An update on brain imaging in parkinsonian dementia

Diverse midbrain dopaminergic neuron subtypes and implications for complex clinical symptoms of Parkinson's disease

Parkinson’s disease (PD), the most common degenerative movement disorder, is clinically manifested with various motor and non-motor symptoms. Degeneration of midbrain substantia nigra pas compacta (SNc) dopaminergic neurons (DANs) is generally attributed to the motor syndrome. The underlying neuronal mechanisms of non-motor syndrome are largely unexplored. Besides SNc, midbrain ventral tegmental area (VTA) DANs also produce and release dopamine and modulate movement, reward, motivation, and memory. Degeneration of VTA DANs also occurs in postmortem brains of PD patients, implying an involvement of VTA DANs in PD-associated non-motor symptoms. However, it remains to be established that there is a distinct segregation of different SNc and VTA DAN subtypes in regulating different motor and non-motor functions, and that different DAN subpopulations are differentially affected by normal ageing or PD. Traditionally, the distinction among different DAN subtypes was mainly based on the location of cell bodies and axon terminals. With the recent advance of single cell RNA sequencing technology, DANs can be readily classified based on unique gene expression profiles. A combination of specific anatomic and molecular markers shows great promise to facilitate the identification of DAN subpopulations corresponding to different behavior modules under normal and disease conditions. In this review, we first summarize the recent progress in characterizing genetically, anatomically, and functionally diverse midbrain DAN subtypes. Then, we provide perspectives on how the preclinical research on the connectivity and functionality of DAN subpopulations improves our current understanding of cell-type and circuit specific mechanisms of the disease, which could be critically informative for designing new mechanistic treatments.

The cerebral blood flow deficits in Parkinson's disease with mild cognitive impairment using arterial spin labeling MRI

Parkinson's disease (PD) with mild cognitive impairment (PD-MCI) is currently diagnosed based on an arbitrarily predefined standard deviation of neuropsychological test scores, and more objective biomarkers for PD-MCI diagnosis are needed. The purpose of this study was to define possible brain perfusion-based biomarkers of not only mild cognitive impairment, but also risky gene carriers in PD using arterial spin labeling magnetic resonance imaging (ASL-MRI). Fifteen healthy controls (HC), 26 cognitively normal PD (PD-CN), and 27 PD-MCI subjects participated in this study. ASL-MRI data were acquired by signal targeting with alternating radio-frequency labeling with Look-Locker sequence at 3 T. Single nucleotide polymorphism genotyping for rs9468 [microtubule-associated protein tau (MAPT) H1/H1 versus H1/H2 haplotype] was performed using a Stratagene Mx3005p real-time polymerase chain-reaction system (Agilent Technologies, USA). There were 15 subjects with MAPT H1/H1 and 11 subjects with MAPT H1/H2 within PD-MCI, and 33 subjects with MAPT H1/H1 and 19 subjects with MAPT H1/H2 within all PD. Voxel-wise differences of cerebral blood flow (CBF) values between HC, PD-CN and PD-MCI were assessed by one-way analysis of variance followed by pairwise post hoc comparisons. Further, the subgroup of PD patients carrying the risky MAPT H1/H1 haplotype was compared with noncarriers (MAPT H1/H2 haplotype) in terms of CBF by a two-sample t test. A pattern that could be summarized as "posterior hypoperfusion" (PH) differentiated the PD-MCI group from the HC group with an accuracy of 92.6% (sensitivity = 93%, specificity = 93%). Additionally, the PD patients with MAPT H1/H1 haplotype had decreased perfusion than the ones with H1/H2 haplotype at the posterior areas of the visual network (VN), default mode network (DMN), and dorsal attention network (DAN). The PH-type pattern in ASL-MRI could be employed as a biomarker of both current cognitive impairment and future cognitive decline in PD.

Approach to the Older Adult With New Cognitive Symptoms

Dementia affects nearly 50 million people worldwide, translating into one new case every 3 seconds. Dementia syndrome is one of the leading causes of disability among older adults, yet it remains vastly underdiagnosed. A timely diagnosis of dementia is essential to ensuring optimal care and support of individuals and their loved ones. Although there is no single test for dementia, health care providers can use a structured approach to the workup and management of new cognitive symptoms. Comprehensive MEDLINE and PubMed searches were performed to develop an unbiased, practical diagnostic approach to these symptoms. This review guides primary care providers in the workup, diagnosis, delivery, and initial management of patients presenting with new cognitive symptoms.

