Replication and reliability of Parkinson's disease clinical subtypes.

BACKGROUND: We recently identified three distinct Parkinson's disease subtypes: "motor only" (predominant motor deficits with intact cognition and psychiatric function); "psychiatric & motor" (prominent psychiatric symptoms and moderate motor deficits); "cognitive & motor" (cognitive and motor deficits). OBJECTIVE: We used an independent cohort to replicate and assess reliability of these Parkinson's disease subtypes. METHODS: We tested our original subtype classification with an independent cohort (N = 100) of Parkinson's disease participants without dementia and the same comprehensive evaluations assessing motor, cognitive, and psychiatric function. Next, we combined the original (N = 162) and replication (N = 100) datasets to test the classification model with the full combined dataset (N = 262). We also generated 10 random split-half samples of the combined dataset to establish the reliability of the subtype classifications. Latent class analyses were applied to the replication, combined, and split-half samples to determine subtype classification. RESULTS: First, LCA supported the three-class solution - Motor Only, Psychiatric & Motor, and Cognitive & Motor- in the replication sample. Next, using the larger, combined sample, LCA again supported the three subtype groups, with the emergence of a potential fourth group defined by more severe motor deficits. Finally, split-half analyses showed that the three-class model also had the best fit in 13/20 (65%) split-half samples; two-class and four-class solutions provided the best model fit in five (25%) and two (10%) split-half replications, respectively. CONCLUSIONS: These results support the reproducibility and reliability of the Parkinson's disease behavioral subtypes of motor only, psychiatric & motor, and cognitive & motor groups.

Sex Differences in Dystonia.

BACKGROUND: Prior studies have indicated that female individuals outnumber male individuals for certain types of dystonia. Few studies have addressed factors impacting these sex differences or their potential biological mechanisms. OBJECTIVES: To evaluate factors underlying sex differences in the dystonias and explore potential mechanisms for these differences. METHODS: Data from individuals with various types of dystonia were analyzed in relation to sex. Data came from two different sources. One source was the Dystonia Coalition database, which contains predominantly idiopathic adult-onset focal and segmental dystonias. The second source was the MDSGene database, which contains predominantly early-onset monogenic dystonias. RESULTS: The 3222 individuals from the Dystonia Coalition included 71% female participants and 29% male participants for an overall female-to-male ratio (F:M) of 2.4. This ratio varied according to body region affected and whether dystonia was task-specific. The female predominance was age-dependent. Sex did not have a significant impact on co-existing tremor, geste antagoniste, depression or anxiety. In the 1377 individuals from the MDSGene database, female participants outnumbered male participants for some genes (GNAL, GCH1, and ANO3) but not for other genes (THAP1, TH, and TOR1A). CONCLUSIONS: These results are in keeping with prior studies that have indicated female individuals outnumber male individuals for both adult-onset idiopathic and early onset monogenic dystonias. These results extend prior observations by revealing that sex ratios depend on the type of dystonia, age, and underlying genetics.

PET Quantification of [18F]VAT in Human Brain and Its Test-Retest Reproducibility and Age Dependence.

Molecular imaging of brain vesicular acetylcholine transporter provides a biomarker to explore cholinergic systems in humans. We aimed to characterize the distribution of, and optimize methods to quantify, the vesicular acetylcholine transporter-specific tracer (-)-(1-(8-(2-[18F]fluoroethoxy)-3-hydroxy-1,2,3,4-tetrahydronaphthalen-2-yl)-piperidin-4-yl)(4-fluorophenyl)methanone ([18F]VAT) in the brain using PET. Methods: Fifty-two healthy participants aged 21-97 y had brain PET with [18F]VAT. [3H]VAT autoradiography identified brain areas devoid of specific binding in cortical white matter. PET image-based white matter reference region size, model start time, and duration were optimized for calculations of Logan nondisplaceable binding potential (BPND). Ten participants had 2 scans to determine test-retest variability. Finally, we analyzed age-dependent differences in participants. Results: [18F]VAT was widely distributed in the brain, with high striatal, thalamic, amygdala, hippocampal, cerebellar vermis, and regionally specific uptake in the cerebral cortex. [3H]VAT autoradiography-specific binding and PET [18F]VAT uptake were low in white matter. [18F]VAT SUVs in the white matter reference region correlated with age, requiring stringent erosion parameters. Logan BPND estimates stabilized using at least 40 min of data starting 25 min after injection. Test-retest variability had excellent reproducibility and reliability in repeat BPND calculations for 10 participants (putamen, 6.8%; r > 0.93). We observed age-dependent decreases in the caudate and putamen (multiple comparisons corrected) and in numerous cortical regions. Finally, we provide power tables to indicate potential mean differences that can be detected between 2 groups of participants. Conclusion: These results validate a reference region for BPND calculations and demonstrate the viability, reproducibility, and utility of using the [18F]VAT tracer in humans to quantify cholinergic pathways.

