Dopaminergic function in familial Parkinson's disease: a clinical and 18F-dopa positron emission tomography study

Imaging modalities in differential diagnosis of Parkinson’s disease: opportunities and challenges

Background

Parkinson’s disease (PD) diagnosis is yet largely based on the related clinical aspects. However, genetics, biomarkers, and neuroimaging studies have demonstrated a confirming role in the diagnosis, and future developments might be used in a pre-symptomatic phase of the disease.

Main text

This review provides an update on the current applications of neuroimaging modalities for PD diagnosis. A literature search was performed to find published studies that were involved on the application of different imaging modalities for PD diagnosis. An organized search of PubMed/MEDLINE, Embase, ProQuest, Scopus, Cochrane, and Google Scholar was performed based on MeSH keywords and suitable synonyms. Two researchers (TM and JPI) independently and separately performed the literature search. Our search strategy in each database was done by the following terms: ((Parkinson [Title/Abstract]) AND ((“Parkinsonian syndromes ”[Mesh]) OR Parkinsonism [Title/Abstract])) AND ((PET [Title/Abstract]) OR “SPECT”[Mesh]) OR ((Functional imaging, Transcranial sonography [Title/Abstract]) OR “Magnetic resonance spectroscopy ”[Mesh]). Database search had no limitation in time, and our last update of search was in February 2021. To have a comprehensive search and to find possible relevant articles, a manual search was conducted on the reference list of the articles and limited to those published in English.

Conclusion

Early diagnosis of PD could be vital for early management and adequate neuroprotection. Recent neuroimaging modalities such as SPECT and PET imaging using radiolabeled tracers, MRI, and CT are used to discover the disease. By the modalities, it is possible to early diagnose dopaminergic degeneration and also to differentiate PD from others parkinsonian syndromes, to monitor the natural progression of the disease and the effect of neuroprotective treatments on the progression. In this regard, functional imaging techniques have provided critical insights and roles on PD.

Imaging Familial and Sporadic Neurodegenerative Disorders Associated with Parkinsonism

In this paper, the structural and functional imaging changes associated with sporadic and genetic Parkinson's disease and atypical Parkinsonian variants are reviewed. The role of imaging for supporting diagnosis and detecting subclinical disease is discussed, and the potential use and drawbacks of using imaging biomarkers for monitoring disease progression is debated. Imaging changes associated with nonmotor complications of PD are presented. The similarities and differences in imaging findings in Lewy body dementia, Parkinson's disease dementia, and Alzheimer's disease are discussed.

Molecular Imaging of the Dopamine Transporter

Dopamine transporter (DAT) single-photon emission tomography (SPECT) with (123)Ioflupane is a widely used diagnostic tool for patients with suspected parkinsonian syndromes, as it assists with differentiating between Parkinson’s disease (PD) or atypical parkinsonisms and conditions without a presynaptic dopaminergic deficit such as essential tremor, vascular and drug-induced parkinsonisms. Recent evidence supports its utility as in vivo proof of degenerative parkinsonisms, and DAT imaging has been proposed as a potential surrogate marker for dopaminergic nigrostriatal neurons. However, the interpretation of DAT-SPECT imaging may be challenged by several factors including the loss of DAT receptor density with age and the effect of certain drugs on dopamine uptake. Furthermore, a clear, direct relationship between nigral loss and DAT decrease has been controversial so far. Striatal DAT uptake could reflect nigral neuronal loss once the loss exceeds 50%. Indeed, reduction of DAT binding seems to be already present in the prodromal stage of PD, suggesting both an early synaptic dysfunction and the activation of compensatory changes to delay the onset of symptoms. Despite a weak correlation with PD severity and progression, quantitative measurements of DAT binding at baseline could be used to predict the emergence of late-disease motor fluctuations and dyskinesias. This review addresses the possibilities and limitations of DAT-SPECT in PD and, focusing specifically on regulatory changes of DAT in surviving DA neurons, we investigate its role in diagnosis and its prognostic value for motor complications as disease progresses.

Dopaminergic Dysfunction in Parkinsonism: New Lessons from Imaging

In recent decades, identification of the dopaminergic deficit in Parkinson's disease has spawned an explo sion of research in the molecular neurobiology of the basal ganglia. In vivo imaging has provided a tool to bridge developments in basic neuroscience and clinicial neurology. Imaging studies have opened a unique window on the neurochemical pathophysiology of Parkinson's disease and more specifically on the onset, progression and physiology of the degenerative process. As we are poised on the brink of new protective and restorative therapies for Parkinson's disease, the potential of imaging to teach us about in vivo brain neurochemistry offers both promise and challenge. NEUROSCIENTIST 5:333-340, 1999

Abnormal Development of Glutamatergic Synapses Afferent to Dopaminergic Neurons of the Pink1(-/-) Mouse Model of Parkinson's Disease

In a preceding study, we showed that in adult pink1^−/− mice, a monogenic animal model of Parkinson’s disease (PD), striatal neurons display aberrant electrical activities that precede the onset of overt clinical manifestations. Here, we tested the hypothesis that the maturation of dopaminergic (DA) neurons of the pink1^−/− substantia nigra compacta (SNc) follows, from early stages on, a different developmental trajectory from age-matched wild type (wt) SNc DA neurons. We used immature (postnatal days P2–P10) and young adult (P30–P90) midbrain slices of pink1^−/− mice expressing the green fluorescent protein in tyrosine hydroxylase (TH)-positive neurons. We report that the developmental sequence of N-Methyl-D-aspartic acid (NMDA) spontaneous excitatory postsynaptic currents (sEPSCs) is altered in pink1^−/− SNc DA neurons, starting from shortly after birth. They lack the transient episode of high NMDA receptor-mediated neuronal activity characteristic of the immature stage of wt SNc DA neurons. The maturation of the membrane resistance of pink1^−/− SNc DA neurons is also altered. Collectively, these observations suggest that electrical manifestations occurring shortly after birth in SNc DA neurons might lead to dysfunction in dopamine release and constitute an early pathogenic mechanism of PD.

