Monosymptomatic resting tremor and Parkinson's disease: a multitracer positron emission tomographic study

Cerebellar involvement in Parkinson's disease: Pathophysiology and neuroimaging

ABSTRACT

Parkinson's disease (PD) is a neurodegenerative disease characterized by various motor and non-motor symptoms. The complexity of its symptoms suggests that PD is a heterogeneous neurological disorder. Its pathological changes are not limited to the substantia nigra-striatal system, but gradually extending to other regions including the cerebellum. The cerebellum is connected to a wide range of central nervous system regions that form essential neural circuits affected by PD. In addition, altered dopaminergic activity and α-synuclein pathology are found in the cerebellum, further suggesting its role in the PD progression. Furthermore, an increasing evidence obtained from imaging studies has demonstrated that cerebellar structure, functional connectivity, and neural metabolism are altered in PD when compared to healthy controls, as well as among different PD subtypes. This review provides a comprehensive summary of the cerebellar pathophysiology and results from neuroimaging studies related to both motor and non-motor symptoms of PD, highlighting the potential significance of cerebellar assessment in PD diagnosis, differential diagnosis, and disease monitoring.

Optimization of [18F]-FDOPA Brain PET Acquisition Times for Assessment of Parkinsonism in the Clinical Setting

BACKGROUND AND PURPOSE

Fluorine 18-fluoro-L-dopa ([18F]-FDOPA) was approved by the FDA in 2019 and reimbursed by the Centers for Medicare & Medicaid Services in 2022 for use with PET to visualize dopaminergic nerve terminals in the striatum for evaluation of parkinsonism. We sought to determine the optimal image acquisition time for [18F]-FDOPA PET by evaluating rater-estimated FDOPA positivity and image quality across 4 time points.

MATERIALS AND METHODS

Brain PET/CT was acquired 90 minutes following injection of 185 megabecquerel (5 mCi) of [18F]-FDOPA. PET was acquired in list mode for 20 minutes, and data were replayed to represent 15-, 10-, and 5-minute acquisitions. By means of MIMneuro, PET/MR imaging or PET/CT was independently graded for FDOPA positivity and image quality by 2 readers, blinded to the clinical report and diagnosis. Expert neuroradiologist clinical reads were used as the criterion standard.

RESULTS

Twenty patients were included, average age 65.6 years, 55% women. Image-quality ratings decreased with shorter acquisition times for both readers (reader 1, ρ = 0.23, P = .044; reader 2, ρ = 0.24, P = .036), but there was no association between abnormality confidence scores and acquisition time (reader 1, ρ = -0.13, P = .250; reader 2, ρ = -0.19, P = .100). There was a high degree of consistency in intra- and interrater agreement and agreement with the expert reads when using acquisition times of ≥10 minutes (maximal confidence score consistency [ρ = 0.92] and interrater agreement [κ = 0.90] were observed at 15 minutes), while image quality was consistently rated as low and FDOPA positivity ratings were inconsistent when using a 5-minute acquisition time.

CONCLUSIONS

Our study suggests that image-quality ratings were stable after 15 minutes and that between-subject abnormality detection rates were highly consistent between the 2 readers when acquired for at least 10 and up to 20 minutes but were inconsistent at 5 minutes. Shorter [18F]-FDOPA PET acquisition times may help maximize patient comfort while increasing throughput in the clinical setting.

Differential influences of rest tremor on brain fiber architecture in essential tremor and Parkinson's disease

BACKGROUND

Rest tremor is a movement disorder commonly found in diseases like Parkinson's disease (PD) and essential tremor (ET). Rest tremor typically shows slower progression in PD, but more severe progression in ET. However, the underlying white matter organization of rest tremor behind PD and ET remains unclear.

METHODS

This study included 57 ET patients (40 without rest tremor (ETWR), 17 with rest tremor (ETRT)), 68 PD patients (34 without rest tremor (PDWR), 34 with rest tremor (PDRT)), and 62 normal controls (NC). Fixel-based analysis was used to evaluate the structural changes of white matter in rest tremor in these different diseases.

RESULTS

The fiber-bundle cross-section (FC) of the right non-decussating dentato-rubro-thalamic tract and several fibers outside the dentato-rubro-thalamic pathway in ETWR were significantly higher than that in NC. The fiber density and cross-section of the left nigro-pallidal in PDWR is significantly lower than that in NC, while the FC of bilateral nigro-pallidal in PDRT is significantly lower than that in NC.

CONCLUSION

ET patients with pure action tremor showed over-activation of fiber tracts. However, when superimposed with rest tremor, ET patients no longer exhibited over-activation of fiber tracts, but rather showed a trend of fiber tract damage. Except for the nigro-pallidal degeneration in all PD, PDRT will not experience further deterioration in fiber organization. These results provide important insights into the unique effects of rest tremor on brain fiber architecture in ET and PD.

Erythrocytic alpha-synuclein as potential biomarker for the differentiation between essential tremor and Parkinson's disease

Introduction

The differentiation between essential tremor (ET) and Parkinson's disease (PD) can be difficult because of the symptom overlaps. Erythrocytes are the major source of peripheral α-synuclein (α-syn), which is the most studied pathological molecular of PD. We have reported that erythrocytic α-syn levels in PD patients are significantly increased compared to those in healthy controls (HCs). However, little is known about the levels of erythrocytic α-syn species in ET patients.

Methods

This study includes 15 patients with ET, 64 patients with PD, and 49 age and sex matched HCs. A well-established electrochemiluminescence assay was used to measure the erythrocytic total and aggregated α-syn levels. The receiver operating characteristic (ROC) curve analysis was applied to evaluate the diagnostic values of erythrocytic α-syn for ET diagnosis and differentiation. The correlations of erythrocytic α-syn levels with disease durations were tested using Spearman's Rank Correlation analysis.

Results

We found that both erythrocytic total and aggregated α-syn concentrations are significantly increased in PD and ET patients compared to those in HCs. Erythrocytic total α-syn levels are significantly higher in ET patients than those in PD group. Furthermore, the ratios of erythrocytic aggregated to total α-syn levels in ET patients are significantly decreased than those in PD and HC subjects. We also found a significant association of erythrocytic aggregated α-syn levels with the disease duration of ET patients.

