What is the diagnostic approach for an older adult, typically over 60 years old, presenting with symptoms such as tremors, rigidity, bradykinesia, and postural instability, suspected of having Parkinson's disease (PD)?

Diagnosing Parkinson's Disease

Core Diagnostic Approach

Parkinson's disease diagnosis requires the presence of bradykinesia (slowness of movement) plus at least one additional cardinal motor sign: resting tremor or rigidity. 1, 2, 3 This is a clinical diagnosis that does not require imaging or laboratory confirmation in typical presentations, though MRI and DaTscan serve important supportive roles when the presentation is unclear. 2

Essential Clinical Features to Identify

Cardinal Motor Signs (Must Document)

Bradykinesia is mandatory - without it, you cannot diagnose Parkinson's disease. 1, 2, 3 Look for:

• Slowness initiating voluntary movements 4
• Decreased amplitude of repetitive movements 4
• Difficulty with fine motor tasks like buttoning clothes or writing 2
• Reduced facial expression (hypomimia) 4
• Decreased blink rate 4

Plus at least ONE of the following:

• Resting tremor: 4-6 Hz "pill-rolling" tremor present when the limb is completely supported and relaxed, typically asymmetric at onset 3, 4
• Rigidity: Constant resistance throughout passive range of motion (lead-pipe) or ratchet-like resistance when combined with tremor (cogwheel) 2, 3

Note: Postural instability typically appears later in disease progression and is no longer considered an early diagnostic criterion. 2, 3, 5

High-Value Historical Features

Ask specifically about these symptoms, which have strong diagnostic value:

• Micrographia (progressively smaller handwriting): positive likelihood ratio 2.8-5.9 6
• Shuffling gait: positive likelihood ratio 3.3-15 6
• Difficulty turning in bed: positive likelihood ratio 13 6
• Trouble opening jars: positive likelihood ratio 6.1 6
• Difficulty rising from a chair: positive likelihood ratio 1.9-5.2 6
• Combined history of rigidity AND bradykinesia: positive likelihood ratio 4.5 6

Physical Examination Maneuvers

Glabellar tap test: Tap repeatedly between the eyebrows; failure to habituate (continued blinking) has a positive likelihood ratio of 4.5 and negative likelihood ratio of 0.13. 6

Heel-to-toe walking: Difficulty performing this has a positive likelihood ratio of 2.9. 6

Rigidity assessment: Passively move limbs while patient relaxes completely, using activation maneuvers (having patient open/close opposite hand) to enhance detection of subtle rigidity. 2

Red Flags That Suggest Alternative Diagnoses

Do NOT diagnose idiopathic Parkinson's disease if any of these are present early in the disease course: 2, 4

• Vertical gaze palsy (especially downward) → suggests Progressive Supranuclear Palsy 2
• Early severe autonomic dysfunction (orthostatic hypotension, urinary incontinence) → suggests Multiple System Atrophy 1, 2
• Cerebellar signs (ataxia) → suggests Multiple System Atrophy 1, 2
• Asymmetric rigidity with alien hand phenomenon → suggests Corticobasal Syndrome 7, 2
• Early dementia or hallucinations → consider Dementia with Lewy Bodies 1
• Poor or absent response to levodopa → suggests atypical parkinsonism 4
• Symmetric presentation at onset is less typical for PD 4

Diagnostic Imaging Algorithm

Step 1: MRI Brain Without Contrast

Obtain MRI brain without contrast as the initial imaging study to rule out structural causes, vascular disease, hydrocephalus, or focal lesions. 1, 2, 3 The MRI is often normal in early Parkinson's disease, but this step is essential to exclude alternative diagnoses before proceeding. 2

Step 2: DaTscan (When Needed)

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

• Clinical presentation is atypical or uncertain 2, 3
• You need to differentiate Parkinson's disease from essential tremor 1, 2
• You need to differentiate from drug-induced parkinsonism 1, 2

Key interpretation points:

• Abnormal DaTscan shows decreased radiotracer uptake in the striatum (usually putamen first, then caudate) 2
• A normal DaTscan essentially excludes parkinsonian syndromes 1, 2
• DaTscan cannot distinguish between different types of parkinsonism (PD vs. PSP vs. MSA vs. CBD) - all show abnormal uptake 2

Imaging to Avoid

Do NOT order:

• Amyloid PET/CT - no supporting evidence for parkinsonism evaluation 2
• Tau PET/CT for initial workup 2
• CT scan as primary modality - poor soft tissue contrast 2

Specialist Referral

Refer to a neurologist or movement disorder specialist for diagnostic confirmation because correctly diagnosing parkinsonian syndromes on clinical features alone is challenging, and misdiagnosis rates are significant even among experienced clinicians. 1, 2 Autopsy studies demonstrate that clinical diagnosis is not confirmed in a substantial proportion of patients. 5

Common pitfalls without specialist involvement:

• Missing atypical parkinsonian syndromes (PSP, MSA, CBD) that have different prognoses and treatment responses 2
• Misinterpreting imaging studies without proper clinical context 2
• Failing to recognize drug-induced parkinsonism 2

Standardized Assessment Tools

Use the Movement Disorder Society-Unified Parkinson's Disease Rating Scale (MDS-UPDRS) for standardized assessment of disease severity, covering activities of daily living, motor examination, and complications of therapy. 2, 3 This provides objective documentation and facilitates monitoring over time.

Critical Diagnostic Pitfalls to Avoid

• Confusing spasticity with rigidity: Spasticity is velocity-dependent (increases with faster stretching), while rigidity shows constant resistance throughout movement 2
• Diagnosing PD based on tremor alone: Tremor without bradykinesia is NOT Parkinson's disease 1, 2, 3
• Missing drug-induced parkinsonism: Always review medication list for antipsychotics, antiemetics (metoclopramide, prochlorperazine), and other dopamine-blocking agents 2, 4
• Failing to assess for non-motor symptoms: These often precede motor symptoms and include REM sleep behavior disorder, constipation, anosmia, depression, and anxiety 8, 4
• Not recognizing that symptoms typically begin asymmetrically: Symmetric onset should raise suspicion for alternative diagnoses 4

Special Populations

In patients over 85 years: Multiple etiologies and mixed pathologies are common, making diagnosis more complex. 1

In patients with history of CAR T-cell therapy (anti-BCMA): Consider Movement and Neurocognitive Treatment-Emergent Adverse Events, which can mimic Parkinson's disease but are levodopa unresponsive. 3