Differentiating Essential Tremor from Parkinson's Disease
Essential tremor presents as a bilateral action tremor of the upper limbs with normal-sized but tremulous handwriting and no other neurological signs, while Parkinson's disease begins unilaterally as a resting tremor with micrographic (small) but non-tremulous handwriting, accompanied by bradykinesia and rigidity. 1
Clinical Examination Algorithm
Step 1: Observe Tremor Characteristics
Tremor timing and distribution:
- Parkinson's disease: Unilateral resting tremor that improves with voluntary movement 1
- Essential tremor: Bilateral action/postural tremor that worsens during voluntary movement 1
Tremor frequency:
- Parkinson's disease: Typically 4-6 Hz resting tremor 2
- Essential tremor: Typically 8-12 Hz action tremor 3
Step 2: Assess for Cardinal Motor Signs
Parkinson's disease requires bradykinesia PLUS at least one of the following: 4
- Resting tremor
- Rigidity
- Postural instability (appears later in disease)
Essential tremor has NO additional neurological signs beyond tremor. 1
Step 3: Perform Handwriting Test
This bedside test is highly discriminatory: 1
- Parkinson's disease: Micrographic (progressively smaller) writing that is NOT tremulous
- Essential tremor: Normal-sized writing that IS tremulous
Step 4: Test for Rigidity
Technique for detecting rigidity: 4
- Have patient completely relax while you passively move their limbs through full range of motion
- Test both upper and lower extremities, comparing sides for asymmetry
- Use activation maneuvers (have patient open/close opposite hand) to enhance detection of subtle rigidity
- Note constant resistance throughout movement (lead-pipe rigidity) or ratchet-like jerky resistance (cogwheel rigidity when combined with tremor)
Rigidity is present in Parkinson's disease but ABSENT in essential tremor. 2, 1
Step 5: Assess for Bradykinesia
Bradykinesia affects: 4
- Fine motor tasks (buttoning clothes, writing)
- Gross motor activities (walking, turning)
- Facial expressions (masked facies)
- Speech (hypophonia)
Bradykinesia is the ESSENTIAL diagnostic feature of Parkinson's disease and is ABSENT in essential tremor. 4, 1
Diagnostic Imaging When Clinical Diagnosis is Uncertain
I-123 ioflupane SPECT/CT (DaTscan) is the definitive test to differentiate these conditions: 5, 4
- Abnormal (decreased striatal uptake): Confirms Parkinsonian syndrome (PD, MSA, PSP, CBD)
- Normal: Essentially excludes Parkinsonian syndromes and supports essential tremor or drug-induced tremor diagnosis
A normal DaTscan essentially rules out Parkinson's disease. 5
MRI brain without contrast is appropriate when structural causes need exclusion but is often normal in early Parkinson's disease. 4
Common Diagnostic Pitfalls
Coexistence of both conditions: With time, essential tremor and Parkinson's disease may coexist within a single patient, complicating diagnosis. 3
Overlapping tremor types: Both conditions can exhibit postural tremor, though the primary tremor type differs (rest vs. action). 3
Drug-induced parkinsonism: Must be excluded through medication history, particularly antipsychotics and antiemetics. 4
Failure to detect subtle rigidity: Not using activation maneuvers or confusing voluntary muscle contraction with true rigidity leads to missed diagnoses. 4
Atypical Parkinsonian syndromes masquerading as Parkinson's disease: 5, 4
- Vertical gaze palsy (especially downward) suggests Progressive Supranuclear Palsy
- Asymmetric rigidity with alien hand phenomenon suggests Corticobasal Syndrome
- Early falls, ataxia, or autonomic dysfunction suggest Multiple System Atrophy