Conclusion

Our findings suggest new insight into the changes of erythrocytic total and aggregated α-syn levels as potential biomarkers for ET patients.

Dopamine Receptors in Parkinson's Disease: A Meta-Analysis of Imaging Studies

Dopamine receptors are abundant along the central nigrostriatal tract and are expressed as 5 subtypes in two receptor families. In PD, compensatory changes in dopamine receptors emerge as a consequence of the loss of dopamine nerve terminals or dopaminergic pharmacotherapy. We performed a systematic review and meta-analysis of the available PET and single-photon emission computed tomography studies that have investigated dopamine receptors in PD, PSP and MSA. The inclusion criteria were studies including human PET or single-photon emission computed tomography imaging; dopamine receptor tracers (D1-like or D2-like) and idiopathic PD, PSP, or MSA patients compared with healthy controls. The 67 included D2-like studies had 1925 patients. Data were insufficient for an analysis of D1-like studies. PD patients had higher striatal binding early in the disease, but after a disease duration of 4.36 years, PD patients had lower binding values than healthy controls. Striatal D2R binding was highest in unmedicated early PD patients and in the striatum contralateral to the predominant motor symptoms. PSP and MSA-P patients had lower striatal D2R binding than PD patients (14.2% and 21.8%, respectively). There is initial upregulation of striatal D2Rs in PD, which downregulate on average 4 years after motor symptom onset, possibly because of agonist-induced effects. The consistent upregulation of D2Rs in the PD striatum contralateral to the predominant motor symptoms indicates that receptor changes are driven by neurodegeneration and loss of striatal neuropil. Both PSP and MSA patients have clearly lower striatal D2R binding values than PD patients, which offers an opportunity for differential diagnostics. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Visualisation of Parkinsonian, essential and physiological tremor planes in 3Dspace

Based on the fact that tremors display some distinct 3D spatial characteristics, we decided to visualise tremor planes in 3D space. We obtained 3-axial linear accelerometer signals of hand tremors from 58 patients with Parkinson´s disease (PD), 37 with isolated resting tremor (iRT), 75 with essential tremor (ET), and 44 healthy volunteers with physiological tremor (Ph). For each group analysis was done with subsequent spatial 3D regression of the input data i.e. along the x, y and z axes; the projected vector lengths in the individual (vertical transversal XY, vertical longitudinal XZ and horizontal YZ) reference frame planes and their angles. Most meaningful and statistically significant differences were found in the analyses of the 3D vector lengths. The tremor of the PD and the iRT group was oriented mainly in the horizontal YZ plane. The tremors of the patients with ET and Ph were oriented approximately in the midway between the all three referential planes with less tilt toward the vertical longitudinal XZ plane.

[Parkinson's disease and glutamate excitotoxicity]

AIM

To measure the concentration of glutamate in the serum of patients with Parkinson's disease (PD) and determine its association with clinical variants of disease course.

MATERIAL AND METHODS

One hundred and ten patients with PD and 90 healthy people were examined. Glutamate concentration in the blood serum was determined with a spectrophotometric method.

RESULTS AND CONCLUSION

Patients with Parkinson's disease had significantly higher levels of serum glutamate compared with healthy subjects (p<0,0001). Patients with a tremor-dominant subtype had significantly higher levels of serum glutamate compared to those in patients with akinetic-rigid and mixed subtypes. The results obtained allow us to expand our understanding of the pathogenesis of this disease. Changes in the concentration of glutamate may reflect neurodegenerative process in PD.

PET imaging of noradrenaline transporters in Parkinson's disease: focus on scan time

OBJECTIVE

In subjects with idiopathic Parkinson's disease (PD) the functional state of the locus coeruleus and the subtle derangements in the finely tuned dopamine-noradrenaline interplay are largely unknown. The PET ligand (S,S)-[¹¹C]-O-methylreboxetine (C-11 MRB) has been described to reliably bind noradrenaline transporters but long scanning protocols might hamper its use, especially in patients with PD. We aimed to assess the feasibility of reducing C-11 MRB scans to 30 min.

METHODS

Ten patients with idiopathic PD underwent dynamic C-11 MRB PET (120 min duration) and brain magnetic resonance imaging. Model-based (i.e., simplified and multilinear reference tissue model 2) non-displaceable binding potentials (BP) of selected brain regions were analyzed for a 90 min scan protocol and compared with BP derived from static 30-min data with different starting times (30, 40, 50 and 60 min) after C-11 MRB injection. Intraclass correlation coefficient and linear regression analysis were used to explore the association between BP of different scan durations. Spearman's ρ served to describe the correlation of BP with demographic and clinical parameters.

RESULTS

With respect to kinetic models, BP_50-80 and BP_60-90 showed the best correlation in several brain areas (R² range 0.95-98; p < 0.001). The thalamus showed the highest BP on average. No correlation between BP, clinical and demographic characteristics was observed.

CONCLUSIONS

An acquisition time of 30 min, starting 50 or 60 min after C-11 MRB injection, allows a reliable estimation of noradrenaline transporter binding values in Parkinsonian people. A short acquisition time can significantly reduce the discomfort of Parkinsonian patients and facilitate PET studies, especially in the medication-off-state.

Analysis of 3D spatial trajectories in Parkinsonian, essential and physiological tremors

The clinical differentiation of the tremor in Parkinson's disease (PD) and essential tremor (ET) can sometimes be challenging, especially in the early stage of the disease. As different neural oscillators are involved in the generation of these two types of tremor, their trajectorial analysis could also be different. The goal of this study was to investigate whether some disease-specific patterns related to their tremor trajectories in fact exist. The three-axial accelerometer signals of the hand tremor obtained from a total of 369 participants [49 with PD, 25 with isolated resting tremor (iRT), 133 with ET, and finally 162 normal subjects with physiological tremor (Ph)] were subjected to vector analysis using a custom-made mathematical program. Subsequently, detailed trajectorial analysis was performed. The key discrimination ability between the PD and ET groups was represented by the ratio of the vector in the y-z plane and the spatial vector. The great majority of the patients with PD and iRT showed significantly higher values as compared to those with ET. The differences between the PD and iRT groups and between ET and Ph were not statistically significant. We suggest that the newly introduced three-axial accelerometry with analysis of tremor trajectories could be beneficial in differentiating between tremors in PD and ET.