PET imaging in neurology: Alzheimer's and Parkinson's diseases

PET studies play an important role in the early detection of Alzheimer's and Parkinson's diseases (AD and PD). Fluorine-18 fluorodeoxyglucose (F-FDG) PET imaging of regional cerebral glucose metabolism and PET amyloid imaging are the two major PET studies for AD. F-FDG PET is highly sensitive for the early diagnosis of AD, in predicting conversion from mild cognitive impairment to AD, and in differentiating AD from other dementias. PET amyloid imaging is positive in the majority of patients with AD. Negative amyloid PET reduces the likelihood of AD. The main limitations of PET amyloid imaging is its high positivity in the normal elderly population and in other medical conditions with amyloid pathologies. Various PET tracers are available to assess motor and cognitive dysfunctions in PD. PET tracers targeting presynaptic dopaminergic function (F-DOPA, radiolabeled PET tracers assessing the availability of dopamine transporters and vesicular monoamine transporters) and postsynaptic dopamine receptors are used to assess motor dysfunction in PD. PET tracers, particularly dopamine transporters, are highly sensitive in the early diagnosis of PD. Uptake of PET tracers in the striatum is inversely correlated with disease severity. PET is valuable in differentiating PD from other movement disorders. PET studies, particularly F-FDG PET, help to evaluate cortical metabolism in PD patients with cognitive dysfunction and dementia.

Position Emission Tomography/Single-Photon Emission Tomography Neuroimaging for Detection of Premotor Parkinson's Disease

Premotor Parkinson's disease (PD) refers to a prodromal stage of Parkinson's disease (PD) during which nonmotor clinical features may be present. Currently, it is difficult to make an early diagnosis for premotor PD. Molecular imaging with position emission tomography (PET) or single-photon emission tomography (SPECT) offers a wide variety of tools for overcoming this difficulty. Indeed, molecular imaging techniques may play a crucial role in diagnosing, monitoring and evaluating the individuals with the risk for PD. For example, dopaminergic dysfunctions can be identified by detecting the expression of vesicular monoamine transporter (VMAT2) and aromatic amino acid decarboxylase (AADC) to evaluate the conditions of dopaminergic terminals functions in high-risk individuals of PD. This detection provides a sensitive and specific measurement of nonmotor symptoms (NMS) such as olfactory dysfunction, sleep disorders, and psychiatric symptoms in the high-risk patients, especially at the premotor phase. Molecular imaging technique is capable of detecting the dysfunction of serotonergic, noradrenergic, and cholinergic systems that are typically associated with premotor manifestations. This review discusses the importance of SPECT/PET applications in the detection of premotor markers preceding motor abnormalities with highlighting their great potential for early and accurate diagnosis of premotor symptoms of PD and its scientific significance.

Imaging of genetic and degenerative disorders primarily causing Parkinsonism

In this chapter the structural and functional imaging changes associated with both genetic causes of Parkinson's disease and the sporadic condition are reviewed. The role of imaging for supporting diagnosis and detecting subclinical disease is discussed and the potential use and drawbacks of using imaging biomarkers for monitoring disease progression are debated. Additionally, the use of imaging for differentiating atypical parkinsonian syndromes from Parkinson's disease is presented.

Nigrostriatal dopamine terminal imaging with dopamine transporter SPECT: an update

This article gives an update on nigrostriatal dopamine terminal imaging, with emphasis on SPECT performed with the presynaptic dopamine transporter (DAT) ligand (123)I-FP-CIT. The paper covers the rational use of this technique in the diagnostic work-up of patients with known or suspected parkinsonian syndromes. In detail, it addresses the impact of the method for the proof or exclusion of neurodegenerative parkinsonism, for its early and preclinical diagnosis, and for the evaluation of disease progression. The importance of normal DAT binding for differentiating symptomatic parkinsonism and relevant tremor syndromes from neurodegeneration is highlighted. Particularly emphasized is the role of DAT SPECT for diagnosing Lewy body dementia and its separation from Alzheimer dementia. Finally, some remarks deal with the economic aspects of the use of these imaging techniques in the clinical setting.

EFNS/MDS-ES/ENS [corrected] recommendations for the diagnosis of Parkinson's disease

BACKGROUND

A Task Force was convened by the EFNS/MDS-ES Scientist Panel on Parkinson's disease (PD) and other movement disorders to systemically review relevant publications on the diagnosis of PD.

METHODS

Following the EFNS instruction for the preparation of neurological diagnostic guidelines, recommendation levels have been generated for diagnostic criteria and investigations.

