Will a Dopamine Transporter (DaT) scan be positive in Multiple System Atrophy (MSA)?

DaT Scan in Multiple System Atrophy (MSA)

Yes, DaT scan will be positive (abnormal) in Multiple System Atrophy, showing decreased dopamine transporter uptake in the striatum due to nigrostriatal dopaminergic degeneration.

Mechanism and Patterns of DaT Scan Abnormalities in MSA

DaT scan (Dopamine Transporter scan) evaluates the integrity of presynaptic dopaminergic neurons by measuring dopamine transporter density in the striatum. In MSA:

• The scan demonstrates reduced radiotracer uptake in the striatum (caudate and putamen), reflecting loss of dopaminergic neurons 1
• Abnormalities typically progress from posterior to anterior (putamen to caudate nuclei) 1
• Studies show significant reductions in mean striatal values in MSA compared to healthy controls 2

Key Diagnostic Considerations

1. Cannot differentiate between Parkinsonian syndromes: While DaT scan is abnormal in MSA, it cannot reliably distinguish MSA from other Parkinsonian syndromes like Parkinson's disease (PD) or Progressive Supranuclear Palsy (PSP) 1

2. Rate of decline: MSA shows a faster rate of decline in specific binding ratio (SBR) compared to Parkinson's disease, which may help in differential diagnosis with longitudinal imaging 3

3. Subtypes of MSA: Both MSA-P (Parkinsonian type) and MSA-C (cerebellar type) show abnormal DaT scans, though MSA-P typically shows lower estimated SBR at symptom onset than MSA-C 3

Complementary Imaging Approaches

Since DaT scan alone cannot differentiate between Parkinsonian syndromes, additional imaging may be helpful:

• Dopamine D2 receptor imaging: D2 receptor binding in the posterior putamen is typically reduced in MSA (below normal range in 71% of cases), while it may be normal or increased in untreated PD 4

• Brain perfusion SPECT: ECD-SPECT imaging shows areas of reduced perfusion in the striatum, brain stem, and cerebellum in MSA compared to PD and controls 5

• MRI: Remains the optimal imaging modality for initial evaluation of Parkinsonian syndromes due to its superior soft-tissue characterization 1

Clinical Implications

The positive DaT scan in MSA reflects the underlying pathophysiology of this synucleinopathy, which involves abnormal cytoplasmic inclusions of ubiquitin and alpha-synuclein in oligodendroglia 6. This distinguishes MSA from essential tremor or drug-induced parkinsonism (which show normal DaT scans) but not from other neurodegenerative Parkinsonian syndromes.

Pitfalls and Limitations

1. DaT scan only shows presynaptic dopaminergic neuron loss and doesn't provide information about postsynaptic receptors 1

2. The pattern of DAT binding alone cannot reliably discriminate between different Parkinsonian syndromes 4

3. Combining DAT imaging with other modalities (D2 receptor imaging, perfusion studies, or MRI) may provide more specific diagnostic information 4, 5

For the most accurate diagnosis of MSA, clinical features (including autonomic dysfunction, cerebellar ataxia, and poor response to levodopa) should be considered alongside imaging findings.