What are the diagnostic criteria for Parkinson's disease?

Diagnostic Criteria for Parkinson's Disease

The diagnosis of Parkinson's disease is primarily clinical, based on the presence of bradykinesia plus either rest tremor, rigidity, or both, with supportive features including good response to dopaminergic therapy and absence of atypical features. 1

Core Clinical Criteria

Cardinal Motor Features:

• Bradykinesia (slowness of movement with progressive reduction in speed and amplitude during repetitive actions) - REQUIRED
• Rest tremor (4-6 Hz tremor when the affected limb is at rest)
• Rigidity (increased resistance to passive movement)
• Postural instability (loss of postural reflexes, typically occurs later in disease)

At least bradykinesia plus either rest tremor or rigidity must be present for diagnosis 2, 1, 3.

Supportive Features:

• Clear and dramatic response to dopaminergic therapy
• Presence of levodopa-induced dyskinesias
• Asymmetric onset of symptoms
• Progressive course
• Disease duration >5 years

Exclusionary Features (Red Flags)

The following features suggest alternative diagnoses:

• Early postural instability (within first 3 years)
• Early dementia or hallucinations
• Prominent autonomic dysfunction early in disease
• Vertical supranuclear gaze palsy
• Cerebellar signs (ataxia)
• Poor or no response to high-dose levodopa
• Rapid disease progression
• Symmetric onset of symptoms
• Presence of structural lesions on brain imaging 2, 4

Clinical Subtypes

Parkinson's disease presents with different phenotypes that affect prognosis:

• Mild motor-predominant (49-53% of patients): Mild symptoms, good response to medication, slower progression
• Intermediate subtype (25-42% of patients): Moderate symptoms and progression
• Diffuse malignant subtype (9-16% of patients): Prominent early motor and non-motor symptoms, poor medication response, faster progression 1

Non-Motor Features

These features may precede motor symptoms by years and should be assessed:

• Prodromal symptoms:
  • REM sleep behavior disorder
  • Hyposmia (reduced sense of smell)
  • Constipation
  • Depression
  • Anxiety
• Cognitive/neuropsychiatric symptoms:
  • Cognitive decline
  • Depression
  • Anxiety
  • Apathy
  • Hallucinations (typically later)
• Autonomic dysfunction:
  • Orthostatic hypotension
  • Urinary dysfunction
  • Sexual dysfunction
  • Constipation 1, 5, 6

Diagnostic Approach

1. Clinical assessment of cardinal motor features
2. Response to dopaminergic therapy - significant improvement supports PD diagnosis
3. Neuroimaging - primarily to exclude other causes:
   • MRI brain (without contrast is preferred) to rule out structural lesions
   • Dopamine transporter SPECT (DaTscan) may be useful when diagnosis is uncertain 2

Important Diagnostic Pitfalls

• Misdiagnosis of essential tremor as PD (essential tremor is typically postural/action tremor rather than rest tremor)
• Missing atypical parkinsonism (MSA, PSP, CBD) which have poorer prognosis and different treatment approaches
• Overlooking drug-induced parkinsonism from antipsychotics, antiemetics, or calcium channel blockers
• Failing to recognize vascular parkinsonism which has more lower body involvement and step-wise progression
• Not considering Lewy body dementia when cognitive symptoms appear within one year of motor symptoms 2, 4

Diagnostic Accuracy

Clinical diagnosis by movement disorder specialists has approximately 80-90% accuracy when compared to pathological confirmation. Diagnostic accuracy improves with:

• Longer disease duration
• Response to dopaminergic therapy
• Development of motor complications
• Presence of classic asymmetric onset 1, 3

When diagnostic uncertainty exists, particularly in early disease stages, follow-up examinations over time and assessment of response to dopaminergic therapy are more valuable than extensive testing.