Ultrasonographically measured atrophy of vagus nerve in Parkinson's Disease: clinical and pathogenetic insights plus systematic review and meta-analysis.

Introduction: According to the current Parkinson's Disease (PD) pathogenesis hypotheses, the vagus nerve (VN) is essential for disease development. It has been identified as a main entry point for misfolded α-synuclein to the central nervous system, and surgical vagotomy appears to limit disease progress both in animal models and in humans. A recent approach tried to assess VN size in PD patients via neck ultrasonography, but the clinical value of this method is yet to be established.

State of the art: A systematic search of the MEDLINE, Scopus, and Web of Science databases was conducted, and 12 case- -control studies were included. Meta-analysis revealed a modest reduction in VN size in PD (effect size - 0.79 SD (95%CI [-1.34, -0.25] p = 0.004)). The atrophy was more pronounced on the right side, and the nerve was smaller in females. In PD patients, VN reduction correlated with cardiac parasympathetic function decline and with advances in motor ratings. The discrimination potential for PD diagnosis, and any association with other non-motor domains, remains unclear.

Clinical implications: VN atrophy in PD could be detected by ultrasound imaging. However, the clinical significance of this phenomenon has yet to be clarified. Size reduction is not readily apparent and is individually variable. However, it may be considered a promising means to improve early PD diagnosis and the recognition of autonomic dysfunction.

Future directions: With more extensive research, VN sonography could provide useful evidence regarding disease origins. Imaging should be performed together with a profound clinical assessment and biomarker testing to establish the role to be played by this method in future practice.