RESULTS

For the clinical diagnosis, we recommend the use of the Queen Square Brain Bank criteria (Level B). Genetic testing for specific mutations is recommended on an individual basis (Level B), taking into account specific features (i.e. family history and age of onset). We recommend olfactory testing to differentiate PD from other parkinsonian disorders including recessive forms (Level A). Screening for pre-motor PD with olfactory testing requires additional tests due to limited specificity. Drug challenge tests are not recommended for the diagnosis in de novo parkinsonian patients. There is an insufficient evidence to support their role in the differential diagnosis between PD and other parkinsonian syndromes. We recommend an assessment of cognition and a screening for REM sleep behaviour disorder, psychotic manifestations and severe depression in the initial evaluation of suspected PD cases (Level A). Transcranial sonography is recommended for the differentiation of PD from atypical and secondary parkinsonian disorders (Level A), for the early diagnosis of PD and in the detection of subjects at risk for PD (Level A), although the technique is so far not universally used and requires some expertise. Because specificity of TCS for the development of PD is limited, TCS should be used in conjunction with other screening tests. Conventional magnetic resonance imaging and diffusion-weighted imaging at 1.5 T are recommended as neuroimaging tools that can support a diagnosis of multiple system atrophy (MSA) or progressive supranuclear palsy versus PD on the basis of regional atrophy and signal change as well as diffusivity patterns (Level A). DaTscan SPECT is registered in Europe and the United States for the differential diagnosis between degenerative parkinsonisms and essential tremor (Level A). More specifically, DaTscan is indicated in the presence of significant diagnostic uncertainty such as parkinsonism associated with neuroleptic exposure and atypical tremor manifestations such as isolated unilateral postural tremor. Studies of [(123) I]MIBG/SPECT cardiac uptake may be used to identify patients with PD versus controls and MSA patients (Level A). All other SPECT imaging studies do not fulfil registration standards and cannot be recommended for routine clinical use. At the moment, no conclusion can be drawn as to diagnostic efficacy of autonomic function tests, neurophysiological tests and positron emission tomography imaging in PD.

CONCLUSIONS

The diagnosis of PD is still largely based on the correct identification of its clinical features. Selected investigations (genetic, olfactory, and neuroimaging studies) have an ancillary role in confirming the diagnosis, and some of them could be possibly used in the near future to identify subjects in a pre-symptomatic phase of the disease.

Examining the motor phenotype of patients with both essential tremor and Parkinson's disease

Background

The subset of patients with essential tremor (ET) that develops Parkinson's disease (PD) has not been fully clinically characterized.

Methods

Motor features were retrospectively reviewed in 18 ET patients who developed PD (ET→PD), 20 ET and 30 PD patients with similar ages and disease durations.

Results

Fewer ET→PD than ET patients had widespread postural and/or action tremor (2/17 [11.8%] vs. 11/17 [64.7%]; p = 0.001) and marginally fewer had cerebellar signs (1/15 [6.7%] vs. 6/18 [33.3%], p = 0.06). ET→PD patients required fewer ET medications than did their counterparts with ET (p = 0.001). ET→PD patients and PD patients did not differ in UPDRS, Hoehn and Yahr, or Schwab and England scores (each p≥0.14).

Discussion

ET patients who develop PD may have distinct pre-PD motor features compared to their counterparts with ET who do not develop co-existing PD. Prospective studies are needed to evaluate the predictive value of these clinical features for the emergence of PD.

Premotor Parkinson's disease: concepts and definitions

Parkinson's disease (PD) has a prodromal phase during which nonmotor clinical features as well as physiological abnormalities may be present. These premotor markers could be used to screen for PD before motor abnormalities are present. The technology to identify PD before it reaches symptomatic Braak Stage 3 (substantia nigra compacta [SNc] involvement) already exists. The current challenge is to define the appropriate scope of use of predictive testing for PD. Imaging technologies such as dopamine transporter imaging currently offer the highest degree of accuracy for identifying premotor PD, but they are expensive as screening tools, and abnormalities on these studies would only be evident at Braak Stage 3 or higher. Efficiency is greatly enhanced by combining imaging with a prescreening test such as olfactory testing. This 2-step process has the potential to greatly reduce costs while retaining diagnostic accuracy. Alternatively, or in concert with this approach, evaluating high-risk populations (eg, patients with rapid eye movement behavior disorder or LRRK2 mutations) would enrich the sample for cases with underlying PD. Ultimately, the role of preclinical detection of PD will be determined by the ability of emerging therapies to influence clinical outcomes. As such, implementation of large-scale screening strategies awaits the arrival of clearly safe and effective therapies that address the underlying pathogenesis of PD. Future research will establish more definitive biomarkers capable of revealing the presence of disease in advance of SNc involvement with the promise of the potential for introducing disease-modifying therapy even before the development of evidence of dopamine deficiency.

Revisiting the relationship between essential tremor and Parkinson's disease

BACKGROUND

The relationship between essential tremor and Parkinson's disease has been a subject of reviews and debates for long time, but there is now growing evidence that the two common movement disorders are pathogenically related, at least in some patient populations.

METHODS

PubMed as well as authors' own files were searched for relevant keywords regarding overlap between the disorders in clinical features as well as on epidemiologic, genetic, imaging, and pathological studies.

RESULTS

New findings in each of these categories are critically reviewed and placed in the context of previously published data.

DISCUSSION

Although we believe that there is compelling evidence for the notion that some patients with "pure" ET evolve into PD, the biologic nature of the association is not well understood. Furthermore, it is not clear what factors predict which ET patients later develop PD and whether patients with PD are more likely to develop ET. Further epidemiologic, clinical, genetic, imaging, and pathological studies are needed to better understand this mixed, ET-PD phenotype.

Development of Parkinson's disease biomarkers

Parkinson's disease (PD) is the most common neurodegenerative movement disorder, affecting over 6 million people worldwide. It is anticipated that the number of affected individuals may increase significantly in the most populous nations by 2030. During the past 20 years, much progress has been made in identifying and assessing various potential clinical, biochemical, imaging and genetic biomarkers for PD. Despite the wealth of information, development of a validated biomarker for PD is still ongoing. It is hoped that reliable and well-validated biomarkers will provide critical clues to assist in the diagnosis and management of Parkinson's disease patients in the near future.