Rest tremor revisited: Parkinson's disease and other disorders

Tremor is the most common movement disorder characterized by a rhythmical, involuntary oscillatory movement of a body part. Since distinct diseases can cause similar tremor manifestations and vice-versa, it is challenging to make an accurate diagnosis. This applies particularly for tremor at rest. This entity was only rarely studied in the past, although a multitude of clinical studies on prevalence and clinical features of tremor in Parkinson's disease (PD), essential tremor and dystonia, have been carried out. Monosymptomatic rest tremor has been further separated from tremor-dominated PD. Rest tremor is also found in dystonic tremor, essential tremor with a rest component, Holmes tremor and a few even rarer conditions. Dopamine transporter imaging and several electrophysiological methods provide additional clues for tremor differential diagnosis. New evidence from neuroimaging and electrophysiological studies has broadened our knowledge on the pathophysiology of Parkinsonian and non-Parkinsonian tremor. Large cohort studies are warranted in future to explore the nature course and biological basis of tremor in common tremor related disorders.

Heterogeneity of Monosymptomatic Resting Tremor in a Prospective Study: Clinical Features, Electrophysiological Test, and Dopamine Transporter Positron Emission Tomography

Background:

The relationship between monosymptomatic resting tremor (mRT) and Parkinson's disease (PD) remains controversial. In this study, we aimed to assess the function of presynaptic dopaminergic neurons in patients with mRT by dopamine transporter positron emission tomography (DAT-PET) and to evaluate the utility of clinical features or electrophysiological studies in differential diagnosis.

Methods:

Thirty-three consecutive patients with mRT were enrolled prospectively. The Unified Parkinson's Disease Rating Scale and electromyography were tested before DAT-PET. Striatal asymmetry index (SAI) was calculated, and a normal DAT-PET was defined as a SAI of <15%. Scans without evidence of dopaminergic deficits (SWEDDs) were diagnosed in patients with a subsequent normal DAT-PET and structural magnetic resonance imaging.

Results:

Twenty-eight mRT patients with a significant reduction in uptake of DAT binding in the striatum were diagnosed with PD, while the remained 5 with a normal DAT-PET scan were SWEDDs. As for UPRDS, the dressing and hygiene score, walking in motor experiences of daily living (Part II) and motor examination (Part III) were significant different between two groups (P < 0.05 and P < 0.01, respectively). Bilateral tremor was more frequent in the SWEDDs group (P < 0.05). The frequency of resting tremor and the amplitude of postural tremor tend to be higher in the SWEDDs group (P = 0.08 and P = 0.05, respectively).

Conclusions:

mRT is heterogeneous in presynaptic nigrostriatal dopaminergic degeneration, which can be determined by DAT-PET brain imaging. Clinical and electrophysiological features may provide clues to distinguish PD from SWEDDs.

Clozapine in Parkinsonian Rest Tremor: A Review of Outcomes, Adverse Reactions, and Possible Mechanisms of Action

BACKGROUND

The pathogenesis of rest tremor in Parkinson's disease (PD) is incompletely understood. This symptom can be resistant to typical anti-PD medications. Therefore, new treatments are needed given the concern that this symptom causes to patients and family. Limited experience suggests that clozapine can have an important antitremor effect in PD. The mechanism(s) underlying this effect is not well understood, but could provide insight and impetus to the development of more-effective and safer antitremor therapies.

METHODS AND RESULTS

Exemplifying the antitremor effects of clozapine, we describe a patient with tremor-predominant PD who obtained prominent reduction of rest tremor with clozapine treatment. We review the responses to this treatment in another 7 of our PD patients with treatment-resistant rest tremor. We also review the published literature on clozapine for tremor in PD and discuss its potential mechanisms of action and possible adverse effects. In our case series, there was a 64% reduction of tremor score after clozapine was initiated. The mechanism of tremor reduction remains unclear with possible involvement of anticholinergic, serotonergic, antihistaminergic, antiadrenergic, and antidopaminergic effects. Clozapine does have potential serious adverse effects.

CONCLUSIONS

Clozapine may be effective in controlling rest tremor in PD. Given the potential fatal side effects, if clozapine is to be initiated in PD patients, it has to be used cautiously with proper monitoring, preferably in specialized centers. We acknowledge that the number of patients in this case series is small. Further studies are needed to understand clozapine's mechanism of action in reducing tremor.

Approach to a tremor patient

Tremors are commonly encountered in clinical practice and are the most common movement disorders seen. It is defined as a rhythmic, involuntary oscillatory movement of a body part around one or more joints. In the majority of the population, tremor tends to be mild. They have varying etiology; hence, classifying them appropriately helps in identifying the underlying cause. Clinically, tremor is classified as occurring at rest or action. They can also be classified based on their frequency, amplitude, and body part involved. Parkinsonian tremor is the most common cause of rest tremor. Essential tremor (ET) and enhanced physiological tremor are the most common causes of action tremor. Isolated head tremor is more likely to be dystonic rather than ET. Isolated voice tremor could be considered to be a spectrum of ET. Psychogenic tremor is not a diagnosis of exclusion; rather, demonstration of various clinical signs is needed to establish the diagnosis. Severity of tremor and response to treatment can be assessed using clinical rating scales as well as using electrophysiological measurements. The treatment of tremor is symptomatic. Medications are effective in half the cases of essential hand tremor and in refractory patients; deep brain stimulation is an alternative therapy. Midline tremors benefit from botulinum toxin injections. It is also the treatment of choice in dystonic tremor and primary writing tremor.

