A 62-year-old man with a 5-year history of Parkinson disease presents with progressive motor symptoms despite optimized dopaminergic therapy. He reports tremor at rest, rigidity, and bradykinesia. His neurologist suspects motor complications and wants to assess the integrity of the dopaminergic system and rule out atypical parkinsonism. Which investigation is most appropriate to confirm dopaminergic denervation and differentiate idiopathic Parkinson disease from atypical parkinsonian syndromes?

Explanation

## Investigation of Choice: 123I-FP-CIT SPECT **Key Point:** 123I-FP-CIT SPECT (also called DaTscan) is the gold standard functional neuroimaging test for confirming dopaminergic denervation in Parkinson disease. It directly visualizes dopamine transporter (DAT) loss in the striatum, which is pathognomonic for nigrostriatal degeneration. **High-Yield:** DaTscan shows a characteristic **asymmetric reduction in striatal uptake** in idiopathic Parkinson disease, with relative sparing of the caudate nucleus ("comma sign"). This pattern helps differentiate PD from: - Essential tremor (normal DAT uptake) - Atypical parkinsonian syndromes (different patterns: symmetric caudate involvement in MSA, preserved putaminal uptake in PSP) - Drug-induced parkinsonism (normal DAT uptake) **Clinical Pearl:** DaTscan is particularly useful when clinical diagnosis is uncertain, when atypical features are present, or when the patient has not yet responded adequately to dopaminergic therapy. It provides objective evidence of nigrostriatal pathology. **Mnemonic:** DAT-scan = **D**opamine **A**ctivity **T**ransporter scan — measures the integrity of dopaminergic neurons. ## Why DaTscan is Superior to Other Modalities | Investigation | Target | Utility in PD | Limitation | | --- | --- | --- | --- | | 123I-FP-CIT SPECT (DaTscan) | Dopamine transporter | Gold standard for DAT loss; differentiates PD from mimics | Requires specialized nuclear medicine facility | | 123I-IBZM SPECT | D2 dopamine receptor | Shows postsynaptic changes; less specific for PD | Cannot distinguish PD from atypical syndromes reliably | | 18F-FDG PET | Glucose metabolism | Useful in atypical parkinsonian syndromes (shows hypometabolism patterns) | Not first-line for confirming dopaminergic denervation in PD | | MRI with SWI | Iron deposition, structural anatomy | Detects red flag features (midbrain atrophy, putaminal rim sign in MSA) | Does not assess functional dopaminergic integrity | **Warning:** Do not confuse DaTscan with 123I-IBZM, which images postsynaptic D2 receptors. IBZM can be abnormal in both PD and atypical syndromes and is less specific for confirming nigrostriatal denervation. [cite:Harrison 21e Ch 429]