Metabolite Study and Structural Authentication for the First-in-Human Use Sphingosine-1-phosphate Receptor 1 Radiotracer.

The sphingosine-1-phosphate receptor 1 (S1PR1) radiotracer [11C]CS1P1 has shown promise in proof-of-concept PET imaging of neuroinflammation in multiple sclerosis (MS). Our HPLC radiometabolite analysis of human plasma samples collected during PET scans with [11C]CS1P1 detected a radiometabolite peak that is more lipophilic than [11C]CS1P1. Radiolabeled metabolites that cross the blood-brain barrier complicate quantitative modeling of neuroimaging tracers; thus, characterizing such radiometabolites is important. Here, we report our detailed investigation of the metabolite profile of [11C]CS1P1 in rats, nonhuman primates, and humans. CS1P1 is a fluorine-containing ligand that we labeled with C-11 or F-18 for preclinical studies; the brain uptake was similar for both radiotracers. The same lipophilic radiometabolite found in human studies also was observed in plasma samples of rats and NHPs for CS1P1 labeled with either C-11 or F-18. We characterized the metabolite in detail using rats after injection of the nonradioactive CS1P1. To authenticate the molecular structure of this radiometabolite, we injected rats with 8 mg/kg of CS1P1 to collect plasma for solvent extraction and HPLC injection, followed by LC/MS analysis of the same metabolite. The LC/MS data indicated in vivo mono-oxidation of CS1P1 produces the metabolite. Subsequently, we synthesized three different mono-oxidized derivatives of CS1P1 for further investigation. Comparing the retention times of the mono-oxidized derivatives with the metabolite observed in rats injected with CS1P1 identified the metabolite as N-oxide 1, also named TZ82121. The MS fragmentation pattern of N-oxide 1 also matched that of the major metabolite in rat plasma. To confirm that metabolite TZ82121 does not enter the brain, we radiosynthesized [18F]TZ82121 by the oxidation of [18F]FS1P1. Radio-HPLC analysis confirmed that [18F]TZ82121 matched the radiometabolite observed in rat plasma post injection of [18F]FS1P1. Furthermore, the acute biodistribution study in SD rats and PET brain imaging in a nonhuman primate showed that [18F]TZ82121 does not enter the rat or nonhuman primate brain. Consequently, we concluded that the major lipophilic radiometabolite N-oxide [11C]TZ82121, detected in human plasma post injection of [11C]CS1P1, does not enter the brain to confound quantitative PET data analysis. [11C]CS1P1 is a promising S1PR1 radiotracer for detecting S1PR1 expression in the CNS.

Structure of alpha-synuclein fibrils derived from human Lewy body dementia tissue.

The defining feature of Parkinson disease (PD) and Lewy body dementia (LBD) is the accumulation of alpha-synuclein (Asyn) fibrils in Lewy bodies and Lewy neurites. Here we develop and validate a method to amplify Asyn fibrils extracted from LBD postmortem tissue samples and use solid state nuclear magnetic resonance (SSNMR) studies to determine atomic resolution structure. Amplified LBD Asyn fibrils comprise a mixture of single protofilament and two protofilament fibrils with very low twist. The protofilament fold is highly similar to the fold determined by a recent cryo-electron microscopy study for a minority population of twisted single protofilament fibrils extracted from LBD tissue. These results expand the structural characterization of LBD Asyn fibrils and approaches for studying disease mechanisms, imaging agents and therapeutics targeting Asyn.

Novel SGCE Mutation in a Patient With Myoclonus-Dystonia: A Case Report.