Dopaminergic correlates of metabolic network activity in Parkinson's disease

Parkinson's disease (PD) is associated with distinct metabolic covariance patterns that relate to the motor and cognitive manifestations of the disorder. It is not known, however, how the expression of these patterns relates to measurements of nigrostriatal dopaminergic activity from the same individuals. To explore these associations, we studied 106 PD subjects who underwent cerebral PET with both (18) F-fluorodeoxyglucose (FDG) and (18) F-fluoro-L-dopa (FDOPA). Expression values for the PD motor- and cognition-related metabolic patterns (PDRP and PDCP, respectively) were computed for each subject; these measures were correlated with FDOPA uptake on a voxel-by-voxel basis. To explore the relationship between dopaminergic function and local metabolic activity, caudate and putamen FDOPA PET signal was correlated voxel-wise with FDG uptake over the entire brain. PDRP expression correlated with FDOPA uptake in caudate and putamen (P < 0.001), while PDCP expression correlated with uptake in the anterior striatum (P < 0.001). While statistically significant, the correlations were only of modest size, accounting for less than 20% of the overall variation in these measures. After controlling for PDCP expression, PDRP correlations were significant only in the posterior putamen. Of note, voxel-wise correlations between caudate/putamen FDOPA uptake and whole-brain FDG uptake were significant almost exclusively in PDRP regions. Overall, the data indicate that PDRP and PDCP expression correlates significantly with PET indices of presynaptic dopaminergic functioning obtained in the same individuals. Even so, the modest size of these correlations suggests that in PD patients, individual differences in network activity cannot be explained solely by nigrostriatal dopamine loss.

Brain network markers of abnormal cerebral glucose metabolism and blood flow in Parkinson's disease

Neuroimaging of cerebral glucose metabolism and blood flow is ideally suited to assay widely-distributed brain circuits as a result of local molecular events and behavioral modulation in the central nervous system. With the progress in novel analytical methodology, this endeavor has succeeded in unraveling the mechanisms underlying a wide spectrum of neurodegenerative diseases. In particular, statistical brain mapping studies have made significant strides in describing the pathophysiology of Parkinson's disease (PD) and related disorders by providing signature biomarkers to determine the systemic abnormalities in brain function and evaluate disease progression, therapeutic responses, and clinical correlates in patients. In this article, we review the relevant clinical applications in patients in relation to healthy volunteers with a focus on the generation of unique spatial covariance patterns associated with the motor and cognitive symptoms underlying PD. These characteristic biomarkers can be potentially used not only to improve patient recruitment but also to predict outcomes in clinical trials.

Parkinson's disease subtypes show a specific link between dopaminergic and glucose metabolism in the striatum

BACKGROUND

Previous studies have shown different clinical and imaging pattern in tremordominant and akinetic-rigid Parkinson's disease (PD) subtypes. The association between dopaminergic and glucose metabolism has in contrast not been investigated yet. Therefore, this study compared PD subtypes with respect to clinical and imaging findings with the aim of establishing a relationship between clinical subtypes, dopamine and glucose metabolism.

METHODS

Two groups of a total of 64 idiopathic PD patients (42 male, 22 female, mean age 56 ± 10.9 years) were analysed: akinetic-rigid (AR, n = 32) and tremor-dominant (TD, n = 32) patients. Both were compared with respect to differential involvement of local striatal dopamine and glucose metabolism using [18F]-fluoro-L-dopa (F-dopa) and [18F]-fluorodeoxyglucose (FDG)-PET.

RESULTS

The analysis of PD subgroups showed significant differences in the F-dopa uptake in the anterior putamen. Using the results of the local striatal dopamine difference as a volume of interest for the FDG-analysis, analysis of AR patients revealed a significantly lower normalised cerebral metabolic rate of glucose (nCMRGlc) within the ventral striatum.

CONCLUSIONS

The dual tracer study illlustrates clear differences between TD and AR subtypes in the ventral striatum. In accordance with previous FP-CIT-SPECT studies, it discloses congruent results for the presynaptic dopaminergic system and extends the knowledge about an additional involvement of local metabolic activity in the caudate and anterior putamen. The findings corroborate the specific role of distinct PD subtypes within the cerebello-thalamo-cortical-circuits. Multitracer PET imaging may thus enhance the knowledge about the clinical segregation into subtypes.

Cerebellar networks with the cerebral cortex and basal ganglia

The dominant view of cerebellar function has been that it is exclusively concerned with motor control and coordination. Recent findings from neuroanatomical, behavioral, and imaging studies have profoundly changed this view. Neuroanatomical studies using virus transneuronal tracers have demonstrated that cerebellar output reaches vast areas of the neocortex, including regions of prefrontal and posterior parietal cortex. Furthermore, it has recently become clear that the cerebellum is reciprocally connected with the basal ganglia, which suggests that the two subcortical structures are part of a densely interconnected network. Taken together, these findings elucidate the neuroanatomical substrate for cerebellar involvement in non-motor functions mediated by the prefrontal and posterior parietal cortex, as well as in processes traditionally associated with the basal ganglia.

The phenomenology of parkinsonian tremor

The definition of Parkinsonian tremor covers all different forms occurring in Parkinson's disease. The most common form is rest tremor, labelled as typical Parkinsonian tremor. Other variants cover also postural and action tremors. Data support the notion that suppression of rest tremor may be more specific for PD tremors. Several differential diagnoses like rest tremor in ET, dystonic tremor, psychogenic tremor and Holmes' tremor may be misinterpreted as PD-tremor. Tests and clinical clues to separate them are presented.

Cerebral causes and consequences of parkinsonian resting tremor: a tale of two circuits?

Tremor in Parkinson's disease has several mysterious features. Clinically, tremor is seen in only three out of four patients with Parkinson's disease, and tremor-dominant patients generally follow a more benign disease course than non-tremor patients. Pathophysiologically, tremor is linked to altered activity in not one, but two distinct circuits: the basal ganglia, which are primarily affected by dopamine depletion in Parkinson's disease, and the cerebello-thalamo-cortical circuit, which is also involved in many other tremors. The purpose of this review is to integrate these clinical and pathophysiological features of tremor in Parkinson's disease. We first describe clinical and pathological differences between tremor-dominant and non-tremor Parkinson's disease subtypes, and then summarize recent studies on the pathophysiology of tremor. We also discuss a newly proposed ‘dimmer-switch model’ that explains tremor as resulting from the combined actions of two circuits: the basal ganglia that trigger tremor episodes and the cerebello-thalamo-cortical circuit that produces the tremor. Finally, we address several important open questions: why resting tremor stops during voluntary movements, why it has a variable response to dopaminergic treatment, why it indicates a benign Parkinson's disease subtype and why its expression decreases with disease progression.

