Metabolic abnormalities associated with mild cognitive impairment in Parkinson disease

Functional imaging of cognitive impairment in Parkinson's disease

Parkinson's disease (PD) is known by most persons to be a neurodegenerative disorder that affects one's motor skills. However, the disease is also characterized by the less recognized cognitive symptoms, including deficits in executive functioning, as well as mood and behavioral problems, which are just as disabling and distressing as the motor symptoms. Imaging methods such as positron emission tomography (PET) have recently enhanced our understanding of cognitive disturbances in PD, and are reviewed in the current article. Furthermore, insights gained from the use of specific radiotracers in the dopaminergic and cholinergic neurotransmitter systems are discussed, as well as findings from in vivo detection of amyloid-beta. We will also discuss the potential use of a metabolic covariance network as a biomarker in clinical trials for the objective assessment of cognitive dysfunction in PD.

Electroencephalographic slowing heralds mild cognitive impairment in idiopathic REM sleep behavior disorder

OBJECTIVE

Patients with idiopathic rapid eye movement (REM) sleep behavior disorder (IRBD) may show electroencephalographic (EEG) slowing reflecting cortical dysfunction and are at risk for developing neurological conditions characterized by cognitive dysfunction including mild cognitive impairment (MCI), dementia with Lewy bodies and Parkinson's disease with associated dementia. We hypothesized that those IRBD patients who later developed MCI had pronounced cortical EEG slowing at presentation.

METHODS

Power EEG spectral analysis was blindly quantified from the polysomnographic studies of 23 IRBD patients without cognitive complaints and 10 healthy controls without RBD. After a mean clinical follow-up of 2.40+/-1.55 years, 10 patients developed MCI (RBD+MCI) and the remaining 13 remained idiopathic.

RESULTS

Patients with RBD+MCI had marked EEG slowing (increased delta and theta activity) in central and occipital regions during wakefulness and REM sleep, particularly in the right hemisphere, when compared with controls and, to a lesser extent, with IRBD subjects who remained idiopathic. The EEG spectral pattern of the RBD+MCI group was similar to that seen in patients with dementia with Lewy bodies and Parkinson's disease associated with dementia.

CONCLUSION

Our findings suggest that the presence of marked EEG slowing on spectral analysis might be indicative of the short-term development of MCI in patients initially diagnosed with IRBD.

Functional imaging studies of non-motoric manifestations of Parkinson's Disease

Positron emission tomography has been widely exploited to study the progression of Parkinson's disease (PD) and the associated motor impairment. It is however now increasingly appreciated that much of the disability of PD arises from non-motor complications, including cognitive and behavioural problems, sleep disturbances, pain and autonomic dysfunction. Recent studies in which these problems were investigated are reviewed in this paper. Although interesting findings are emerging, much work remains to be done. This will depend upon the ability to investigate non dopaminergic mechanisms as well as extra-striatal release of dopamine and the capacity to assess the impact of behaviourally relevant stimuli on neurotransmitter release and on networks of brain activation. Functional imaging studies, including PET, SPECT and functional MRI may provide valuable insights into the pathophysiology of disorders such as Parkinson's disease (PD), where structural changes are limited. Until recently, the major focus of such studies has been either on patterns of cerebral blood flow or glucose metabolism at rest or following a variety of activation paradigms, or on studies of the dopamine (DA) system. This article will review non-motor aspects of PD. While there is an enormous volume of imaging literature on the motor aspects of PD or motoric complications, that will not be addressed here.