Objectives: Characterize the presentation, workup, and management of SGCE myoclonus-dystonia, a rare genetic condition, in a patient with atypical presenting symptoms and no family history of movement abnormalities. Methods: A woman with myoclonus and dystonia was identified based on clinical history and physical examination. Workup was conducted to determine the cause of her symptoms, including whole-exome sequencing. Myoclonus-dystonia is associated with more than 100 distinct mutations in MYC/DYT-SGCE that account for only half of the total myoclonus-dystonia patients. As such, this case required intensive genetic analyses rather than screening only for a small subset of well-characterized mutations. Results: Childhood onset myoclonus and worsening dystonia with age were identified in a young woman. She underwent screening for common causes of twitching movements, followed by whole-exome sequencing which identified a de novo novel variant in the SGCE gene, resulting in a diagnosis of SGCE myoclonus-dystonia. Discussion: Myoclonus-dystonia should be considered in patients with symptoms of head and upper extremity myoclonus early in life, especially with co-occurring dystonia, even in the absence of a family history of similar symptoms. Diagnosis of this condition should take place using sequencing, as new mutations continue to be discovered.

Longitudinal predictors of health-related quality of life in isolated dystonia.

OBJECTIVE: To determine longitudinal predictors of health-related quality of life (HR-QoL) in an international multicenter cohort of patients with isolated dystonia. METHODS: Out of 603 dystonia patients prospectively enrolled in the Natural History Dystonia Coalition study, 155 were assessed three times within 2 years for HR-QoL, symptoms of depression, generalized anxiety disorder (GAD), and social anxiety disorder (SAD), as well as dystonia severity and dystonic tremor. In addition, the impact of botulinum neurotoxin (BoNT) injections on HR-QoL was evaluated after 1 year. RESULTS: Depressive symptoms at baseline predicted lower HR-QoL on all subscales after 2 years (all p ≤ 0.001). Higher GAD scores at baseline predicted lower HR-QoL related to general health, pain and emotional well-being, whereas higher SAD scores predicted higher pain-related QoL after 2 years (all p ≤ 0.006). Dystonia severity at baseline predicted social functioning (p = 0.002). Neither dystonic tremor, age, or sex predicted HR-QoL at 2 years. Two latent categories were revealed across the three-time points: Category 1 with higher total HR-QoL scores (mean HR-QoL = 74.4% ± 16.1), susceptible to symptoms of depression and SAD, and Category 2 with lower total HR-QoL scores (mean HR-QoL = 45.5% ± 17.6), susceptible to symptoms of GAD. HR-QoL improved over the course of 1 year irrespective of the use of BoNT. CONCLUSION: The longitudinal impact of psychiatric symptoms on HR-QoL emphasizes the importance of incorporating mental health treatment, in particular also the therapy of anxiety disorders, into treatment regimens for dystonia.

Cell-free RNA signatures predict Alzheimer's disease.

There is a need for affordable, scalable, and specific blood-based biomarkers for Alzheimer's disease that can be applied to a population level. We have developed and validated disease-specific cell-free transcriptomic blood-based biomarkers composed by a scalable number of transcripts that capture AD pathobiology even in the presymptomatic stages of the disease. Accuracies are in the range of the current CSF and plasma biomarkers, and specificities are high against other neurodegenerative diseases.

An Empirical Comparison of Commonly Used Universal Rating Scales for Dystonia.

Background: There are several widely used clinical rating scales for documenting the severity and distribution of various types of dystonia. Objectives: The goal of this study was to evaluate the performance of the most commonly used scales in a large group of adults with the most common types of isolated dystonia. Methods: Global Dystonia Rating Scale (GDRS) and the Burke-Fahn-Marsden Dystonia Rating Scale (BFM) scores were obtained for 3067 participants. Most had focal or segmental dystonia, with smaller numbers of multifocal or generalized dystonia. These scales were also compared for 209 adults with cervical dystonia that had Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS) scores and 210 adults with blepharospasm that had Blepharospasm Severity Scale (BSRS) scores. Results: There were strong correlations between the GDRS and BFM total scores (r = 0.79) and moderate correlations for their sub scores (r > 0.5). Scores for both scales showed positive skew, with an overabundance of low scores. BFM sub-scores were not normally distributed, due to artifacts caused by the provoking factor. Relevant sub-scores of the GDRS and BFM also showed moderate correlations with the TWSTRS (r > 0.5) for cervical dystonia and the BSRS (r > 0.5) for blepharospasm. Conclusions: The BFM is more widely used than the GDRS, but these results suggest the GDRS may be preferable for focal and segmental dystonias. The overabundance of very low scores for both scales highlights challenges associated with discriminating very mild dystonia from other abnormal movements or variants of normal behavior.

