacity to find effective strategies. The purpose of this workshop was to define overall lines of research that could lead to the development of effective novel therapeutic solutions. Research has long focused on diseases per se rather than on cognitive and behavioural dimensions common to several diseases. Their expression is often partial and variable, but can today be well-characterised using neurophysiological or imaging methods. This dimensional or syndromic vision should enable a new insight to the question, taking a transnosographic approach to re-position research and to propose: translational models exploring the same functions in animal models and in humans; identification of homogeneous groups of patients defined according to the clinical, anatomico-functional and molecular characteristics; and preclinical and clinical developments enriched by the use of cognitive-behavioural, biological neurological, and imaging biomarkers. For this mutation to be successful, it must be accompanied by synchronised action from the public authorities and by ad hoc measures from the regulatory agencies.

Antidepressants in Parkinson's disease. Recommendations by the movement disorder study group of the Neurological Association of Madrid

INTRODUCTION

Although antidepressants are widely used in Parkinson's disease (PD), few well-designed studies to support their efficacy have been conducted.

DEVELOPMENT

These clinical guidelines are based on a review of the literature and the results of an AMN movement disorder study group survey.

CONCLUSIONS

Evidence suggests that nortriptyline, venlafaxine, paroxetine, and citalopram may be useful in treating depression in PD, although studies on paroxetine and citalopram yield conflicting results. In clinical practice, however, selective serotonin reuptake inhibitors are usually considered the treatment of choice. Duloxetine may be an alternative to venlafaxine, although the evidence for this is less, and venlafaxine plus mirtazapine may be useful in drug-resistant cases. Furthermore, citalopram may be indicated for the treatment of anxiety, atomoxetine for hypersomnia, trazodone and mirtazapine for insomnia and psychosis, and bupropion for apathy. In general, antidepressants are well tolerated in PD. However, clinicians should consider the anticholinergic effect of tricyclic antidepressants, the impact of serotonin-norepinephrine reuptake inhibitors on blood pressure, the extrapyramidal effects of antidepressants, and any potential interactions between monoamine oxidase B inhibitors and other antidepressants.

Microbiota-gut-brain signalling in Parkinson's disease: Implications for non-motor symptoms

Parkinson's disease is the second most common neurodegenerative disorder, affecting 1-2% of the population over 65 years of age. The primary neuropathology is the loss of midbrain dopaminergic neurons, resulting in characteristic motor deficits, upon which the clinical diagnosis is based. However, a number of significant non-motor symptoms (NMS) are also evident that appear to have a greater impact on the quality of life of these patients. In recent years, it has become increasingly apparent that neurobiological processes can be modified by the bi-directional communication that occurs along the brain-gut axis. The microbiota plays a key role in this communication throughout different routes in both physiological and pathological conditions. Thus, there has been an increasing interest in investigating how microbiota changes within the gastrointestinal tract may be implicated in health and disease including PD. Interestingly α-synuclein-aggregates, the cardinal neuropathological feature in PD, are present in both the submucosal and myenteric plexuses of the enteric nervous system, prior to their appearance in the brain, indicating a possible gut to brain route of "prion-like" spread. In this review we highlight the potential importance of gut to brain signalling in PD with particular focus on the role of the microbiota as major player in this communication.

Evaluation of Nonmotor Symptoms in Diagnosis of Parkinsonism and Tremor

Background. Nonmotor symptoms particularly olfactory dysfunction, RBD, depression, hallucinations, and constipation are currently not included in the typical clinical criteria for diagnosing Lewy body Parkinsonian disorders (LBPD). The aim of this study is to determine the diagnostic value of nonmotor symptoms in patients presenting with Parkinsonism and tremor. Methods. All new patients seen between January 2007 and May 2013 in the Movement Disorders Specialist Clinics of the Royal Melbourne Hospital (RMH), who were referred with a possible neurodegenerative syndrome or concerns of Parkinsonism and/or tremor, were included. Patients underwent routine evaluation with the four-minute “Sniffin Sticks” test, RBD, depression, and constipation. Results. 291 patients were included in the analysis. Conclusion. We found that lower olfaction scores based on “Sniffin Sticks” testing combined with reports of depression and constipation are independent predictors for the diagnosis of the spectrum of Lewy body Parkinsonian disorders (LBPD). Parkinson's disease (PD) cannot be reliably clinically differentiated from other causes of Parkinsonism that share symptomatology and structural abnormalities.

Trait Impulsivity and Anhedonia: Two Gateways for the Development of Impulse Control Disorders in Parkinson's Disease?