A role for locus coeruleus in Parkinson tremor

We analyzed rest tremor, one of the etiologically most elusive hallmarks of Parkinson disease (PD), in 12 consecutive PD patients during a specific task activating the locus coeruleus (LC) to investigate a putative role of noradrenaline (NA) in tremor generation and suppression. Clinical diagnosis was confirmed in all subjects by reduced dopamine reuptake transporter (DAT) binding values investigated by single photon computed tomography imaging (SPECT) with [¹²³I] N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl) tropane (FP-CIT). The intensity of tremor (i.e., the power of Electromyography [EMG] signals), but not its frequency, significantly increased during the task. In six subjects, tremor appeared selectively during the task. In a second part of the study, we retrospectively reviewed SPECT with FP-CIT data and confirmed the lack of correlation between dopaminergic loss and tremor by comparing DAT binding values of 82 PD subjects with bilateral tremor (n = 27), unilateral tremor (n = 22), and no tremor (n = 33). This study suggests a role of the LC in Parkinson tremor.

Cerebral oxygen metabolism in patients with early Parkinson's disease

AIM

Decreased activity of the mitochondrial electron transport chain (ETC) has been implicated in the pathogenesis of Parkinson's disease (PD). This model would most likely predict a decrease in the rate of cerebral oxygen consumption (CMRO(2)). To test this hypothesis, we compared CMRO(2) and cerebral blood flow (CBF) PET scans from PD patients and healthy controls.

MATERIALS AND METHODS

Nine early-stage PD patients and 15 healthy age-matched controls underwent PET scans for quantitative mapping of CMRO(2) and CBF. Between-group differences were evaluated for absolute data and intensity-normalized values.

RESULTS

No group differences were detected in regional magnitudes of CMRO(2) or CBF. Upon normalization using the reference cluster method, significant relative CMRO(2) decreases were evident in widespread prefrontal, parieto-occipital, and lateral temporal regions. Sensory-motor and subcortical regions, brainstem, and the cerebellum were spared. A similar pattern was evident in normalized CBF data, as described previously.

CONCLUSION

While the data did not reveal substantially altered absolute CMRO(2) in brain of PD patients, employing data-driven intensity normalization revealed widespread relative CMRO(2) decreases in cerebral cortex. The detected pattern was very similar to that reported in earlier CBF and CMRglc studies of PD, and in the CBF images from the same subjects. Thus, the present results are consistent with the occurrence of parallel declines in CMRO(2), CBF, and CMRglc in spatially contiguous cortical regions in early PD, and support the hypothesis that ETC dysfunction could be a primary pathogenic mechanism in early PD.

Differential effects of activating D1 and D2 receptors on electrophysiology of neostriatal neurons in a rat model of Parkinson's disease induced by paraquat and maneb

Neostriatum plays an important role in the pathophysiology of Parkinson's disease (PD). However, the changes of sensitivity of dopamine receptors of neostriatal neurons in PD have been less addressed in vivo. In the present study, systemic exposure to paraquat and maneb induced Parkinsonian symptoms and neuronal loss of substantia nigra pars compacta. Using single-unit recording methods, three types of neostriatal neurons were recorded including medium spiny-like neurons, large aspiny-like neurons and fast-spiking interneurons. In the exposed rats, increased firing activity of neostriatal neurons was revealed when compared to control rats. Following D1 receptor agonist, SKF38393 and D2 receptor agonist, LY171555 iontophoretically administrated respectively, effects of increase and decrease in firing activity were both observed in neostriatal neurons. However, stronger inhibitory effects of activating D1 receptors and weaker excitatory effects of activating D2 receptors were found in the exposed rats as compared to controls. It indicated that differential changes of sensitivity of D1 and D2 receptors in Parkinson's disease were related to the modulation of the imbalance between D1-receptor-dependent striatonigral direct pathway and D2-receptor-dependent striatopallidal indirect pathway. Our results illustrate the electrophysiological changes of in vivo neostriatal neurons in Parkinson's disease, thereby providing insight into the regulatory mechanisms of dopamine-mediated physiology.

Improvements in both psychosis and motor signs in Parkinson's disease, and changes in regional cerebral blood flow after electroconvulsive therapy

PURPOSE

Psychotic symptoms in Parkinson's disease (PD) are relatively common and, in addition to creating a disturbance in patients' daily lives, have consistently been shown to be associated with poor outcome. The use of anti-PD medications has been the most widely identified risk factor for PD psychosis (PDP). However, the pathophysiology of PDP remains unclear. Although the efficacy of electroconvulsive therapy (ECT) for PD had been pointed out, only one study has demonstrated the effectiveness of ECT on both psychotic symptoms and motor symptoms. The aim of this study was to examine the acute effectiveness of ECT on PD and to identify the brain areas associated with PDP.

METHODS

The study was conducted at Juntendo University Hospital in Tokyo. Eight patients with L-DOPA- or dopamine (DA) agonist-induced PDP, who were resistant to quetiapine treatment, were enrolled. Severity of PD was evaluated using the Hoehn and Yahr stage. Psychotic symptoms were evaluated using multiple measures from the Scale for the Assessment of Positive Symptoms (SAPS). Technetium-99m ethyl cysteinate dimer single photon emission computed tomography (99mTc ECD SPECT) was used to assess regional cerebral blood flow (rCBF) before and after a course of ECT. A voxel-by-voxel group analysis was performed using Statistical Parametric Mapping (SPM5).

RESULTS

Our study clearly demonstrated that PDP was significantly less severe after ECT than before ECT, as indicated by change in mean SAPS total domain score (t=7.2, P=0.0002). Furthermore, the patients showed significant improvement in Hoehn and Yahr stage after ECT (t=11.7, P<0.0001). A further notable observation was significant increase in rCBF in the right middle frontal gyrus after ECT.

CONCLUSION

We conclude that a course of ECT produced notable improvements not only in PDP but also in the severity of PD. The findings of change in rCBF suggest implications for dysfunction in the middle frontal region for patients with PDP.