Reduced uptake of [¹⁸F]FDOPA PET in asymptomatic welders with occupational manganese exposure

BACKGROUND

Welding exposes workers to manganese (Mn) fumes, but it is unclear if this exposure damages dopaminergic neurons in the basal ganglia and predisposes individuals to develop parkinsonism. PET imaging with 6-[(18)F]fluoro-l-dopa (FDOPA) is a noninvasive measure of nigrostriatal dopaminergic neuron integrity. The purpose of this study is to determine whether welding exposure is associated with damage to nigrostriatal neurons in asymptomatic workers.

METHODS

We imaged 20 asymptomatic welders exposed to Mn fumes, 20 subjects with idiopathic Parkinson disease (IPD), and 20 normal controls using FDOPA PET. All subjects were examined by a movement disorders specialist. Basal ganglia volumes of interest were identified for each subject. The specific uptake of FDOPA, K(i), was generated for each region using graphical analysis method.

RESULTS

Repeated measures general linear model (GLM) analysis demonstrated a strong interaction between diagnostic group and region (F(4,112) = 15.36, p < 0.001). Caudate K(i)s were lower in asymptomatic welders (0.0098 + 0.0013 minutes(-1)) compared to control subjects (0.0111 + 0.0012 minutes(-1), p = 0.002). The regional pattern of uptake in welders was most affected in the caudate > anterior putamen > posterior putamen. This uptake pattern was anatomically reversed from the pattern found in subjects with IPD.

CONCLUSIONS

Active, asymptomatic welders with Mn exposure demonstrate reduced FDOPA PET uptake indicating dysfunction in the nigrostriatal dopamine system. The caudate K(i) reduction in welders may represent an early (asymptomatic) marker of Mn neurotoxicity and appears to be distinct from the pattern of dysfunction found in symptomatic IPD.

Characteristics of depressive symptoms in essential tremor

Depressive symptoms are common in essential tremor (ET) and may be a primary feature of the underlying disease. However, it is still unclear whether depression in ET and depression in primary affective disorders share common clinical manifestations. Sixty-one depressed ET patients and 112 depressed patients without ET were assessed using the Montgomery-Asberg Depression Rating Scale (MADRS). We compared the individual depressive symptoms of the two groups by comparing MADRS subitem scores. Although there was no significant difference between the level of cognitive function and the severity of depression, patients with ET had a lower score on items "reported sadness", "inability to feel" and "pessimistic thoughts", and a higher score on items "concentration difficulties" and "lassitude" than those of patients without ET. These results show that depressive symptoms in patients with ET possess distinct characteristics compared to those in depressed patients without ET.

Genome-wide association study confirms extant PD risk loci among the Dutch

In view of the population-specific heterogeneity in reported genetic risk factors for Parkinson's disease (PD), we conducted a genome-wide association study (GWAS) in a large sample of PD cases and controls from the Netherlands. After quality control (QC), a total of 514,799 SNPs genotyped in 772 PD cases and 2024 controls were included in our analyses. Direct replication of SNPs within SNCA and BST1 confirmed these two genes to be associated with PD in the Netherlands (SNCA, rs2736990: P = 1.63 × 10(-5), OR = 1.325 and BST1, rs12502586: P = 1.63 × 10(-3), OR = 1.337). Within SNCA, two independent signals in two different linkage disequilibrium (LD) blocks in the 3' and 5' ends of the gene were detected. Besides, post-hoc analysis confirmed GAK/DGKQ, HLA and MAPT as PD risk loci among the Dutch (GAK/DGKQ, rs2242235: P = 1.22 × 10(-4), OR = 1.51; HLA, rs4248166: P = 4.39 × 10(-5), OR = 1.36; and MAPT, rs3785880: P = 1.9 × 10(-3), OR = 1.19).

Neuroimaging for Lewy body disease: is the in vivo molecular imaging of α-synuclein neuropathology required and feasible?

Alpha-synuclein aggregation is a neuropathological hallmark of many neurodegenerative diseases including Parkinson's disease (PD), Parkinson's disease with dementia (PDD) and dementia with Lewy bodies (DLB), collectively termed the α-synucleinopathies. Substantial advances in clinical criteria and neuroimaging technology over the last 20 years have allowed great strides in the detection and differential diagnosis of these disorders. Nevertheless, it is clear that whilst the array of different imaging modalities in clinical use allow for a robust diagnosis of α-synucleinopathy in comparison to healthy subjects, there is no clear diagnostic imaging marker that affords a reliable differential diagnosis between the different forms of Lewy body disease (LBD) or that could facilitate tracking of disease progression. This has led to a call for a biomarker based on the pathological hallmarks of these diseases, namely α-synuclein-positive Lewy bodies (LBs). This potentially may be advantageous in terms of early disease detection, but may also be leveraged into a potential marker of disease progression. We here aim to firstly review the current status of neuroimaging biomarkers in PD and related synucleinopathies. Secondly, we outline the rationale behind α-synuclein imaging as a potential novel biomarker as well as the potential benefits and limitations of this approach. Thirdly, we attempt to illustrate the likely technical hurdles to be overcome to permit successful in vivo imaging of α-synuclein pathology in the diseased brain. Our overriding aim is to provide a framework for discussion of how to address this major unmet clinical need.

