Can Plasma α-Synuclein Help Us to Differentiate Parkinson's Disease from Essential Tremor?

Distinguishing essential tremor from Parkinson's disease: clinical and experimental tools

INTRODUCTION

Essential tremor (ET) and Parkinson's disease (PD) are the most common causes of tremor and the most prevalent movement disorders, with overlapping clinical features that can lead to diagnostic challenges, especially in the early stages.

AREAS COVERED

In the present paper, the authors review the clinical and experimental studies and emphasized the major aspects to differentiate between ET and PD, with particular attention to cardinal phenomenological features of these two conditions. Ancillary and experimental techniques, including neurophysiology, neuroimaging, fluid biomarker evaluation, and innovative methods, are also discussed for their role in differential diagnosis between ET and PD. Special attention is given to investigations and tools applicable in the early stages of the diseases, when the differential diagnosis between the two conditions is more challenging. Furthermore, the authors discuss knowledge gaps and unsolved issues in the field.

EXPERT OPINION

Distinguishing ET and PD is crucial for prognostic purposes and appropriate treatment. Additionally, accurate diagnosis is critical for optimizing clinical and experimental research on pathophysiology and innovative therapies. In a few years, integrated technologies could enable accurate, reliable diagnosis from early disease stages or prodromal stages in at-risk populations, but further research combining different techniques is needed.

Exploring serum α‑synuclein and its autoantibodies in essential tremor: implications for diagnosis and symptom correlations

There is no definite biomarker for confirming the diagnosis of essential tremor (ET) or differentiating it from other diseases, particularly Parkinson's disease. The present study aimed to investigate the serum levels of the α-synuclein protein (α-syn) and its autoantibodies in patients with ET compared with healthy controls and its relation to motor and non-motor symptoms in patients with ET. Serum α-syn and its autoantibodies were measured in 32 patients with ET and 32 age- and sex-matched controls. Both groups were assessed using the non-motor symptoms scale, MoCA, Beck Depression Inventory, Hamilton Anxiety Rating Scale, and the Short Form 36 Health Survey Questionnaire. Tremor was assessed using the Fahn-Tolosa-Marin Tremor Rating Scale. The serum α-syn concentration in patients with ET was significantly lower than that in healthy controls (P<0.001), with a positive predictive value of 0.81 and a negative predictive value of 0.75, while the serum anti-a-syn autoantibody concentration was not significantly different between the two groups. There were no correlations between serum α-syn or its autoantibodies and patients' clinical characteristics. Furthermore, patients with ET had worse cognitive impairment, depression, anxiety, non-motor symptoms and quality of life. The serum α-syn concentration was lower in patients with ET than in controls, with favorable predictive values, suggesting that it could serve as a biomarker for ET diagnosis.

Alpha-synuclein in peripheral body fluid as a biomarker for Parkinson's disease

OBJECTIVE

Whether alpha-synuclein in peripheral body fluids can be used for the diagnosis of Parkinson's disease (PD) remains in controversy. This study evaluates diagnostic potential of alpha-synuclein for PD in various peripheral body fluids using a meta-analysis approach.

METHODS

Studies published before October 2022 were searched in Web of Science and PubMed databases. The results were computed using the STATA 12.0 statistical software.

RESULTS

In plasma, PD patients exhibited elevated alpha-synuclein levels relative to healthy controls (HCs) [standard mean difference (SMD) = 0.78, 95% confidence interval (CI) = 0.42 to 1.15] with a sensitivity of 0.79 (95% CI: 0.64-0.89) and a specificity of 0.95 (95% CI: 0.90-0.98). Higher plasma alpha-synuclein levels were correlated with longer disease durations, higher Unified Parkinson's Disease Rating Scale motor scores, and higher Hoehn and Yahr stages in PD patients. Plasma neural-derived exosomal alpha-synuclein levels (SMD = 1.82, 95% CI = 0.30 to 3.35), ratio of plasma neural-derived exosomal alpha-synuclein to total alpha-synuclein (SMD = 1.26, 95% CI = 0.19 to 2.33), and erythrocytic alpha-synuclein levels were also increased in PD patients (SMD = 6.57, 95% CI = 3.55 to 9.58). In serum, there was no significant difference in alpha-synuclein levels between PD patients and HCs (SMD = 0.54, 95% CI = - 0.27 to 1.34). In saliva, reduced alpha-synuclein levels were observed in PD patients (SMD = - 0.85, 95% CI = - 1.67 to - 0.04).

CONCLUSIONS

Alpha-synuclein levels in plasma, plasma neural-derived exosome, erythrocyte, and saliva may serve as potential biomarkers for the diagnosis of 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.

Detection and assessment of alpha-synuclein in Parkinson disease

PURPOSE

Different studies have reported varying alpha-synuclein values in the cerebrospinal fluid (CSF), serum, and plasma, making determination of the alpha-synuclein cutoff value for Parkinson's disease difficult and rendering identifying the cause of variation essential.

METHOD

We searched PubMed from inception to June 2021 and identified 76 eligible studies. Included studies reported data on total, phosphorylated, and oligomeric alpha-synuclein in the CSF, serum, or plasma from individuals with Parkinson's disease and healthy controls. The mean or median alpha-synuclein values from the included studies were summarized and categorized through laboratory assays to visualize potential trends.

RESULTS

The enzyme-linked immunosorbent assay (ELISA) is the most common assay used to determine alpha-synuclein concentrations. Less common assays include Luminex, single molecule arrays, electrochemiluminescence, and immunomagnetic reduction (IMR). IMR is a single-antibody and wash-free immunoassay designed for determining the extremely low concentration of bio-molecules. For patients with Parkinson's disease, the median or mean testing values ranged from 60.9 to 55,000 pg/mL in the CSF, 0.446 to 1,777,100 pg/mL in plasma, and 0.0292 to 38,200,000 pg/mL in serum. The antibody selection was diverse between studies. The tendency of distribution was more centralized among studies that used the same kit. Studies adopting specific antibodies or in-house assays contribute to the extreme values. Only a few studies on phosphorylated and oligomeric alpha-synuclein were included.

CONCLUSION

The type of assay and antibody selection in the laboratory played major roles in the alpha-synuclein variation. Studies that used the same assay and kit yielded relatively unanimous results. Furthermore, IMR may be a promising assay for plasma and serum alpha-synuclein quantification. A consensus on sample preparation and testing protocol unification is warranted in the future.

Correction: Can Plasma α-Synuclein Help Us to Differentiate Parkinson's Disease from Essential Tremor?

[This corrects the article DOI: 10.5334/tohm.600.].