Molecular imaging and the neuropathologies of Parkinson's disease

The main motor symptoms of Parkinson's disease (PD) are linked to degeneration of the nigrostriatal dopamine (DA) fibers, especially those innervating the putamen. This degeneration can be assessed in molecular imaging studies with presynaptic tracers such as [(18)F]-fluoro-L-DOPA (FDOPA) and ligands for DA transporter ligands. However, the pathologies of PD are by no means limited to nigrostriatal loss. Results of post mortem and molecular imaging studies reveal parallel degenerations of cortical noradrenaline (NA) and serotonin (5-HT) innervations, which may contribute to affective and cognitive changes of PD. Especially in advanced PD, cognitive impairment can come to resemble that seen in Alzheimer's dementia, as can the degeneration of acetylcholine innervations arising in the basal forebrain. The density of striatal DA D(2) receptors increases in early untreated PD, consistent with denervation upregulation, but there is an accelerated rate of DA receptor loss as the disease advances. Animal studies and post mortem investigations reveal changes in brain opioid peptide systems, but these are poorly documented in imaging studies of PD. Relatively minor changes in the binding sites for GABA are reported in cortex and striatum of PD patients. There remains some controversy about the expression of the 18 kDa translocator protein (TSPO) in activated microglia as an indicator of an active inflammatory component of neurodegeneration in PD. A wide variety of autonomic disturbances contribute to the clinical syndrome of PD; the degeneration of myocardial sympathetic innervation can be revealed in SPECT studies of PD patients with autonomic failure. Considerable emphasis has been placed on investigations of cerebral blood flow and energy metabolism in PD. Due to the high variance of these physiological estimates, researchers have often employed normalization procedures for the sensitive detection of perturbations in relatively small patient groups. However, a widely used normalization to the global mean must be used with caution, as it can result in spurious findings of relative hypermetabolic changes in subcortical structures. A meta-analysis of the quantitative studies to date shows that there is in fact widespread hypometabolism and cerebral blood flow in the cerebral cortex, especially in frontal cortex and parietal association areas. These changes can bias the use of global mean normalization, and probably represent the pathophysiological basis of the cognitive impairment of PD.

Tremor--some controversial aspects

The commonest cause of pathological tremor is essential tremor (ET). However, it has proved difficult to identify genetic mutations causing ET, particularly because other causes of tremor continue to be misdiagnosed as ET. Whether subjects with dystonia or Parkinson's disease (PD) carry an increased genetic risk of developing ET, or vice versa, is controversial. In addition, the notion of a separate disorder of benign tremulous parkinsonism (BTP) has been debated. This article gives a selective viewpoint on some areas of uncertainty and controversy in tremor.

The cerebellum and basal ganglia are interconnected

The cerebellum and the basal ganglia are major subcortical nuclei that control multiple aspects of behavior largely through their interactions with the cerebral cortex. Discrete multisynaptic loops connect both the cerebellum and the basal ganglia with multiple areas of the cerebral cortex. Interactions between these loops have traditionally been thought to occur mainly at the level of the cerebral cortex. Here, we review a series of recent anatomical studies in nonhuman primates that challenge this perspective. We show that the anatomical substrate exists for substantial interactions between the cerebellum and the basal ganglia. Furthermore, we discuss how these pathways may provide a useful framework for understanding cerebellar contributions to the manifestation of two prototypical basal ganglia disorders, Parkinson's disease and dystonia.

Cortical hypometabolism and hypoperfusion in Parkinson's disease is extensive: probably even at early disease stages

Recent cerebral blood flow (CBF) and glucose consumption (CMRglc) studies of Parkinson's disease (PD) revealed conflicting results. Using simulated data, we previously demonstrated that the often-reported subcortical hypermetabolism in PD could be explained as an artifact of biased global mean (GM) normalization, and that low-magnitude, extensive cortical hypometabolism is best detected by alternative data-driven normalization methods. Thus, we hypothesized that PD is characterized by extensive cortical hypometabolism but no concurrent widespread subcortical hypermetabolism and tested it on three independent samples of PD patients. We compared SPECT CBF images of 32 early-stage and 33 late-stage PD patients with that of 60 matched controls. We also compared PET FDG images from 23 late-stage PD patients with that of 13 controls. Three different normalization methods were compared: (1) GM normalization, (2) cerebellum normalization, (3) reference cluster normalization (Yakushev et al.). We employed standard voxel-based statistics (fMRIstat) and principal component analysis (SSM). Additionally, we performed a meta-analysis of all quantitative CBF and CMRglc studies in the literature to investigate whether the global mean (GM) values in PD are decreased. Voxel-based analysis with GM normalization and the SSM method performed similarly, i.e., both detected decreases in small cortical clusters and concomitant increases in extensive subcortical regions. Cerebellum normalization revealed more widespread cortical decreases but no subcortical increase. In all comparisons, the Yakushev method detected nearly identical patterns of very extensive cortical hypometabolism. Lastly, the meta-analyses demonstrated that global CBF and CMRglc values are decreased in PD. Based on the results, we conclude that PD most likely has widespread cortical hypometabolism, even at early disease stages. In contrast, extensive subcortical hypermetabolism is probably not a feature of PD.

Data-driven intensity normalization of PET group comparison studies is superior to global mean normalization

BACKGROUND

Global mean (GM) normalization is one of the most commonly used methods of normalization in PET and SPECT group comparison studies of neurodegenerative disorders. It requires that no between-group GM difference is present, which may be strongly violated in neurodegenerative disorders. Importantly, such GM differences often elude detection due to the large intrinsic variance in absolute values of cerebral blood flow or glucose consumption. Alternative methods of normalization are needed for this type of data.

MATERIALS AND METHODS

Two types of simulation were performed using CBF images from 49 controls. Two homogeneous groups of 20 subjects were sampled repeatedly. In one group, cortical CBF was artificially decreased moderately (simulation I) or slightly (simulation II). The other group served as controls. Ratio normalization was performed using five reference regions: (1) Global mean; (2) An unbiased VOI; (3) Data-driven region extraction (Andersson); (4-5) Reference cluster methods (Yakushev et al.). Using voxel-based statistics, it was determined how much of the original signal was detected following each type of normalization.

RESULTS

For both simulations, global mean normalization performed poorly, with only a few percent of the original signal recovered. Global mean normalization moreover created artificial increases. In contrast, the data-driven reference cluster method detected 65-95% of the original signal.

CONCLUSION

In the present simulation, the reference cluster method was superior to GM normalization. We conclude that the reference cluster method will likely yield more accurate results in the study of patients with early to moderate stage neurodegenerative disorders.