A quick test of cognitive speed can predict development of dementia in Parkinson's disease

Parkinson’s disease (PD) patients frequently develop cognitive impairment. There is a need for brief clinical assessments identifying PD patients at high risk of progressing to dementia. In this study, we look into predicting dementia in PD and underlying structural and functional correlates to cognitive decline in PD. We included 175 patients with PD, 30 with PD dementia, 51 neurologically healthy controls and 121 patients with Alzheimer’s disease (AD) from Skane University Hospital, BIOFINDER cohorts. All underwent cognitive tests, including MMSE, 10-word list delayed recall (ADAS-cog), A Quick Test of cognitive speed (AQT), Letter S fluency, Clock Drawing Test (CDT) and pentagon copying. In non-demented patients with PD, abnormal AQT and CDT results predicted an increased risk of subsequent development of dementia (hazard ratio 2.2 for both). When comparing the cognitive profile between PD and AD, decreased performance on AQT, which measures attention and processing speed, was more typical in PD. Lastly, we investigated the underlying structural and functional correlates for the PD-specific test AQT with magnetic resonance imaging. In PD patients, decreased performance on AQT was associated with i) cortical thinning in temporoparietal regions, ii) changes in diffusion MRI, especially in the cingulum tract, and iii) decreased functional connectivity in posterior brain networks.

Interactions of Hallucinogens with the Glutamatergic System: Permissive Network Effects Mediated Through Cortical Layer V Pyramidal Neurons

Recordings made from layer V (L5) pyramidal cells of the prefrontal cortex (PFC) and neocortex in rodent slice preparations have shown that serotonin (5-hydroxytryptamine, 5-HT) and serotonergic hallucinogens induce an increase in the frequency of spontaneous excitatory postsynaptic currents (EPSCs) in the apical dendritic field by activating 5-HT_2A receptors. Serotonergic hallucinogens induce late EPSCs and increase recurrent network activity when subcortical or mid-cortical regions are stimulated at low frequencies (e.g., 0.1 Hz). A range of agonists or positive allosteric modulators (PAMs) for mostly G_i/o-coupled receptors, including metabotropic glutamate₂ (mGlu₂), adenosine A₁, or μ-opioid receptors, suppress these effects of 5-HT_2A receptor stimulation. Furthermore, a range of mostly G_q/11-coupled receptors (including orexin₂ [OX₂]; α₁-adrenergic, and mGlu₅ receptors) similarly induce glutamate (Glu) release onto L5 pyramidal cells. Evidence implicates a number of brain regions in mediating these effects of serotonergic hallucinogens and G_q/11-coupled receptors including the midline and intralaminar thalamic nuclei, claustrum, and neurons in deep PFC. These effects on 5-HT_2A receptors and related GPCRs appear to play a major role in the behavioral effects of serotonergic hallucinogens, such as head twitches in rodents and higher order behaviors such as rodent lever pressing on the differential-reinforcement-of-low rate 72-s (DRL 72-s) schedule. This implies that the effects of 5-HT_2A receptor activation on the activity of L5 pyramidal cells may be responsible for mediating a range of behaviors linked to limbic circuitry with connectivity between the PFC, striatum, thalamus, claustrum, striatum, amygdala, and the hippocampal formation.

Cognitive impairment in Parkinson's disease: a report from a multidisciplinary symposium on unmet needs and future directions to maintain cognitive health

People with Parkinson's disease (PD) and their care partners frequently report cognitive decline as one of their greatest concerns. Mild cognitive impairment affects approximately 20-50% of people with PD, and longitudinal studies reveal dementia in up to 80% of PD. Through the Parkinson's Disease Foundation Community Choice Research Award Program, the PD community identified maintaining cognitive function as one of their major unmet needs. In response, a working group of experts across multiple disciplines was organized to evaluate the unmet needs, current challenges, and future opportunities related to cognitive impairment in PD. Specific conference goals included defining the current state in the field and gaps regarding cognitive issues in PD from patient, care partner, and healthcare professional viewpoints; discussing non-pharmacological interventions to help maintain cognitive function; forming recommendations for what people with PD can do at all disease stages to maintain cognitive health; and proposing ideas for how healthcare professionals can approach cognitive changes in PD. This paper summarizes the discussions of the conference, first by addressing what is currently known about cognitive dysfunction in PD and discussing several non-pharmacological interventions that are often suggested to people with PD. Second, based on the conference discussions, we provide considerations for people with PD for maintaining cognitive health and for healthcare professionals and care partners when working with people with PD experiencing cognitive impairment. Furthermore, we highlight key issues and knowledge gaps that need to be addressed in order to advance research in cognition in PD and improve clinical care.

