What are the diagnostic criteria for Parkinson's disease?

How to Diagnose Parkinson's Disease

Parkinson's disease is diagnosed clinically by identifying bradykinesia (slowness of movement) plus at least one of the following: resting tremor or rigidity, with I-123 ioflupane SPECT/CT (DaTscan) reserved for cases where the clinical diagnosis is uncertain. 1, 2

Essential Diagnostic Criteria

The diagnosis requires bradykinesia as the mandatory feature, which must be present alongside at least one additional cardinal motor sign 1, 3:

• Resting tremor (typically 4-6 Hz, present at rest and diminishing with action) 3, 4
• Rigidity (constant resistance throughout passive movement, often with cogwheel phenomenon) 1, 4
• Postural instability (though this typically appears later in disease progression and is not required for initial diagnosis) 5, 4

Clinical Examination Technique

Assessing Bradykinesia

Evaluate slowness across multiple domains 1:

• Fine motor tasks (buttoning clothes, writing)
• Gross motor activities (walking, turning)
• Facial expressions (hypomimia)
• Speech (hypophonia)

Assessing Rigidity

Use proper technique to avoid false positives 1:

• Have the patient relax completely while you passively move their limbs
• Test both upper and lower extremities through full range of motion
• Compare sides for asymmetry (PD typically begins asymmetrically)
• Use activation maneuvers (have patient open/close the opposite hand) to enhance detection of subtle rigidity
• Note lead-pipe rigidity (constant resistance) or cogwheel phenomenon (ratchet-like resistance when combined with tremor)

When to Use Diagnostic Imaging

I-123 ioflupane SPECT/CT (DaTscan) is the definitive imaging test when clinical diagnosis is uncertain, particularly to differentiate PD from essential tremor or drug-induced parkinsonism 6, 1, 2:

• Abnormal DaTscan: Shows decreased radiotracer uptake in the striatum (typically putamen before caudate), confirming parkinsonian syndrome 6, 2
• Normal DaTscan: Essentially excludes parkinsonian syndromes and supports alternative diagnoses like essential tremor 6, 1, 2

MRI brain without contrast is the optimal structural imaging modality when you need to exclude other causes, though it is often normal in early PD 6, 1:

• Helps rule out structural lesions, vascular disease, or focal atrophy
• Advanced sequences (susceptibility-weighted imaging) may show loss of the "swallow tail" sign in the substantia nigra, though sensitivity/specificity remain unclear 6

Red Flags Suggesting Alternative Diagnoses

Watch for features that indicate atypical parkinsonian syndromes rather than idiopathic PD 1, 4:

Progressive Supranuclear Palsy (PSP):

• Vertical gaze palsy, especially downward 1
• Early falls and axial rigidity 1

Multiple System Atrophy (MSA):

• Early severe autonomic dysfunction (orthostatic hypotension, urinary incontinence) 1
• Cerebellar signs (ataxia) 1
• Pyramidal signs 1

Corticobasal Degeneration (CBD):

• Asymmetric rigidity with alien hand phenomenon 1
• Cortical sensory loss 1

Drug-Induced Parkinsonism:

• Recent use of antipsychotics, antiemetics, or other dopamine-blocking agents 1, 2
• Typically more symmetric than PD 4

Critical Diagnostic Pitfalls to Avoid

• Poor response to levodopa strongly suggests an alternative diagnosis to idiopathic PD 4
• Early dementia, hallucinations, or severe autonomic dysfunction within the first year suggests atypical parkinsonism 4
• Symmetric presentation at onset is atypical for PD (which usually begins asymmetrically) 4
• Failure to obtain complete medication history may miss drug-induced parkinsonism 1, 2
• Not using activation maneuvers during rigidity assessment may cause you to miss subtle rigidity 1
• Confusing spasticity (velocity-dependent resistance) with rigidity (constant resistance) 1

Specialist Referral

General neurologists or movement disorder specialists should confirm the diagnosis because correctly diagnosing parkinsonian syndromes on clinical features alone is challenging 1:

• Atypical parkinsonian syndromes (PSP, MSA, CBD) have different prognoses and treatment responses
• DaTscan interpretation requires clinical context and expertise
• Early accurate diagnosis impacts treatment planning and prognosis

Prodromal Features

While not required for diagnosis, recognizing prodromal symptoms may facilitate earlier detection 3, 7, 8:

• REM sleep behavior disorder (acting out dreams)
• Hyposmia (reduced sense of smell)
• Constipation
• Depression or anxiety

These symptoms typically appear years before motor symptoms, when approximately 40-50% of dopaminergic neurons have already been lost 1.