Morphological differences in Parkinson's disease with and without rest tremor

BACKGROUND

Rest tremor is a hallmark of Parkinson's disease (PD), but its pathogenesis remains incompletely understood. Nigro-striatal dopamine deficiency correlates best with bradykinesia, but not with tremor. Oscillating neurons in one or multiple localizations within the basal gangliathalamo-cortical loop may cause rest tremor, and an active contribution of the cerebellum and the cerebello-thalamo-cortical projections has been postulated.

OBJECTIVE

To compare the pattern of grey matter volume in PD patients with and without tremor to identify structural correlates of rest tremor.

METHODS

Voxel-based morphometry (VBM) of a high-resolution 3 Tesla, T1-weighted MR images, pre-processed according to an optimized protocol using SPM2, was performed in 24 patients with mild to moderate PD comparing local grey matter volume in patients with (n = 14) and without rest tremor (n = 10).

RESULTS

Grey matter volume is decreased in the right quadrangular lobe and declive of the cerebellum in PD with tremor compared to those without (PFDR < 0.05).

CONCLUSIONS

These results demonstrate for the first time morphological changes in the cerebellum in PD patients with rest tremor and highlight the involvement of the cerebellum and cerebello- thalamo-cortical circuit in the pathogenesis of parkinsonian rest tremor.

Clinical and cerebral activity changes induced by subthalamic nucleus stimulation in advanced Parkinson's disease: a prospective case-control study

BACKGROUND

High-frequency stimulation of the subthalamic nucleus (STN-DBS) improves motor symptoms in advanced Parkinson's disease (PD), but the mechanisms are still unclear. Functional imaging evidenced pathological overactivity in motor cortical areas in advanced PD that can be normalized by effective therapies.

PATIENTS AND METHODS

We studied resting state cerebral blood flow pre-operatively and 12 months after surgery in 40 patients with advanced PD using ECD-SPECT. SPECT scans were also acquired 1 year apart in 21 matched PD controls who did not undergo surgery. Statistical analysis was performed using statistical parametric mapping (SPM2) software. In addition, we correlated brain perfusion changes after surgery with clinical improvement, assessed using the unified PD rating scale motor score (UPDRS-III).

RESULTS

Patients showed marked motor improvement and medication reduction after surgery. Stimulated PD patients revealed bilateral rCBF decrements in motor cortical areas and prefrontal cortex bilaterally compared to pre-surgical condition as well as versus PD controls (p<.01 FDR corrected). Perfusion increases were found in cerebellum, temporal and occipital lobes. Clinical improvement was associated with perfusion decrements in primary motor and premotor cortices.

CONCLUSIONS

Effective STN-DBS is associated with neuronal activity changes in brain regions implicated in movement programming and performance. We hypothesize that clinical benefit might be associated with stimulation-induced normalization of the abnormal overactivity within the cortico-basal ganglia-thalamo-cortical motor loop in advanced PD.

Levodopa responsiveness in disorders with parkinsonism: a review of the literature

A literature review was conducted to investigate whether or not levodopa (LD) responsiveness (LR) is a useful criterion in the diagnosis of parkinsonian disorders. Although LR does appear to differ among the parkinsonian disorders, there is considerable confusion in the literature. While most patients with Parkinson's disease (PD) have a sustained benefit from LD, a small minority of patients with documented PD do not respond. The literature suggests that the LR rate is higher for multiple system atrophy (MSA), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD) than based on published diagnostic criteria. Magnitude and duration of response to LD and tolerability (time course, type and distribution of dyskinesias, mental effects and motor worsening) may be useful features in distinguishing PD, MSA, PSP, and CBD. Efforts should be directed toward better defining LR when used for diagnostic purposes and in scientific publications.

Normalization in PET group comparison studies--the importance of a valid reference region

INTRODUCTION

In positron emission tomography (PET) studies of cerebral blood flow (CBF) and metabolism, the large interindividual variation commonly is minimized by normalization to the global mean prior to statistical analysis. This approach requires that no between-group or between-state differences exist in the normalization region. Given the variability typical of global CBF and the practical limit on sample size, small group differences in global mean easily elude detection, but still bias the comparison, with profound consequences for the physiological interpretation of the results.

MATERIALS AND METHODS

Quantitative [15O]H2O PET recordings of CBF were obtained in 45 healthy subjects (21-81 years) and 14 patients with hepatic encephalopathy (HE). With volume-of-interest (VOI) and voxel-based statistics, we conducted regression analyses of CBF as function of age in the healthy group, and compared the HE group to a subset of the controls. We compared absolute CBF values, and CBF normalized to the gray matter (GM) and white matter (WM) means. In additional simulation experiments, we manipulated the cortical values of 12 healthy subjects and compared these to unaltered control data.

RESULTS

In healthy aging, CBF was shown to be unchanged in WM and central regions. In contrast, with normalization to the GM mean, CBF displayed positive correlation with age in the central regions. Very similar artifactual increases were seen in the HE comparison and also in the simulation experiment.

CONCLUSION

Ratio normalization to the global mean readily elevates CBF in unchanged regions when a systematic between-group difference exists in gCBF, also when this difference is below the detection threshold. We suggest that the routine normalization to the global mean in earlier studies resulted in spurious interpretations of perturbed CBF. Normalization to central WM yields less biased results in aging and HE and could potentially serve as a normalization reference region in other disorders as well.

[123I]FP-CIT striatal binding in early Parkinson's disease patients with tremor vs. akinetic-rigid onset

We performed [123I]FP-CIT/SPECT in 20 drug-naive Parkinson's disease (PD) patients, 10 with unilateral akinesia/rigidity at onset (arPD) and 10 with additional tremor-at-rest (tPD), to evaluate whether resting tremor at onset is associated with differences in striatal dopamine transporter binding. Patients of the two cohorts were matched for age, disease duration (<3 years) and severity of non-tremor motor symptoms; 31 healthy participants served as controls. Mean striatal dopamine transporter binding reduction in PD patients vs. controls was 42% for arPD and 50% for tPD; mean ipsilateral striatum and caudate nucleus uptake values were lower by 12 and 24%, respectively, in tPD than arPD. We conclude that widespread degeneration of the nigrostriatal dopaminergic pathway might be necessary for the development of parkinsonian tremor-at-rest.