Characteristics of executive function impairment in Parkinson's disease patients without dementia

Executive function impairments in Parkinson's disease (PD) are well documented. However, uncertainties remain regarding the impact of these deficits on other areas of cognitive functioning. The goal of this study was to provide a comprehensive assessment of cognitive characteristics in patients with PD without dementia and to assess how any such deficits affected other areas of cognitive functioning. Forty PD patients without dementia were compared to healthy controls using measures of attention and speed of processing and a comprehensive set of executive function tests including working memory, planning, and problem solving. Measures of memory/learning and visuospatial skills were also included to examine the relationship between aspects of executive function and other areas of cognition. Patients with PD showed deficits on measures of executive function, problem solving, and visuospatial skills. However, they were unimpaired on measures of planning, attention, and memory/learning. Deficits in problem solving were only evident for tasks with a high visuospatial content and were no longer significant when visuospatial skills were controlled for. While deficits in executive function and visuospatial skills were apparent for PD patients compared to controls, many aspects of cognition remained intact. These can provide a focus for cognitive intervention strategies that can be effective in delaying decline for PD patients.

Abnormalities in metabolic network activity precede the onset of motor symptoms in Parkinson's disease

Imaging studies show that Parkinson's disease (PD) alters the activity of motor- and cognition-related metabolic brain networks. However, it is not known whether the network changes appear at or before symptom onset. In this study, we examined 15 hemiparkinsonian patients who underwent serial metabolic imaging with [(18)F]-fluorodeoxyglucose (FDG) PET at baseline and again 2.1 +/- 0.6 (mean +/- SD) and 3.9 +/- 0.7 years later. We assessed longitudinal changes in network activity in each cerebral hemisphere, focusing specifically on the "presymptomatic" hemisphere--ipsilateral to the initially involved body side. At the network level, the activity of the PD motor-related pattern (PDRP) increased symmetrically in both hemispheres over time (p < 0.001), with significant bilateral elevations at each of the three time points. Hemispheric expression of the PD cognition-related pattern likewise increased symmetrically (p < 0.001), although significant elevations were not evident on either side until 4 years. At the regional level, putamen metabolism contralateral to the initially affected body side was elevated at all three time points, without longitudinal change. In contrast, in the initially presymptomatic hemisphere, putamen metabolic activity increased steadily over time, reaching abnormal levels only at 4 years. Metabolic activity in the contralateral precuneus fell to subnormal levels by the final time point. These findings suggest that abnormal PDRP activity antecedes the appearance of motor signs by approximately 2 years. The timing and laterality of symptom onset relates to focal asymmetric metabolic changes at the putamenal node of this network.

Metabolic networks for assessment of therapy and diagnosis in Parkinson's disease

Neuroimaging and modern computational techniques like spatial covariance analysis have contributed greatly to the understanding of neural system abnormalities in neurodegenerative disorders such as Parkinson's disease (PD). The application of network analysis to metabolic PET data obtained from patients with PD has led to the identification and validation of two distinct spatial covariance patterns associated with the motor and cognitive manifestations of the disease. Quantifying the activity of these patterns in individual subjects has provided an objective tool for the assessment of treatment efficacy and differential diagnosis. We have found that activity of the PD motor-related network is modulated by antiparkinsonian treatments such as dopaminergic therapy, deep brain stimulation (DBS), and subthalamic nucleus (STN) gene therapy. By contrast, the cognitive-related network is not altered by these interventions for PD motor symptoms. This pattern may however change in response to therapies targeting the cognitive symptoms of this disorder. Recent work has focused on the identification of specific network biomarkers for atypical parkinsonian conditions such as multiple system atrophy (MSA) and progressive supranuclear palsy (PSP). These disease-related patterns can potentially be used in an automated imaging-based algorithm to classify patients with these disorders.

FDG PET in the Evaluation of Parkinson's Disease

Network analysis of (18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) is an innovative approach for the study of in movement disorders, such as Parkinson's disease (PD). Spatial covariance analysis of imaging data acquired from PD patients has revealed characteristic regional patterns associated with the motor and cognitive features of disease. Quantification of pattern expression in individual patients can be used for diagnosis, assessment of disease severity, and evaluation of novel medical and surgical therapies. Identification of disease-specific patterns in other parkinsonian syndromes, such as multiple system atrophy and progressive supranuclear palsy, has improved diagnostic accuracy in patients with difficult to diagnose parkinsonism. Further developments of these techniques are likely to enhance the role of functional imaging in investigating underlying abnormalities and potential new therapies in these neurodegenerative diseases.