Apathy and impulsivity are two major comorbid syndromes of Parkinson's disease (PD) that may represent two extremes of a behavioral spectrum modulated by dopamine-dependent processes. PD is characterized by a progressive loss of dopaminergic neurons in the substantia nigra pars compacta to which are attributed the cardinal motor symptoms of the disorder. Dopamine replacement therapy (DRT), used widely to treat these motor symptoms, is often associated with deficits in hedonic processing and motivation, including apathy and depression, as well as impulse control disorders (ICDs). ICDs comprise pathological gambling, hypersexuality, compulsive shopping, binge eating, compulsive overuse of dopaminergic medication, and punding. More frequently observed in males with early onset PD, ICDs are associated not only with comorbid affective symptoms, such as depression and anxiety, but also with behavioral traits, such as novelty seeking and impulsivity, as well as with personal or familial history of alcohol use. This constellation of associated risk factors highlights the importance of inter-individual differences in the vulnerability to develop comorbid psychiatric disorders in PD patients. Additionally, withdrawal from DRT in patients with ICDs frequently unmasks a severe apathetic state, suggesting that apathy and ICDs may be caused by overlapping neurobiological mechanisms within the cortico-striato-thalamo-cortical networks. We suggest that altered hedonic and impulse control processes represent distinct prodromal substrates for the development of these psychiatric symptoms, the etiopathogenic mechanisms of which remain unknown. Specifically, we argue that deficits in hedonic and motivational states and impulse control are mediated by overlapping, yet dissociable, neural mechanisms that differentially interact with DRT to promote the emergence of ICDs in vulnerable individuals. Thus, we provide a novel heuristic framework for basic and clinical research to better define and treat comorbid ICDs in PD.

Association of specific frequency bands of functional MRI signal oscillations with motor symptoms and depression in Parkinson's disease

A novel empirical mode decomposition method was adopted to investigate the dissociative or interactive neural impact of depression and motor impairments in Parkinson’s disease (PD). Resting-state fMRI data of 59 PD subjects were first decomposed into characteristic frequency bands, and the main effects of motor severity and depression and their interaction on the energy of blood-oxygen-level-dependent signal oscillation in specific frequency bands were then evaluated. The results show that the severity of motor symptoms is negatively correlated with the energy in the frequency band of 0.10–0.25 Hz in the bilateral thalamus, but positively correlated with 0.01–0.027 Hz band energy in the bilateral postcentral gyrus. The severity of depression, on the other hand, is positively correlated with the energy of 0.10–0.25 Hz but negatively with 0.01–0.027 Hz in the bilateral subgenual gyrus. Notably, the interaction between motor and depressive symptoms is negatively correlated with the energy of 0.10–0.25 Hz in the substantia nigra, hippocampus, inferior orbitofrontal cortex, and temporoparietal junction, but positively correlated with 0.02–0.05 Hz in the same regions. These findings indicate unique associations of fMRI band signals with motor and depressive symptoms in PD in specific brain regions, which may underscore the neural impact of the comorbidity and the differentiation between the two PD-related disorders.

Screening for Mild Cognitive Impairment in Parkinson's Disease: Comparison of the Italian Versions of Three Neuropsychological Tests

Mild cognitive impairment (MCI) is frequent in Parkinson's disease (PD). Recently proposed criteria for MCI in PD (PD-MCI) indicate level I diagnosis based on abbreviated assessment and level II based on comprehensive neuropsychological evaluation. The study explored the sensitivity and specificity of the Italian versions of three neuropsychological tests for level I diagnosis of PD-MCI. We recruited 100 consecutive PD patients. After screening for inclusion criteria, 43 patients were included. The sensitivity and specificity of the Mini Mental State Examination (MMSE), the Montreal Cognitive Assessment (MoCA), and the Addenbrooke's Cognitive Examination Revised (ACE-R) in comparison to level II diagnosis of PD-MCI were examined. PD-MCI was diagnosed (level II) in 51% of patients. Disease duration was significantly longer and PD motor scales were more severely impaired in MCI group. The receiver-operator characteristics curve documented nonsignificant difference in the performance of the three tests, with slight advantage of MMSE (corrected data). The time of administration favored MMSE. In Italian-speaking PD patients, MMSE might represent a good screening tool for PD-MCI, because of the shorter time of administration and the performance comparable to those of MoCA and ACE-R. Further studies are needed to validate the new PD-MCI criteria across different languages and cultures.

Neuroimaging distinction between neurological and psychiatric disorders

BACKGROUND

It is unclear to what extent the traditional distinction between neurological and psychiatric disorders reflects biological differences.

AIMS

To examine neuroimaging evidence for the distinction between neurological and psychiatric disorders.

METHOD

We performed an activation likelihood estimation meta-analysis on voxel-based morphometry studies reporting decreased grey matter in 14 neurological and 10 psychiatric disorders, and compared the regional and network-level alterations for these two classes of disease. In addition, we estimated neuroanatomical heterogeneity within and between the two classes.

RESULTS

Basal ganglia, insula, sensorimotor and temporal cortex showed greater impairment in neurological disorders; whereas cingulate, medial frontal, superior frontal and occipital cortex showed greater impairment in psychiatric disorders. The two classes of disorders affected distinct functional networks. Similarity within classes was higher than between classes; furthermore, similarity within class was higher for neurological than psychiatric disorders.

CONCLUSIONS

From a neuroimaging perspective, neurological and psychiatric disorders represent two distinct classes of disorders.