A 62-year-old man presents with a 3-year history of progressive tremor at rest, rigidity, and bradykinesia. Clinical examination confirms a resting tremor of 4–5 Hz, cogwheel rigidity, and slow gait. The diagnosis of Parkinson disease is suspected. Which investigation is most appropriate to confirm the diagnosis and rule out secondary causes?

Explanation

## Diagnosis of Parkinson Disease: Imaging Approach **Key Point:** Clinical diagnosis of Parkinson disease is based on cardinal motor features (resting tremor, rigidity, bradykinesia, postural instability) and response to levodopa. However, when diagnosis is uncertain or atypical features are present, functional neuroimaging is indicated to confirm dopaminergic dysfunction. ### Investigation Comparison for PD Diagnosis | Investigation | Principle | Sensitivity | Clinical Use | Limitation | |---|---|---|---|---| | **SPECT with 123I-ioflupane** | Presynaptic dopamine transporter imaging | 90–95% | Gold standard for confirming nigrostriatal dopaminergic loss | Expensive, not widely available in India | | **PET with 18F-fluorodopa** | Presynaptic dopamine synthesis | 95–98% | Research standard, highest specificity | Cyclotron required, very limited availability | | **MRI brain (SWI)** | Structural imaging | N/A | Rules out secondary causes (atrophy, infarcts, tumors) | Does NOT confirm dopaminergic dysfunction | | **Transcranial ultrasound** | Echogenicity of substantia nigra | 85–90% | Operator-dependent, non-invasive screening | Poor specificity, not diagnostic | **High-Yield:** SPECT with 123I-ioflupane (DaTscan) is the **most appropriate confirmatory test** for Parkinson disease in clinical practice. It detects presynaptic dopamine transporter loss, which is the hallmark of PD and distinguishes it from essential tremor, drug-induced parkinsonism, and Parkinson-plus syndromes. ### When to Use Each Investigation 1. **SPECT 123I-ioflupane** — Diagnostic uncertainty, atypical presentation, need to exclude mimics (essential tremor, dystonia). 2. **PET 18F-fluorodopa** — Research, when SPECT unavailable, suspected nigrostriatal denervation in early/atypical PD. 3. **MRI brain** — First-line structural imaging to exclude secondary causes (stroke, tumor, hydrocephalus, atrophy patterns suggestive of atypical parkinsonian syndromes). 4. **Transcranial ultrasound** — Screening tool in research; operator-dependent, not diagnostic. **Clinical Pearl:** In India, where SPECT availability may be limited, MRI is often performed first to exclude structural lesions, followed by SPECT or PET if diagnosis remains uncertain. However, the question asks for the **most appropriate confirmatory test**, which is SPECT DaTscan. **Warning:** Do not confuse MRI (structural) with functional imaging (SPECT/PET). MRI rules out mimics but does NOT confirm dopaminergic loss. [cite:Harrison 21e Ch 428]