Clinicopathologic mismatch in young-onset parkinsonism: Multiple system atrophy masquerading as a neurotransmitter synthesis disorder.

BACKGROUND: Multiple system atrophy (MSA) is a progressive adult-onset synucleinopathy that remains difficult to diagnose clinically, particularly in younger patients and during early disease stages. Brainstem nuclei degeneration resulting in reduced monoamines in cerebrospinal fluid (CSF) analysis may further complicate diagnostic interpretation. OBJECTIVES: To describe a clinicopathologic case of early-onset MSA found to have decreased CSF monoamines suggestive of a primary neurotransmitter synthesis disorder. MATERIALS AND METHODS: Clinical records, neuroimaging findings, CSF neurotransmitter analysis, and postmortem neuropathologic examination were reviewed. RESULTS: A 41-year-old woman developed rapidly progressive parkinsonism, dystonia, dysphagia, and speech impairment. Broad CSF analysis was significant for reduced homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA), and tetrahydrobiopterin, raising concern for a monoamine synthesis defect and prompting treatment for sepiapterin reductase deficiency (SRD). Despite directed therapy, neurologic decline continued. Postmortem examination demonstrated neuropathologic findings consistent with MSA, parkinsonian subtype (MSA-P). Early cortical and allocortical amyloid-β deposition corresponding to Thal phase 2 was also identified. No additional proteinopathies were present. CONCLUSION: Degeneration of dopaminergic and serotonergic nuclei in MSA can reduce CSF monoamine metabolite levels and mimic disorders of neurotransmitter biosynthesis despite correlation with tetrahydrobiopterin levels. This case highlights a potential diagnostic pitfall when interpreting CSF neurotransmitter studies in adults with parkinsonism.