Diagnosing premotor Parkinson's disease using a two-step approach combining olfactory testing and DAT SPECT imaging

Extranigral neuropathological changes may precede the degeneration of nigrostriatal dopaminergic neurones in Parkinson's disease (PD). Therefore, non-motor disturbances are an interesting target for the development of tests aimed at identifying individuals with premotor PD. An impaired sense of smell occurs with high prevalence in the clinical motor stages of PD patients and has also been reported in first-degree relatives of PD patients. In a prospective study in 361 asymptomatic first-degree relatives of PD patients, we studied the value of a two-step approach, combining olfactory testing and dopamine transporter (DAT) SPECT imaging, in detecting patients in the premotor phase of PD. Unexplained hyposmia alone was associated with a 12.5% risk of developing PD within a five year period. Cox regression analysis revealed that odour discrimination performance was most strongly correlated with the risk of future PD. Furthermore, all relatives that later developed PD had both hyposmia and abnormally reduced striatal DAT binding at baseline. The results of our studies provide the proof-of-principle that a two-step approach of olfactory testing and DAT SPECT imaging may serve to diagnose PD in its premotor phase. Yet, the low positive predictive value of hyposmia indicates that a wider application of this approach for screening purposes would require too many DAT SPECT scans in healthy individuals. Therefore, future studies in larger populations are necessary to further characterize premotor PD and identify additional genetic and/or clinical susceptibility markers to be used in conjunction with olfactory testing as additional screening steps toward diagnosing PD in its earliest stages.

Premotor Parkinson's disease: clinical features, detection, and prospects for treatment

The period immediately before the onset of motor symptoms of Parkinson's disease (PD) often has a recognizable phenotype with features including autonomic dysfunction and impaired olfaction. Subclinical dopaminergic cell loss can also be detected at this time using molecular imaging techniques. A greater recognition of the features of premotor PD and improvements in screening technologies have opened the possibility of predictive testing for PD. In addition to molecular imaging of the dopamine system, screening tests that can potentially be used to identify the physiological abnormalities in premotor PD include olfactory testing, imaging of the sympathetic innervation of the heart, transcranial ultrasound, and genetic testing for mutations known to cause hereditary PD. All of these technologies have trade-offs as screening tests for accuracy, availability, and costs. Using these tests in combination may produce a more favorable combination of reasonable cost and accuracy than using any single test alone. Ultimately, the value of screening for PD depends on development of neuroprotective treatments for PD that would create an imperative for early identification and treatment.

Imaging neurodegeneration in Parkinson's disease

Neuroimaging techniques have evolved over the past several years giving us unprecedented information about the degenerative process in Parkinson's disease (PD) and other movement disorders. Functional imaging approaches such as positron emission tomography (PET) and single photon emission computerised tomography (SPECT) have been successfully employed to detect dopaminergic dysfunction in PD, even while at a preclinical stage, and to demonstrate the effects of therapies on function of intact dopaminergic neurons within the affected striatum. PET and SPECT can also monitor PD progression as reflected by changes in brain levodopa and glucose metabolism and dopamine transporter binding. Structural imaging approaches include magnetic resonance imaging (MRI) and transcranial sonography (TCS). Recent advances in voxel-based morphometry and diffusion-weighted MRI have provided exciting potential applications for the differential diagnosis of parkinsonian syndromes. Substantia nigra hyperechogenicity, detected with TCS, may provide a marker of susceptibility to PD, probably reflecting disturbances of iron metabolism, but does not appear to correlate well with disease severity or change with disease progression. In the future novel radiotracers may help us assess the involvement of non-dopaminergic brain pathways in the pathology of both motor and non-motor complications in PD.

Exploring the relationship between essential tremor and Parkinson's disease

Although essential tremor (ET) and Parkinson's disease (PD) are considered distinct disorders, there is overlap in some clinical features. In some PD patients, a long-standing postural tremor in the hands may precede the onset of parkinsonian features by several years or decades. Furthermore, large families with both ET and PD phenotypes have been described and autopsy studies have demonstrated Lewy body pathology in brains of ET patients. Functional neuroimaging suggests that some ET patients have dopaminergic deficit. We examine here the evidence for and against an association between ET and PD, and critically review data supporting the notion that a subset of ET patients is predisposed to developing PD.

Prodromal non-motor symptoms of Parkinson's disease

The motor symptoms of Parkinson's disease (PD), bradykinesia, muscular rigidity, and tremor depend upon degeneration of the dopaminergic neurons in the substantia nigra pars compacta. Recent neuropathological studies show that the Lewy bodies, the intraneuronal landmark of PD, accumulate in several neuronal cell types in the brain. An ascending gradient of pathological involvement, from the medulla oblongata to neocortical areas has been reported. Thus the original view of PD as a disease characterized by selective damage of the dopaminergic neurons in the mesencephalon should be updated into the concept of a severe multisystemic neurodegenerative disorder. Additionally, the neuropathological alterations outside the substantia nigra are soundly correlated with the non-motor symptoms of PD. As a result of these findings, interest is growing in the identification of prodromal non-motor symptoms of PD. Indeed, data from the literature suggest that autonomic disturbances, olfactory dysfunctions, depression and sleep disorders (in particular REM-sleep behavior disorder) may represent prodromal non-motor symptoms of PD. Several tests are available to detect most of these symptoms. Thus, the identification of prodromal non-motor symptoms may contribute to the precocious diagnosis of PD, and might be useful in the future to test the efficacy of neuroprotective agents.