Midbrain transcranial sonography in Korean patients with Parkinson's disease

Transcranial sonography (TCS) is potentially useful for the diagnosis of Parkinson's disease (PD). However, studies on TCS have so far been restricted to European populations. To investigate the efficacy of TCS in Korean PD patients and its correlation with the clinical features, we carried out midbrain TCS in 43 PD patients and 35 normal controls and evaluated the area of the substantia nigra (SN) hyperechogenicity and its ratio to the area of the whole midbrain. In 16 subjects (21%), TCS was unsuccessful due to insufficient acoustic temporal bone windows. The mean area of bilateral SN hyperechogenicity and its ratio to the midbrain area were greater in the PD patients than that in the controls (P < 0.01). In the PD patients, the area of SN hyperechogenicity and its ratio to the individual midbrain area were moderately correlated with the PD duration (r = 0.526 and 0.536, P = 0.01, respectively) but not with the age, UPDRS motor scores or H-Y stage. There was no difference in the SN hyperechogenicity between the tremor-dominant, akinetic-rigid, and mixed-type PD patients. In conclusion, midbrain TCS is an effective diagnostic tool for detecting PD in the Korean population. However, it does not reflect the severity or phenotypes of parkinsonism.

Regional CBF changes in Parkinson’s disease: a correlation with motor dysfunction

PURPOSE

The purpose of this study was to further localize cerebral perfusion abnormalities, and to better correlate these abnormalities with the clinical severity of Parkinson's disease (PD).

METHODS

A single-photon emission computed tomography (SPECT) study was performed on 27 patients with PD and 24 age-matched controls. SPECT images were spatially normalized, concatenated, and then decomposed using Infomax independent component analysis (ICA). The resulting image components were separated by logistic regression into two subspaces: "disease-related" components whose subject weights differed between groups, and "disease-unrelated" components. The resultant regional cerebral blood flow (rCBF) subspace images were normalized to global CBF for each subject, and then processed using statistical parametric mapping to compare rCBF values between PD and control subjects.

RESULTS

In the disease-related image subspace, patients with PD exhibited significantly higher adjusted rCBF in the putamen, globus pallidum, thalamus, brainstem, and the anterior lobe of the cerebellum, and significant hypoperfusion in the parieto-temporo-occipital cortex, the dorsolateral prefrontal cortex, the insula, and the cingulate gyrus. The motor Unified Parkinson's Disease Rating Scale scores correlated negatively with rCBF in the insula and cingulate gyrus. In the disease-unrelated image subspace, no brain voxels exhibited a significant group difference.

CONCLUSION

ICA-based separation of normalized images into disease-related and disease-unrelated subspaces revealed many disease-related group blood flow differences. The regions revealed by ICA are consistent with the current model of PD. These rCBF changes in PD have not been fully demonstrated in any single functional imaging study previously.

Transcranial brain sonography findings in clinical subgroups of idiopathic Parkinson's disease

To investigate whether transcranial brain sonography (TCS) discriminates different courses of idiopathic Parkinson's disease (PD), 101 patients with clinically definite PD were studied. In four patients, TCS was not possible due to insufficient acoustic temporal bone windows. Substantia nigra (SN) hyperechogenicity was found in 96% of assessable patients. Larger SN echogenic size correlated with younger age at PD onset (Spearman correlation, r = -0.383; P < 0.001), but not with age, PD duration, or severity. Marked bilateral SN hyperechogenicity indicated early-onset rather than late-onset PD, and akinetic-rigid (AR) or mixed-type (MX) PD rather than tremor-dominant PD. SN echogenic sizes were larger contralateral to the clinically more affected side in AR PD and MX PD patients. Reduced echogenicity of brainstem raphe was associated with depression (RR = 1.61; 95% CI = 1.05-2.46; P = 0.044) but not with other clinical features. Caudate nucleus hyperechogenicity was, independently from PD duration, related to drug-induced psychosis (RR = 2.40; CI = 1.36-4.22; P = 0.001), but not to motor fluctuations. Lenticular nucleus hyperechogenicity indicated AR PD rather than tremor-dominant PD (RR = 1.44; CI = 1.11-1.86; P = 0.040). Frontal horn dilatation > 15.4 mm (mean of bilateral measurements) indicated increased risk of dementia (RR = 4.11; CI = 1.51-11.2; P = 0.001). We conclude that TCS displays characteristic changes of deep brain structures in different clinical manifestations of PD.

What has been learnt from study of dopamine receptors in Parkinson's disease?

Since the introduction of dopamine replacement therapy using L-3,4-dihydroxyphenyalanine (L-DOPA) to treat Parkinson's disease and the recognition of the problems associated with L-DOPA use, numerous studies have investigated dopamine receptor regulation and function in Parkinson's disease. These studies have provided insight into the pathological process of the disorder and the molecular consequences of chronic dopaminergic treatment, but they have been less successful in identifying new pharmacological targets or treatment regimes that are as effective as L-DOPA at alleviating the symptoms of Parkinson's disease. This review will present a summary of the reported changes in dopamine receptor regulation and function that occur in Parkinson's disease and will discuss their contribution to the current pharmacological management of Parkinson's disease.

The diagnosis of Parkinson's disease

The correct diagnosis of Parkinson's disease is important for prognostic and therapeutic reasons and is essential for clinical research. Investigations of the diagnostic accuracy for the disease and other forms of parkinsonism in community-based samples of patients taking antiparkinsonian medication confirmed a diagnosis of parkinsonism in only 74% of patients and clinically probable Parkinson's disease in 53% of patients. Clinicopathological studies based on brain bank material from the UK and Canada have shown that clinicians diagnose the disease incorrectly in about 25% of patients. In these studies, the most common reasons for misdiagnosis were presence of essential tremor, vascular parkinsonism, and atypical parkinsonian syndromes. Infrequent diagnostic errors included Alzheimer's disease, dementia with Lewy bodies, and drug-induced parkinsonism. Increasing knowledge of the heterogeneous clinical presentation of the various parkinsonisms has resulted in improved diagnostic accuracy of the various parkinsonian syndromes in specialised movement-disorder units. Also genetic testing and various other ancillary tests, such as olfactory testing, MRI, and dopamine-transporter single-photon-emission computed-tomography imaging, help with clinical diagnostic decisions.