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 identification of cardinal motor signs including bradykinesia, rigidity, resting tremor, and postural instability, with bradykinesia being an essential requirement for diagnosis.

Clinical Diagnostic Features

Core Motor Symptoms

• Bradykinesia (required): Slowness of movement and progressive reduction in speed and amplitude during repetitive actions
• At least one of the following:
  • Resting tremor (4-6 Hz)
  • Rigidity
  • Postural instability (typically occurs later in disease progression)

Supportive Clinical Features

• Response to dopaminergic therapy: Significant improvement with medications like levodopa 1
• Asymmetric onset of motor symptoms
• Progressive course of symptoms over time
• Presence of non-motor symptoms:
  • Hyposmia (reduced sense of smell)
  • REM sleep behavior disorder
  • Constipation
  • Depression/anxiety
  • Cognitive changes

Diagnostic Approach

Step 1: Clinical Assessment

1. History taking focusing on:

   • Presence of cardinal motor symptoms
   • Asymmetry of symptoms
   • Prodromal features (hyposmia, REM sleep behavior disorder, constipation)
   • Family history
   • Response to levodopa (if previously treated)

2. Physical examination to assess:

   • Bradykinesia: Finger tapping, hand movements, pronation-supination
   • Tremor: Resting (4-6 Hz), postural, and action tremors
   • Rigidity: Cogwheel or lead-pipe rigidity in limbs and neck
   • Gait: Shuffling, festination, freezing
   • Postural stability: Pull test

Step 2: Exclude Alternative Diagnoses

• Rule out secondary causes of parkinsonism:
  • Cerebrovascular disease
  • Medication-induced parkinsonism
  • Normal pressure hydrocephalus
  • Multiple system atrophy
  • Progressive supranuclear palsy
  • Corticobasal degeneration

Step 3: Consider Supportive Testing

• Neuroimaging:

  • MRI brain (to exclude structural lesions) 2
  • Dopamine transporter (DaT) SPECT scan if diagnosis is uncertain 2

• Functional testing:

  • Response to levodopa challenge (significant improvement supports PD diagnosis)

Red Flags That Suggest Alternative Diagnoses

• Early falls and postural instability
• Symmetrical onset of symptoms
• Rapid progression of symptoms
• Poor or no response to levodopa
• Early severe autonomic dysfunction
• Early cognitive decline or hallucinations
• Abnormal eye movements (especially vertical gaze)
• Cerebellar signs

Diagnostic Confidence Levels

Based on clinical presentation, patients can be classified as having:

1. Clinically Established PD:

   • Presence of bradykinesia plus either rest tremor or rigidity
   • Clear and dramatic response to dopaminergic therapy
   • No red flags suggesting alternative diagnosis
   • At least 3 supportive features

2. Clinically Probable PD:

   • Presence of bradykinesia plus either rest tremor or rigidity
   • No red flags suggesting alternative diagnosis
   • At least 2 supportive features

3. Possible PD:

   • Presence of bradykinesia plus either rest tremor or rigidity
   • 1-2 red flags present
   • Some supportive features present

Common Pitfalls in Diagnosis

• Misdiagnosis of essential tremor as PD (essential tremor is typically postural/action tremor rather than rest tremor)
• Overlooking drug-induced parkinsonism (antipsychotics, antiemetics)
• Missing atypical parkinsonian syndromes that may initially present similarly to PD
• Relying solely on tremor for diagnosis (10-15% of PD patients may not have tremor)
• Failure to recognize non-motor symptoms that often precede motor manifestations

The diagnostic accuracy improves with disease duration and response to dopaminergic therapy. When clinical features are atypical or response to treatment is poor, additional imaging studies like DaT-SPECT should be considered to improve diagnostic certainty 2, 3.