Imaging in Parkinson's disease: the role of monoamines in behavior

Positron emission tomography (PET) and single photon emission computed tomography (SPECT) can measure striatal dopamine (DA) terminal function in vivo as reflected by DA storage capacity and transporter binding. In Parkinson's disease (PD) posterior dorsal putamen DA terminals are initially targeted, the anterior putamen and head of caudate subsequently becoming affected. In contrast, dopaminergic function in pallidal, amygdala, and cingulate regions is upregulated in early PD and only later becomes reduced. Rigidity and bradykinesia in PD have been shown to correlate with loss of putamen dopaminergic function, whereas performance on executive and working memory tasks correlates with integrity of caudate dopaminergic terminals. 11C-RTI32 PET, a marker of noradrenergic and dopaminergic transporter binding, can be used to assess noradrenergic along with dopaminergic terminal function. Serotonergic transporter binding can be assessed with 11C-DASB PET and 123I-beta CIT SPECT, whereas HT1A binding can be measured with 11C-WAY100635 PET. With these modalities, the relationship between mood, noradrenergic and serotonergic function can be examined in PD. The functional effects of focal DA replacement on DA storage capacity and patterns of brain activation via implantation of fetal midbrain cells or glial derived neurotrophic factor (GDNF) infusion into putamen of PD patients has been examined with PET. Both approaches lead to consistently increased levels of putamen 18F-dopa uptake, and cell implantation can restore levels of frontal activation. Clinical outcome, however, has proved to be variable and off-medication dyskinesias are an unwanted side effect in transplanted cases. Dopamine release after pharmacological challenges or during behavioral tasks can be assessed indirectly by studying changes in receptor availability to PET radioligands. Stereotyped sequential movements are associated with striatal DA release, and this increases with more complex behaviors and the presence of financial incentives, which also increase frontal DA levels. Parkinson patients release less putamen DA than healthy control subjects during stereotyped finger movements. Interestingly, those PD patients who develop a dopa dependency syndrome, craving their medication, generate significantly greater levels of ventral striatal DA compared with similarly disabled patients without such a psychological dependency. In the future, functional imaging is likely to throw light on the roles of peptide transmission in regulating mood and behavior as non-peptide analogue ligands become available. Novel markers of amyloid plaque load will also help clarify the etiology of dementia in PD.

Assessing neuroprotection in Parkinson's disease: from the animal models to molecular neuroimaging in vivo

An important goal in Parkinson's Disease research is to identify neuroprotective therapy, and the interaction between basic science and clinical research is needed to discover drugs that can slow or halt the disorder progression. At present there is not a perfect animal model of PD to test neuroprotective strategies, however the models that portray the basic characteristics needed are toxin-induced and gene-based models. The first group comprehends 6-OHDA e MPTP and recently rotenone, paraquat and epoxomicin treated animals that shows some of human disease characteristics. Gene-based models are various and, even if with limits, they seem suitable models to test neuroprotection in PD since they present replicable lesions, a predictable pattern of neurodegeneration and a well-characterized behavior, biochemistry and morphology to assist in the understanding of induced changes. In clinical trials researchers have first used as marker of disease progression clinical scores and motor tasks which are limited by the potential symptomatic effect of tested drugs and are not useful in the pre-clinical phases of PD. Recently has emerged the important role of neuroimaging (Dopamine Transporter SPECT, 18FDopa-PET) as surrogate biomarker of PD progression. Even if there are still concerns about the influence of regulatory effects of tested drugs, neuroimaging features could represent a good outcome measure to evaluate PD progression and putative neuroprotective effect of pharmacological and non-pharmacological manipulations.

Marker for a preclinical diagnosis of Parkinson's disease as a basis for neuroprotection

Neuroprotective therapy is a pivotal aim in the treatment of the relentlessly progressive disorder Parkinson's disease. However, more than 60% of the dopaminergic neurons of the substantia nigra have already degenerated, when the diagnosis may be established. At this "advanced stage" neuroprotective strategies will - if at all - only have limited effect. It is, therefore, essential to establish markers to identify subjects at risk before motor manifestation. A number of such "premotor" signs have been discovered and investigated lately. Such signs include a genetic vulnerability and hyperechogenicity of the substantia nigra as well as premotor symptoms like olfactory and autonomic dysfunction, depression, REM sleep behaviour disorder, visual and neuropsychological impairment. Moreover, first signs of affection of the substantia nigra like PET and SPECT abnormalities and slight motor signs can be included, as they may be detected before a definite diagnosis can be made. Although most of these signs and symptoms are unspecific if singularly evaluated a combination of these features may indeed be valuable to detect a subgroup of the population at risk for PD. However, future studies are necessary to establish the predictive value of these "markers" singularly and in combination.

Positron emission tomography and single-photon emission computed tomography in central nervous system drug development

In this review, the value of functional imaging [positron emission tomography (PET)/single-photon emission computed tomography (SPECT)] in drug development is considered. Radionuclide imaging can help establish the diagnosis of neurodegenerative disorders where this is in doubt and provides a potential biomarker for following drug effects on disease progression. PET and SPECT can help understand mechanisms of disease and determine the functional effects of therapeutic approaches on neurotransmission and metabolism. Synthesizing radiotracer analogs of novel drugs can provide proof of principle that these agents reach their enzyme or receptor targets and delineate their regional brain distribution. If such radiotracers do not prove to have ideal properties for imaging, the concept of microdosing potentially allows multiple other drug analogs to be tested with less stringent regulatory requirements than for novel medicinals. Finally, PET tracers can provide receptor and enzyme active site dose occupancy profiles, thereby guiding dosage selection for phase 1 and phase 2 trials. The eventual hope is that radiotracer imaging will provide a surrogate marker for drug efficacy, although this has yet to be realized, and progress the concept of personalized medicine where receptor/enzyme binding profiles help predict therapeutic outcome.