Quantitative Analysis of S1PR1 Expression in the Postmortem Multiple Sclerosis Central Nervous System.

Multiple sclerosis (MS) is an immune-mediated disease that is characterized by demyelination and inflammation in the central nervous system (CNS). Previous studies demonstrated that sphingosine-1-phosphate receptor (S1PR) modulators effectively inhibit S1PR1 in immune cell trafficking and reduce entry of pathogenic cells into the CNS. Studies have also implicated a nonimmune, inflammatory role of S1PR1 within the CNS in MS. In this study, we explored the expression of S1PR1 in the development and progression of demyelinating pathology of MS by quantitative assessment of S1PR1 expression using our S1PR1-specific radioligand, [3H]CS1P1, in the postmortem human CNS tissues including cortex, cerebellum, and spinal cord of MS cases and age- and sex-matched healthy cases. Immunohistochemistry with whole slide scanning for S1PR1 and various myelin proteins was also performed. Autoradiographic analysis using [3H]CS1P1 showed that the expression of S1PR1 was statistically significantly elevated in lesions compared to nonlesion regions in the MS cases, as well as normal healthy controls. The uptake of [3H]CS1P1 in the gray matter and nonlesion white matter did not significantly differ between healthy and MS CNS tissues. Saturation autoradiography analysis showed an increased binding affinity (Kd) of [3H]CS1P1 to S1PR1 in both gray matter and white matter of MS brains compared to healthy brains. Our blocking study using NIBR-0213, a S1PR1 antagonist, indicated [3H]CS1P1 is highly specific to S1PR1. Our findings demonstrated the activation of S1PR1 and an increased uptake of [3H]CS1P1 in the lesions of MS CNS. In summary, our quantitative autoradiography analysis using [3H]CS1P1 on human postmortem tissues shows the feasibility of novel imaging strategies for MS by targeting S1PR1.

Assessing the Severity of Cervical Dystonia: Ask the Doctor or Ask the Patient?

Background: Assessing disease severity can be performed using either clinician-rated scales (CRS) or patient-rated outcome (PRO) tools. These two measures frequently demonstrate poor correlations. Objectives: To determine if the correlation between a CRS and PRO for motor features of cervical dystonia (CD) improves by accounting for non-motor features. Methods: Subjects with CD (N = 209) were evaluated using a CRS (Toronto Western Spasmodic Torticollis Rating Scale, TWSTRS) and a PRO (Cervical Dystonia Impact Profile, CDIP-58). Results: Linear regression revealed a weak correlation between the two measures, even when considering only the motor subscales of each. The strength of this relationship improved with a regression model that included non-motor symptoms of pain, depression, and disability. Conclusions: These results argue that the results of motor assessments in a PRO for CD cannot be fully appreciated without simultaneous assessment of non-motor co-morbidities. This conclusion might apply to other disorders, especially those with frequent non-motor co-morbidities.

Interrater reliability of motor severity scales for hemifacial spasm.

To compare the inter-rater reliability (IRR) of five clinical rating scales for video-based assessment of hemifacial spasm (HFS) motor severity. We evaluated the video recordings of 45 HFS participants recruited through the Dystonia Coalition. In Round 1, six clinicians with expertise in HFS assessed the participants' motor severity with five scales used to measure motor severity of HFS: the Jankovic rating scale (JRS), Hemifacial Spasm Grading Scale (HSGS), Samsung Medical Center (SMC) grading system for severity of HFS spasms (Lee's scale), clinical grading of spasm intensity (Chen's scale), and a modified version of the Abnormal Involuntary Movement Scale (Tunc's scale). In Round 2, clinicians rated the same cohort with simplified scale wording after consensus training. For each round, we evaluated the IRR using the intraclass correlation coefficient [ICC (2,1) single-rater, absolute-agreement, 2-way random model]. The scales exhibited IRR that ranged from "poor" to "moderate"; the mean ICCs were 0.41, 0.43, 0.47, 0.43, and 0.65 for the JRS, HSGS, Lee's, Chen's, and Tunc's scales, respectively, for Round 1. In Round 2, the corresponding IRRs increased to 0.63, 0.60, 0.59, 0.53, and 0.71. In both rounds, Tunc's scale exhibited the highest IRR. For clinical assessments of HFS motor severity based on video observations, we recommend using Tunc's scale because of its comparative reliability and because clinicians interpret the scale easily without modifications or the need for consensus training.

