Nigral neuropathology of Parkinson's motor subtypes coincide with circuitopathies: a scoping review

Mitochondrial complex I deficiency stratifies idiopathic Parkinson's disease

Idiopathic Parkinson's disease (iPD) is believed to have a heterogeneous pathophysiology, but molecular disease subtypes have not been identified. Here, we show that iPD can be stratified according to the severity of neuronal respiratory complex I (CI) deficiency, and identify two emerging disease subtypes with distinct molecular and clinical profiles. The CI deficient (CI-PD) subtype accounts for approximately a fourth of all cases, and is characterized by anatomically widespread neuronal CI deficiency, a distinct cell type-specific gene expression profile, increased load of neuronal mtDNA deletions, and a predilection for non-tremor dominant motor phenotypes. In contrast, the non-CI deficient (nCI-PD) subtype exhibits no evidence of mitochondrial impairment outside the dopaminergic substantia nigra and has a predilection for a tremor dominant phenotype. These findings constitute a step towards resolving the biological heterogeneity of iPD with implications for both mechanistic understanding and treatment strategies.

18F-FP-DTBZ PET/CT detectable associations between monoaminergic depletion in the putamen with rigidity and the pallidus with tremor in Parkinson's disease

INTRODUCTION

The motor subtypes of Parkinson's disease (PD) are widely accepted and implemented. However, the motor subtypes have been thought to represent different stages of PD recently because some patients experience tremor-dominant (TD) conversion to the non-tremor-dominant subtype, such as postural instability-gait difficulty (PIGD). In this study, we explore the monoaminergic denervation features of the striatal and extra-striatal areas in patients with different subtypes of PD with 18F-9-fluoropropyl-(+)-dihydrotetrabenazine (18F-FP-DTBZ) PET/CT.

METHODS

Sixty-five patients diagnosed with PD were included and classified as TD (n = 25) and PIGD (n = 40). We evaluated the difference of monoaminergic features of each subregion of brain between motor subtypes of PD, as well as associations between these features and Parkinsonian motor symptoms.

RESULTS

The striatal standardized uptake value ratios (SUVR) showed that dopaminergic disruption of patients with PIGD was more symmetrical in the posterior ventral putamen (p < 0.001) and more severe in the ipsilateral posterior dorsal putamen (p < 0.001 corrected) compared with that of patients with TD. The severity of PIGD scores was associated with striatal dopaminergic depletion, while tremor was associated with monoaminergic changes in extra-striatal areas, including pallidus, thalamus, and raphe nuclie.

CONCLUSION

These results indicate that patients with different motor subtypes may have different underlying mechanisms of PD pathogenesis. Therefore, accurate diagnosis of PD subtypes can aid prognosis evaluation and treatment decision-making.

Diagnostic utility of 7T neuromelanin imaging of the substantia nigra in Parkinson's disease

Parkinson's disease (PD) is a prevalent neurodegenerative disorder that presents a diagnostic challenge due to symptom overlap with other disorders. Neuromelanin (NM) imaging is a promising biomarker for PD, but adoption has been limited, in part due to subpar performance at standard MRI field strengths. We aimed to evaluate the diagnostic utility of ultra-high field 7T NM-sensitive imaging in the diagnosis of PD versus controls and essential tremor (ET), as well as NM differences among PD subtypes. A retrospective case-control study was conducted including PD patients, ET patients, and controls. 7T NM-sensitive 3D-GRE was acquired, and substantia nigra pars compacta (SNpc) volumes, contrast ratios, and asymmetry indices were calculated. Statistical analyses, including general linear models and ROC curves, were employed. Twenty-one PD patients, 13 ET patients, and 18 controls were assessed. PD patients exhibited significantly lower SNpc volumes compared to non-PD subjects. SNpc total volume showed 100% sensitivity and 96.8% specificity (AUC = 0.998) for differentiating PD from non-PD and 100% sensitivity and 95.2% specificity (AUC = 0.996) in differentiating PD from ET. Contrast ratio was not significantly different between PD and non-PD groups (p = 0.07). There was also significantly higher asymmetry index in SNpc volume in PD compared to non-PD cohorts (p < 0.001). NM signal loss in PD predominantly involved the inferior, posterior, and lateral aspects of SNpc. Akinetic-rigid subtype showed more significant NM signal loss compared to tremor dominant subtype (p < 0.001). 7T NM imaging demonstrates potential as a diagnostic tool for PD, including potential distinction between subtypes, allowing improved understanding of disease progression and subtype-related characteristics.

Nigrosome and Neuromelanin Imaging as Tools to Differentiate Parkinson's Disease and Parkinsonism

Parkinson's disease (PD) lacks a definitive diagnosis due to a lack of pathological validation of patients at antemortem. The risk of misdiagnosis is high in the early stages of PD, often eluded by atypical parkinsonian symptoms. Neuroimaging and laboratory biomarkers are being sought to aid in the clinical diagnosis of PD. Nigrosome imaging and neuromelanin (NM)-sensitive magnetic resonance imaging (MRI) are the new emerging tools, both technically simple plus cost-effective for studying nigral pathology, and have shown potential for authenticating the clinical diagnosis of PD. Visual assessment of the nigrosome-1 appearance, at 3 or 7 Tesla, yields excellent diagnostic accuracy for differentiating idiopathic PD from healthy controls. Moreover, midbrain atrophy and putaminal hypointensity in nigrosome-1 imaging are valid pointers in distinguishing PD from allied parkinsonian disorders. The majority of studies employed T2 and susceptibility-weighted imaging MRI sequences to visualize nigrosome abnormalities, whereas T1-weighted fast-spin echo sequences were used for NM imaging. The diagnostic performance of NM-sensitive MRI in discriminating PD from normal HC can be improved further. Longitudinal studies with adequate sampling of varied uncertain PD cases should be designed to accurately evaluate the sensitivity and diagnostic potential of nigrosome and NM imaging techniques. Equal weightage is to be given to uniformity and standardization of protocols, data analysis, and interpretation of results. There is tremendous scope for identifying disease-specific structural changes in varied forms of parkinsonism with these low-cost imaging tools. Nigrosome-1 and midbrain NM imaging may not only provide an accurate diagnosis of PD but could mature into tools for personally tailored treatment and prognosis.