What are the steps to diagnose Parkinson's disease in a patient presenting with symptoms such as resting tremor, rigidity, bradykinesia, and postural instability?

How to Diagnose Parkinson's Disease

Parkinson's disease is diagnosed clinically when you identify bradykinesia (slowness of movement) plus at least one of the following: resting tremor or rigidity—no laboratory test or imaging can make the diagnosis alone. 1, 2

Essential Clinical Criteria

The diagnosis requires bradykinesia as the mandatory feature, accompanied by at least one additional cardinal sign 1, 2, 3:

• Resting tremor: 4-6 Hz "pill-rolling" tremor present when the limb is completely supported and relaxed, typically starting asymmetrically 2, 4
• Rigidity: Constant resistance throughout passive range of motion, often with "cogwheel" quality when combined with tremor 2, 4
• Postural instability: Appears later in disease progression, not typically present at initial diagnosis 2, 5

Critical Examination Technique for Rigidity

To properly assess rigidity in your patient 1:

• Instruct the patient to relax completely while you passively move their limbs through full range of motion 1
• Test both upper and lower extremities at varying speeds, comparing sides for asymmetry 1
• Use an activation maneuver: Have the patient open and close the opposite hand while you test for rigidity—this brings out subtle rigidity that might otherwise be missed 1
• Note "lead-pipe" rigidity (constant resistance) or "cogwheel" phenomenon (ratchet-like resistance when combined with tremor) 1

Common pitfall: Failure to have the patient completely relax leads to false positives from voluntary muscle contraction 1

Diagnostic Imaging Algorithm

Order MRI brain without contrast first as your initial imaging study 1, 2:

• MRI is optimal for ruling out structural causes, focal lesions, vascular disease, or white matter changes 1, 2
• MRI is often normal in early Parkinson's disease but essential to exclude alternative diagnoses 1

Order I-123 ioflupane SPECT/CT (DaTscan) when 1, 2:

• Clinical presentation is atypical or uncertain 2
• You need to differentiate Parkinson's disease from essential tremor or drug-induced tremor 1, 2
• A normal DaTscan essentially excludes Parkinsonian syndromes 1, 2
• Abnormal scan shows decreased radiotracer uptake in the striatum, usually beginning in the putamen and progressing to caudate 1

Do not order: Amyloid PET/CT or tau PET/CT—there is no supporting evidence for their use in Parkinsonian syndrome evaluation 1

Red Flags Suggesting Alternative Diagnoses

Watch for these features that indicate not idiopathic Parkinson's disease 6, 1, 4:

• Vertical gaze palsy (especially downward): Suggests Progressive Supranuclear Palsy 6, 1
• Early severe autonomic dysfunction, cerebellar signs, or pyramidal signs: Suggests Multiple System Atrophy 1
• Asymmetric rigidity with alien hand phenomenon: Suggests Corticobasal Syndrome 6, 1
• Ataxia: Excludes typical Parkinson's disease 1
• Poor or no response to levodopa: Consider atypical Parkinsonian syndromes 3, 7
• Early severe dementia, hallucinations, or postural instability: Suggests alternative diagnosis 7

Timing and Pathophysiology

By the time motor symptoms appear, approximately 40-50% of dopaminergic neurons in the substantia nigra have already been lost, typically about 5 years after initial neurodegeneration begins 1, 4

Specialist Referral

Refer to a neurologist or movement disorder specialist for diagnostic confirmation 1:

• Correctly diagnosing a Parkinsonian syndrome on clinical features alone is challenging 1
• Specialists are needed to distinguish Parkinson's disease from atypical Parkinsonian syndromes (PSP, MSA, CBD) that have different prognoses and treatment responses 1
• Proper interpretation of DaTscan imaging requires specialist expertise 1

Standardized Assessment Tools

Use the Movement Disorder Society-Unified Parkinson's Disease Rating Scale (MDS-UPDRS) for standardized assessment of disease severity, including activities of daily living, motor examination, and complications 1, 2

Special Consideration

In patients with history of anti-BCMA CAR T-cell therapy, consider Movement and Neurocognitive Treatment-Emergent Adverse Events (MNTs), which can mimic Parkinson's disease but are levodopa unresponsive 2