Chronic striatal cholinergic interneuron excitation induces clinically-relevant dystonic behavior in mice.

Dystonia is common, debilitating, often medically refractory, and difficult to diagnose. The gold standard for both clinical and mouse model dystonia evaluation is subjective assessment, ideally by expert consensus. However, this subjectivity makes translational quantification of clinically-relevant dystonia metrics across species nearly impossible. Many mouse models of genetic dystonias display abnormal striatal cholinergic interneuron excitation, but few display subjectively dystonic features. Therefore, whether striatal cholinergic interneuron pathology causes dystonia remains unknown. To address these critical limitations, we first demonstrate that objectively quantifiable leg adduction variability correlates with leg dystonia severity in people. We then show that chemogenetic excitation of striatal cholinergic interneurons in mice causes comparable leg adduction variability in mice. This clinically-relevant dystonic behavior in mice does not occur with acute excitation, but rather develops after 14 days of ongoing striatal cholinergic interneuron excitation. This requirement for prolonged excitation recapitulates the clinically observed phenomena of a delay between an inciting brain injury and subsequent dystonia manifestation and demonstrates a causative link between chronic striatal cholinergic interneuron excitation and clinically-relevant dystonic behavior in mice. Therefore, these results support targeting striatal ChIs for dystonia drug development and suggests early treatment in the window following injury but prior to dystonia onset. One Sentence Summary: Chronic excitation of dorsal striatal cholinergic interneuron causes clinically-relevant dystonic phenotypes in mice.

Proteome wide association studies of LRRK2 variants identify novel causal and druggable proteins for Parkinson's disease.

Common and rare variants in the LRRK2 locus are associated with Parkinson's disease (PD) risk, but the downstream effects of these variants on protein levels remain unknown. We performed comprehensive proteogenomic analyses using the largest aptamer-based CSF proteomics study to date (7006 aptamers (6138 unique proteins) in 3107 individuals). The dataset comprised six different and independent cohorts (five using the SomaScan7K (ADNI, DIAN, MAP, Barcelona-1 (Pau), and Fundació ACE (Ruiz)) and the PPMI cohort using the SomaScan5K panel). We identified eleven independent SNPs in the LRRK2 locus associated with the levels of 25 proteins as well as PD risk. Of these, only eleven proteins have been previously associated with PD risk (e.g., GRN or GPNMB). Proteome-wide association study (PWAS) analyses suggested that the levels of ten of those proteins were genetically correlated with PD risk, and seven were validated in the PPMI cohort. Mendelian randomization analyses identified GPNMB, LCT, and CD68 causal for PD and nominate one more (ITGB2). These 25 proteins were enriched for microglia-specific proteins and trafficking pathways (both lysosome and intracellular). This study not only demonstrates that protein phenome-wide association studies (PheWAS) and trans-protein quantitative trail loci (pQTL) analyses are powerful for identifying novel protein interactions in an unbiased manner, but also that LRRK2 is linked with the regulation of PD-associated proteins that are enriched in microglial cells and specific lysosomal pathways.

Human striatal glia differentially contribute to AD- and PD-specific neurodegeneration.

The commonalities and differences in cell-type-specific pathways that lead to Alzheimer disease (AD) and Parkinson disease (PD) remain unknown. Here, we performed a single-nucleus transcriptome comparison of control, AD and PD striata. We describe three astrocyte subpopulations shared across different brain regions and evolutionarily conserved between humans and mice. We reveal common features between AD and PD astrocytes and regional differences that contribute toward amyloid pathology and neurodegeneration. In contrast, we found that transcriptomic changes in microglia are largely unique to each disorder. Our analysis identified a population of activated microglia that shared molecular signatures with murine disease-associated microglia (DAM) as well as disease-associated and regional differences in microglia transcriptomic changes linking microglia to disease-specific amyloid pathology, tauopathy and neuronal death. Finally, we delineate undescribed subpopulations of medium spiny neurons (MSNs) in the striatum and provide neuronal transcriptomic profiles suggesting disease-specific changes and selective neuronal vulnerability.

Discovery of a Promising Fluorine-18 Positron Emission Tomography Radiotracer for Imaging Sphingosine-1-Phosphate Receptor 1 in the Brain.