Lewy Body Dementias: Dementia With Lewy Bodies and Parkinson Disease Dementia

PURPOSE OF REVIEW

This article provides an overview of the clinical features, neuropathologic findings, diagnostic criteria, and management of dementia with Lewy bodies (DLB) and Parkinson disease dementia (PDD), together known as the Lewy body dementias.

RECENT FINDINGS

DLB and PDD are common, clinically similar syndromes that share characteristic neuropathologic changes, including deposition of α-synuclein in Lewy bodies and neurites and loss of tegmental dopamine cell populations and basal forebrain cholinergic populations, often with a variable degree of coexisting Alzheimer pathology. The clinical constellations of DLB and PDD include progressive cognitive impairment associated with parkinsonism, visual hallucinations, and fluctuations of attention and wakefulness. Current clinical diagnostic criteria emphasize these features and also weigh evidence for dopamine cell loss measured with single-photon emission computed tomography (SPECT) imaging and for rapid eye movement (REM) sleep behavior disorder, a risk factor for the synucleinopathies. The timing of dementia relative to parkinsonism is the major clinical distinction between DLB and PDD, with dementia arising in the setting of well-established idiopathic Parkinson disease (after at least 1 year of motor symptoms) denoting PDD, while earlier cognitive impairment relative to parkinsonism denotes DLB. The distinction between these syndromes continues to be an active research question. Treatment for these illnesses remains symptomatic and relies on both pharmacologic and nonpharmacologic strategies.

SUMMARY

DLB and PDD are important and common dementia syndromes that overlap in their clinical features, neuropathology, and management. They are believed to exist on a spectrum of Lewy body disease, and some controversy persists in their differentiation. Given the need to optimize cognition, extrapyramidal function, and psychiatric health, management can be complex and should be systematic.

Subcortical whiter matter hyperintensities within the cholinergic pathways of patients with dementia and parkinsonism

BACKGROUND AND PURPOSE

White matter hyperintensities (WMHs) in the cholinergic pathways are associated with cognitive performance in Alzheimer's disease (AD) and Parkinson disease dementia (PDD). This study aimed to evaluate the relationship between loss of white matter cholinergic pathways and cognitive function in patients with AD, diffuse Lewy body disease (DLB), and PDD.

METHODS

The subjects included 20 patients with AD, 17 with DLB, 21 with PDD, and 20 healthy controls. The extent of WMHs within cholinergic pathways was assessed using the Cholinergic Pathways Hyperintensities Scale (CHIPS) and was compared among the different diseases.

RESULTS

The mean CHIPS scores were similar among the three dementia groups (AD vs. DLB vs. PDD = 34.6 ± 17.9 vs. 32.4 ± 14.1 vs. 31.8 ± 14.5, p = 0.781 by ANCOVA) and higher than those of controls (11.5 ± 7.6, p = 0.001 by ANCOVA).

CONCLUSIONS

Losses of cholinergic pathways were similar among AD, DLB, and PDD groups, and more severe cognitive dysfunction was associated with elevated WMHs. These findings suggest that interruption of acetylcholine pathways may be related to cognitive dysfunction in these three diseases, even though they have different pathological mechanisms.

Cholinergic dysfunction in Parkinson's disease

There is increasing interest in the clinical effects of cholinergic basal forebrain and tegmental pedunculopontine complex (PPN) projection degeneration in Parkinson's disease (PD). Recent evidence supports an expanded role beyond cognitive impairment, including effects on olfaction, mood, REM sleep behavior disorder, and motor functions. Cholinergic denervation is variable in PD without dementia and may contribute to clinical symptom heterogeneity. Early in vivo imaging evidence that impaired cholinergic integrity of the PPN associates with frequent falling in PD is now confirmed by human post-mortem evidence. Brainstem cholinergic lesioning studies in primates confirm the role of the PPN in mobility impairment. Degeneration of basal forebrain cholinergic projections correlates with decreased walking speed. Cumulatively, these findings provide evidence for a new paradigm to explain dopamine-resistant features of mobility impairments in PD. Recognition of the increased clinical role of cholinergic system degeneration may motivate new research to expand indications for cholinergic therapy in PD.