Early Parkinson's disease: longitudinal changes in brain activity during sequence learning

Impairment of sequence learning is common in Parkinson's disease (PD), but the time course of this cognitive abnormality is not known. We assessed longitudinal changes in sequence learning performance and associated task-related cerebral blood flow in 13 early stage PD patients who underwent H(2)(15)O PET at baseline and again 2 years later. Ten healthy volunteer subjects served as controls. A trend toward decline in learning performance (p=0.08) was evident over the 2 years of follow-up. During this interval, significant declines in learning-related activation were detected in parietal and temporo-occipital association areas and in the right dorsolateral prefrontal cortex. Learning-related activation in these regions was normal at baseline, but declined to subnormal levels (p<0.01) at 2 years. Significant hippocampal activation (p<0.005) was present in the subjects with high learning performance over time. The findings are consistent with a decline in learning-related neural activity in cortical areas with prominent Lewy body formation.

Metabolic brain networks in neurodegenerative disorders: a functional imaging approach

Network analysis of functional brain imaging data is an innovative approach to study circuit abnormalities in neurodegenerative diseases. In Parkinson's disease, spatial covariance analysis of resting-state metabolic images has identified specific regional patterns associated with motor and cognitive symptoms. With functional imaging, these metabolic networks have recently been used to measure system-related progression and to evaluate novel treatment strategies. Network analysis is also being used to characterize specific functional biomarkers for Huntington's disease and Alzheimer's disease. These networks have been particularly helpful in uncovering compensatory mechanisms in genetically at-risk individuals. Ongoing developments in network applications are likely to enhance the role of functional imaging in the investigation of neurodegenerative disorders.

Cognitive impairment in Parkinson's disease: tools for diagnosis and assessment

Cognitive impairment (CI) and dementia are frequent and debilitating features associated with Parkinson's disease (PD). Formal neuropsychological examination is required to ascertain the degree and pattern of CI over the course of the disease. The use of different tools may explain heterogeneous data obtained from studies to date. Normative data for extensively used scales [Mattis Dementia Rating Scale (MDRS), Mini-Mental State Examination (MMSE)] is incomplete in PD populations. According to sample characteristics, statistical analyses, and methodological quality, 33 studies using scales not specific to PD (MDRS, MMSE, Cambridge Cognitive Assessment, FAB) or PD-specific scales (Mini-Mental Parkinson, Scales for Outcomes of Parkinson's disease-Cognition, Parkinson's Disease-Cognitive Rating Scale, and Parkinson Neuropsychometric Dementia Assessment) were eligible for the critical analysis of their appropriateness to assess cognition in PD. Of the four scales specifically designed for PD, the SCOPA-COG and the PD-CRS have undergone extensive and rigorous validation processes. While the SCOPA-COG mainly assesses "frontal-subcortical" cognitive defects, the PD-CRS also assesses "instrumental-cortical" functions, allowing better characterization of the different patterns of CI that may be present in PD from the earliest stages. The MMP and PANDA scales were designed as brief screening tests for CI and have not yet been subjected to extensive clinimetric evaluations. Further research on PD-specific tools seems mandatory to help establish accurate cut-off scores for the diagnosis of mild PDD, detect cognitive profiles more prone to the future development of dementia, and allow comparisons between different descriptive or interventional studies.

Distinct patterns of regional cerebral glucose metabolism in Parkinson's disease with and without mild cognitive impairment

There is no consensus with regard to the clinical and neuroimaging characteristics of prodromal dementia in Parkinson's disease (PD). To delineate functional neuroimaging features of PD with mild cognitive impairment (PDMCI) and with no cognitive impairment (PDNC), we compared regional cerebral glucose metabolism (CMRglc) amongst 13 patients with PDMCI, 27 with PDNC, and 13 healthy controls. The PDNC patients had limited areas of hypometabolism in the frontal and occipital cortices. In the PDMCI patients, there were extensive areas of hypometabolism in the posterior cortical regions, including the temporo-parieto-occipital junction, medial parietal, and inferior temporal cortices. The present results suggest that posterior cortical dysfunction is the primary neuroimaging feature of PD patients at risk for dementia.