Sphingosine-1-phosphate receptor 1 (S1PR1) is recognized as a novel therapeutic and diagnostic target in neurological disorders. We recently transferred the S1PR1 radioligand [11C]CS1P1 into clinical investigation for multiple sclerosis. Herein, we reported the design, synthesis and evaluation of novel F-18 S1PR1 radioligands. We combined the structural advantages of our two lead S1PR1 radioligands and synthesized 14 new S1PR1 compounds, then performed F-18 radiochemistry on the most promising compounds. Compound 6h is potent (IC50 = 8.7 nM) and selective for S1PR1. [18F]6h exhibited a high uptake in macaque brain (SUV > 3.0) and favorable brain washout pharmacokinetics in positron emission tomography (PET) study. PET blocking and displacement studies confirmed the specificity of [18F]6h in vivo. Radiometabolite analysis confirmed no radiometabolite of [18F]6h entered into the brain to confound the PET measurement. In summary, [18F]6h is a promising radioligand to image S1PR1 and worth translational clinical investigation for humans with brain disorders.

Advance Care Planning and Health-Related Quality of Life in Huntington Disease: Results from a Multicenter National Study.

Objective: With Huntington disease (HD), a fatal neurodegenerative disease where the prevalence of suicidal thoughts and behavior (STB) remains elevated as compared to other neurological disorders, it is unknown whether STB and health-related quality of life (HRQoL) affect plans for the end of life or more broadly, advance care planning (ACP). Conversely, it is unknown whether ACP would provoke future changes to STB and HRQoL. Therefore, we sought to evaluate whether STB and HRQoL patient-reported outcomes (PROs) contribute to ACP and whether ACP relates to changes in STB and HRQoL at 24 months. Methods: HD-validated clinician- and patient-assessments (i.e., HRQoL PROs) were obtained at baseline enrollment, 12 and 24 months through our multi-center study (HDQLIFE™) throughout the United States among people with premanifest, early-stage, and late-stage manifest HD. We used linear mixed-effects models to determine the relationships between STB and HRQoL at baseline and HDQLIFE End of Life Planning at follow-up. Separate linear mixed-effects models were used to assess the relationship between HDQLIFE End of Life Planning at baseline, and HRQoL and STB at 12 and 24 months. False discovery rate adjustments were used to account for multiple comparisons. Results: At baseline enrollment, STB and HRQoL were not related to HDQLIFE End of Life Planning at 12 or 24 months. Similarly, at baseline, HDQLIFE End of Life Planning demonstrated no association with STB or HRQoL at 12 or 24 months. Interpretation: STB and HRQoL PROs do not significantly affect patient engagement with ACP. Most importantly, engaging in ACP does not cause untoward effects on HRQoL or STB for this rare neurodegenerative disease where the lifetime prevalence of STB approaches 30%.

Imaging in Movement Disorders.

OBJECTIVE: This article reviews commonly used imaging modalities in movement disorders, particularly parkinsonism. The review includes the diagnostic utility, role in differential diagnosis, reflection of pathophysiology, and limitations of neuroimaging in the setting of movement disorders. It also introduces promising new imaging modalities and describes the current status of research. LATEST DEVELOPMENTS: Iron-sensitive MRI sequences and neuromelanin-sensitive MRI can be used to directly assess the integrity of nigral dopaminergic neurons and thus may reflect disease pathology and progression throughout the full range of severity in Parkinson disease (PD). The striatal uptake of presynaptic radiotracers in their terminal axons as currently assessed using clinically approved positron emission tomography (PET) or single-photon emission computed tomography (SPECT) imaging correlates with nigral pathology and disease severity only in early PD. Cholinergic PET, using radiotracers that target the presynaptic vesicular acetylcholine transporter, constitutes a substantial advance and may provide crucial insights into the pathophysiology of clinical symptoms such as dementia, freezing, and falls. ESSENTIAL POINTS: In the absence of valid, direct, objective biomarkers of intracellular misfolded α-synuclein, PD remains a clinical diagnosis. The clinical utility of PET- or SPECT-based striatal measures is currently limited given their lack of specificity and inability to reflect nigral pathology in moderate to severe PD. These scans may be more sensitive than clinical examination to detect nigrostriatal deficiency that occurs in multiple parkinsonian syndromes and may still be recommended for clinical use in the future to identify prodromal PD if and when disease-modifying treatments become available. Multimodal imaging to evaluate underlying nigral pathology and its functional consequences may hold the key to future advances.