Dopaminergic function in a family with the PARK6 form of autosomal recessive Parkinson’s syndrome

A G309D mutation in the PINK1 gene in a consanguineous Spanish kindred with seven siblings, three of whom are clinically affected, has recently been shown to be a cause of the PARK6 form of autosomal-recessive Parkinson's syndrome. In this family, we studied pre- and postsynaptic dopaminergic function using 123I-FP-CIT- and 123I-iodobenzamide-SPECT to determine binding to the presynaptic dopamine transporter (DAT) and postsynaptic D2 receptors respectively. All three PARK6 patients showed reduced striatal DAT binding with posterior preponderance similar to sporadic idiopathic PD, but only one patient showed significant striatal asymmetry. In two of the siblings, DAT binding was markedly increased. IBZM-SPECT was normal in both patients and sibs. Our findings indicate that 123I-FP-CIT-SPECT shows similar DAT binding in PARK6 patients compared to idiopathic Parkinson's disease. The increased DAT binding in heterozygous PARK6 carriers may be a new very early preclinical finding, but its significance is still unclear.

Role of [18F]-dopa-PET imaging in assessing movement disorders

FD-PET has proved to be an extremely useful technique for the noninvasive evaluation of nigrostriatal pathophysiology in patients with PD and other movement disorders. The development of ratio methods for image analysis has greatly reduced the complexity of these PET studies and has facilitated data analysis. With the recent advances in cyclotron targetry and automated synthesis modules FD-PET will soon become an important component of the clinical armamentarium.

PET and SPECT functional imaging studies in Parkinsonian syndromes: from the lesion to its consequences

Functional imaging techniques provide major insights into understanding the pathophysiology, progression, complications, and differential diagnosis of Parkinson's disease (PD). The dopaminergic system has been particularly studied allowing now early, presymptomatic diagnoses, which is of interest for future neuroprotective strategies. The existence of a compensatory hyperactivity of dopa-decarboxylase at disease onset has been recently demonstrated in the nigrostriatal and also extrastriatal dopaminergic pathways. Modification of dopamine receptors expression is observed during PD, but the respective contribution of dopaminergic drugs and the disease process towards these changes is still debated. Abnormalities of cerebral activation are seen and are clearly task-dependent, but the coexistence of hypoactivation in some areas and hyperactivation in others is also now well established. Such hyperactivation may be compensatory but could also reflect an inability to select appropriate motor circuits and inhibit inappropriate ones by PD patients. Interestingly, dopaminergic medications or surgical therapy reverse such abnormalities of brain activation.

Manganese-induced parkinsonism and Parkinson's disease

It has long been appreciated that manganese exposure can cause neurotoxicity and a neurologic syndrome that resembles Parkinson's disease (PD). Current evidence indicates that manganese-induced parkinsonism can be differentiated from PD because of its predilection to accumulate in and damage the pallidum and striatum rather than the SNc. The clinical syndrome, response to levodopa, imaging studies with MRI and PET, and pathologic features all help to distinguish these two conditions and permit the correct diagnosis to be established. This is of particular relevance in differentiating patients with parkinsonism due to manganese intoxication from patients with idiopathic PD who have incidental manganese exposure.

Neuroimaging in Parkinson's disease

In this review, the potential role of positron emission tomography and single photon emission computed tomography as biological markers for diagnosing and following the progression of Parkinson's disease (PD) is discussed. Their value for assessing the efficacy of putative neuroprotective agents in PD and for revealing the pharmacological changes underlying the symptomatology and complications of this disorder is also considered. It is concluded that in the future functional imaging will provide a valuable adjunct to clinical assessment when judging the efficacy of putative neuroprotective approaches to PD.

The sensitivity of 18-fluorodopa positron emission tomography and magnetic resonance imaging in Parkinson's disease

Parkinson's disease (PD) as the most important movement disorder is characterized by a progressive loss of nigral dopamine neurons and a subsequent degeneration within several other transmitter systems. Functional brain imaging with positron emission tomography (PET) and the radiotracer 18-fluorodopa (FDOPA) is capable to quantify the deficiency of dopamine synthesis and storage within pre-synaptic striatal nerve terminals. Therefore, FDOPA-PET allows the diagnosis of PD in early disease stages and the differentiation of clinically unclear cases from other movement disorders, e.g. essential tremor. Additionally, FDOPA-PET imaging permits the follow-up of disease progression, the assessment of medical and surgical PD therapy strategies with possible neuroprotective properties and the detection of pre-clinical disease in subjects at risk for the disorder. The classical domain of morphological magnetic resonance imaging (MRI) is the differentiation of symptomatic Parkinsonism from PD. However, recent advances in MRI data acquisition and analysis techniques demonstrated MRI to be also a valuable tool for detection of nigral pathology in PD and for differentiation of neurodegenerative disorders with atypical Parkinsonism.

The role of positron emission tomography imaging in movement disorders

PET imaging provides the means to study neurochemical, hemodynamic, or metabolic processes that underlie movement disorders in vivo. Because the extent of presynaptic nigrostriatal dopaminergic denervation can be quantified in PD even at an early or preclinical stage of the disease, PET imaging may allow the selection of at-risk subjects for neuroprotective intervention trials. These techniques may also provide markers to follow progression of disease or evaluate the effects of neurorestorative interventions in patients who have more advanced disease. PET is expected to play an increasing role in the selection of patients who have PD for deep brain stimulation. Dopaminergic studies may have a limited clinical role in the diagnosis of patients who have symptoms that suggestive of PD yet do not respond to typical dopaminergic drugs, such as patients who have vascular parkinsonism or ET with mild resting tremor who may have normal dopaminergic innervation. The differential diagnosis between PD and multiple system atrophy, progressive supranuclear palsy, or corticobasal degeneration is not yet clearly established by PET, but combined pre- and postsynaptic dopaminergic imaging may be able to distinguish early idiopathic PD from atypical parkinsonian disorders, in general. Huntington's chorea is characterized by more prominent striatal dopamine receptor loss, whereas nigrostriatal denervation is present to a lesser degree. Patients who have TS may have enhanced synaptic dopamine release in the putamen. Functional imaging studies have generally failed to demonstrate nigrostriatal denervation in essential tremor or idiopathic dystonia. Studies have shown striatal dopamine receptor loss in selected subtypes of dystonic patients. In conclusion, it is expected that PET will help us to better understand the pathophysiology of movement disorders, increase the diagnostic accuracy, allow preclinical diagnosis, monitor disease progression, and evaluate the efficacy of therapeutic agents. Pharmacologic radioligand displacement studies and the development of new nondopaminergic ligands may further aid in the unraveling of cerebral mechanisms that underlie movement disorders.