Amnestic mild cognitive impairment in Parkinson's disease: a brain perfusion SPECT study

The purpose of this study was to investigate cortical dysfunction in Parkinson's disease (PD) patients with amnestic deficit (PD-MCI). Perfusion single photon emission computed tomography was performed in 15 PD-MCI patients and compared (statistical parametric mapping [SPM2]) with three groups, i.e., healthy subjects (CTR), cognitively intact PD patients (PD), and common amnestic MCI patients (aMCI). Age, depression, and UPDRS-III scores were considered as confounding variables. PD-MCI group (P < 0.05, false discovery rate-corrected for multiple comparisons) showed relative hypoperfusion in bilateral posterior parietal lobe and in right occipital lobe in comparison to CTR. As compared to aMCI, MCI-PD demonstrated hypoperfusion in bilateral posterior parietal and occipital areas, mainly right cuneus and angular gyrus, and left precuneus and middle occipital gyrus. With a less conservative threshold (uncorrected P < 0.01), MCI-PD showed hypoperfusion in a left parietal region, mainly including precuneus and inferior parietal lobule, and in a right temporal-parietal-occipital region, including middle occipital and superior temporal gyri, and cuneus-precuneus, as compared to PD. aMCI versus PD-MCI showed hypoperfusion in bilateral medial temporal lobe, anterior cingulate, and left orbitofrontal cortex. PD-MCI patients with amnestic deficit showed cortical dysfunction in bilateral posterior parietal and occipital lobes, a pattern that can be especially recognized versus both controls and common aMCI patients, and to a lesser extent versus cognitively intact PD. The relevance of this pattern in predicting dementia should be evaluated in longitudinal studies.

Gait dynamics in Parkinson's disease: common and distinct behavior among stride length, gait variability, and fractal-like scaling

Parkinson's disease (PD) is a common, debilitating neurodegenerative disease. Gait disturbances are a frequent cause of disability and impairment for patients with PD. This article provides a brief introduction to PD and describes the gait changes typically seen in patients with this disease. A major focus of this report is an update on the study of the fractal properties of gait in PD, the relationship between this feature of gait and stride length and gait variability, and the effects of different experimental conditions on these three gait properties. Implications of these findings are also briefly described. This update highlights the idea that while stride length, gait variability, and fractal scaling of gait are all impaired in PD, distinct mechanisms likely contribute to and are responsible for the regulation of these disparate gait properties.

Increased oxidation of certain glycolysis and energy metabolism enzymes in the frontal cortex in Lewy body diseases

Lipoxidative damage of aldolase A, enolase 1, and glyceraldehyde dehydrogenase (GAPDH) was found in the frontal cortex in a percentage of aged controls by bidimensional gel electrophoresis, Western blot test, in-gel digestion, and mass spectrometry. Aldolase A and enolase 1 were altered in 12 of 19 cases, whereas oxidation of GAPDH was found in 6 of 19 controls. The three enzymes were oxidized in the frontal cortex in the majority of cases of incidental Parkinson's disease (iPD), PD, and dementia with Lewy bodies (DLB). Differences were statistically significant (chi(2) test) for GAPDH in PD and DLB. Densitometric studies have shown that the ratio of oxidized protein per spot is higher in iPD, PD, and DLB compared with controls. These findings show oxidation of three enzymes linked with glycolysis and energy metabolism in the adult human brain as well as increased oxidation of aldolase A, enolase 1, and GAPDH in the frontal cortex in Lewy body diseases. Modifications of these enzymes may result in decreased activity and may partly account for impaired metabolism and function of the frontal lobe in PD.