Diagnostic Reclassification From Parkinson's Disease to Multiple System Atrophy Based on Longitudinal Clinical and Imaging Findings: A Case Report.

It is often challenging to differentiate Parkinson's disease (PD) from multiple system atrophy (MSA) - particularly, the parkinsonian subtype (MSA-P) - in the early disease stages because of overlapping clinical features. Here, we report the case of a 72-year-old man who was initially diagnosed with PD but whose diagnosis was later changed to MSA-P following the emergence of autonomic dysfunction and characteristic magnetic resonance imaging (MRI) findings. He presented with tremors, rigidity, bradykinesia, and postural instability, and dopamine transporter imaging demonstrated bilateral presynaptic dopaminergic dysfunction. Cardiac 123I-metaiodobenzylguanidine (MIBG) scintigraphy revealed reduced uptake on initial evaluation, which was considered supportive of PD. However, the response to levodopa was limited. During follow-up, several atypical features became apparent, including Pisa syndrome, early postural instability, orthostatic hypotension, and neurogenic bladder. Follow-up MRI revealed a hyperintense rim along the lateral margins of the bilateral putamina, suggesting MSA-P; these findings provided a key objective basis for the diagnostic reclassification. Repeat MIBG scintigraphy performed after selegiline discontinuation revealed an improvement in the heart-to-mediastinum ratio, suggesting a possible drug-related effect on tracer uptake. This case illustrates the limitations of single-time-point diagnostic assessments and highlights the value of longitudinal, multimodal evaluation. It also suggests that MIBG abnormalities may be reversible, underscoring the need for cautious interpretation of MIBG findings in the differential diagnosis of PD and MSA.