Looking Backward to Move Forward: Early Detection of Neurodegenerative Disorders

Early detection of neurodegenerative disorders would provide clues to the underlying pathobiology of these diseases and would enable more effective diagnosis and treatment of patients. Recent advances in molecular neuroscience have begun to provide the tools to detect diseases like Alzheimer's disease, Parkinson's disease, and others early in their course and potentially even before the development of clinical manifestations of disease. These genetic, imaging, clinical, and biochemical tools are being validated in a number of studies. Early detection of these slowly progressive diseases offers the promise of presymptomatic diagnosis and, ultimately, of disease-modifying medications for use early in disease and during the presymptomatic period.

Use of functional imaging in parkinsonism and dementia

Neuropsychiatric symptoms, including dementia, frequently coexist with parkinsonian disorders and may cause diagnostic confusion as well as management problems. Functional imaging studies include single photon emission computerised tomography (SPECT), positron emission tomography (PET), proton magnetic resonance spectroscopy, diffusion tensor imaging, and functional magnetic resonance imaging. This review addresses the utility of these techniques, from the clinician's perspective, focusing on the most common causes of parkinsonism and cognitive impairment, Parkinson's disease with dementia, dementia with Lewy bodies, and Alzheimer's disease. The potential and limitations of these techniques for accurate and early diagnosis, monitoring disease progression, and establishing the pathophysiological basis underlying key clinical features are considered. The development of new probes for SPECT and PET cameras capable of labeling protein aggregates (e.g., beta-amyloid) will offer exciting new insights into the spatial and temporal pattern of pathophysiological processes. Longitudinal studies with clinicopathological correlation represent the "gold standard" for fully evaluating functional imaging techniques.

Genetic and environmental factors in the cause of Parkinson's disease

Despite being the subject of intense study, the pathogenesis of Parkinson's disease still remains unclear. In recent years, however, there has been increasing evidence to support a role for genetic factors in its cause. This has come from twin and family studies, the mapping and cloning of PARK genes that are associated with the development of PD, and analysis of potential susceptibility genes. There is also evidence indicating that environmental factors may play a role in the disease process. It is likely that for most cases, there is a complex interplay between these genetic and environmental influences in the causation of Parkinson's disease. This article reviews the evidence in support of genetic and environmental factors in the cause of PD.

Clinical and subclinical dopaminergic dysfunction in PARK6-linked parkinsonism: an 18F-dopa PET study

PARK6, a locus for early-onset recessive parkinsonism, has been causally implicated in nine unrelated families from four different countries. The gene is still unidentified and hence the importance of PARK6 as a cause of Parkinson's disease is unknown. To date, no pathology or functional imaging studies are available on PARK6-linked parkinsonism. We have used (18)F-dopa positron emission tomography to study four patients who are homozygous and three asymptomatic relatives who are heterozygous for PARK6. The clinically affected PARK6 subjects had a similar 85% reduction in posterior dorsal putamen (18)F-dopa uptake to a group of idiopathic Parkinson's disease patients matched for clinical disease severity and duration but showed significantly greater involvement of head of caudate and anterior putamen. The group of asymptomatic PARK6 carriers showed a significant mean 20 to 30% reduction in caudate and putamen (18)F-dopa uptake in comparison with controls, individual values falling toward the bottom of the normal range. Our results indicate that PARK6 pathology results in a more uniform loss of striatal dopamine terminal function than Parkinson's disease. The subclinical loss of striatal dopamine storage capacity found in the PARK6 carriers implies that the unidentified gene on the short arm of chromosome 1 exhibits either haploinsufficency or a dominant negative effect.

Do dopamine agonists or levodopa modify Parkinson's disease progression?

During the past decade, in vivo imaging of the nigrostriatal dopaminergic system has been developed as a research tool to monitor progressive dopaminergic neuron loss in Parkinson's disease (PD) and to assess the effect of medication on imaging outcomes. Recently two similar studies compared the effect of initial treatment with a dopamine agonist (pramipexole (CALM-PD CIT) or ropinirole (REAL-PET)) or levodopa on the progression of PD as measured by [123I]beta-CIT or [18F]Dopa imaging. These two clinical imaging studies targeting dopamine function with different imaging ligands and technology both demonstrate slowing in the rate of loss of [123I]beta-CIT or [18F]Dopa uptake in early PD patients treated with dopamine agonists compared with levodopa. The relative reduction in the per cent loss from baseline of [123I]beta-CIT uptake in the pramipexole versus the levodopa group was 47% at 22 months, 44% at 34 months and 37% at 46 months after initiating treatment. The relative reduction of 18F-dopa uptake in the ropinirole group versus the levodopa group was 35% at 24 months. These results should be very cautiously interpreted with regard to the effect of dopamine agonists or levodopa on clinical disease progression. These data highlight the need to compare imaging outcomes of dopamine neuronal loss with multiple meaningful clinical endpoints of disease progression in placebo controlled, larger and long-term studies.