How do you differentiate essential tremor from Parkinson's disease?

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, while Parkinson's disease begins unilaterally with resting tremor, micrographic handwriting, and accompanying bradykinesia and rigidity. 1

Key Clinical Features to Distinguish the Two Conditions

Tremor Characteristics

• Essential tremor is a bilateral action tremor (occurs during voluntary movement) affecting primarily the upper limbs, with no signs other than tremor present on examination 1
• Parkinson's disease begins unilaterally with a resting tremor (present when the limb is at rest and disappears with movement), though postural tremor can also occur 1
• The tremor in PD is accompanied by bradykinesia (slowness of movement) and rigidity, which are essential diagnostic features 2, 1

Handwriting Test (Simple Bedside Differentiator)

• PD handwriting: Small and micrographic (progressively smaller letters) but without tremor 1
• ET handwriting: Tremulous but normal-sized letters 1
• This simple test can usually distinguish between the two conditions at the bedside 1

Cardinal Motor Signs Required for PD Diagnosis

• Bradykinesia is mandatory for PD diagnosis, plus at least one of the following: resting tremor, rigidity, or postural instability 2
• In ET, tremor exists in isolation without bradykinesia or rigidity 1
• Test for rigidity by passively moving the patient's relaxed limbs through their full range of motion, noting constant resistance (lead-pipe rigidity) or ratchet-like resistance when combined with tremor (cogwheel rigidity) 2

Diagnostic Algorithm

Step 1: Clinical Examination

• Observe tremor at rest, during posture holding, and during action 1, 3
• Check for unilateral versus bilateral onset (PD starts unilaterally, ET is bilateral) 1
• Test for bradykinesia and rigidity (present in PD, absent in ET) 2, 1
• Obtain handwriting sample 1

Step 2: Imaging When Clinical Diagnosis is Uncertain

• I-123 ioflupane SPECT/CT (DaTscan) is the first-line imaging modality recommended by the American College of Radiology for differentiating PD from essential tremor 2, 4
• DaTscan shows decreased radiotracer uptake in the striatum in PD, while it remains normal in essential tremor 2, 4
• A normal DaTscan essentially excludes Parkinsonian syndromes, making it highly valuable for ruling out PD 2, 4
• MRI brain without contrast can rule out structural causes but is often normal in early PD 2

Common Pitfalls and Caveats

Overlapping Features

• Both conditions can have rest and postural tremor, though the predominant tremor type differs 5, 6
• Some patients may have both ET and PD coexisting (ET-PD), particularly when long-standing ET is followed by development of parkinsonian features 5, 6
• A subset of PD patients may have postural tremor preceding other parkinsonian features by years 6

Red Flags Suggesting Alternative Parkinsonian Syndromes

• Vertical gaze palsy (especially downward) suggests Progressive Supranuclear Palsy rather than idiopathic PD 7, 2
• Asymmetric rigidity with alien hand phenomenon suggests Corticobasal Syndrome 7, 2
• Ataxia suggests other diagnoses such as Multiple System Atrophy 2

Assessment Errors to Avoid

• Failure to have the patient completely relax during rigidity testing can produce false positives from voluntary muscle contraction 2
• Not using activation maneuvers (having the patient move the contralateral limb) may cause you to miss subtle rigidity 2
• Confusing spasticity (velocity-dependent resistance) with rigidity (constant resistance throughout movement) 2

When to Proceed with Advanced Testing

• When clinical features are ambiguous or atypical 2, 4
• When tremor characteristics overlap between the two conditions 3
• When early diagnosis is critical for treatment planning, as PD symptoms typically appear only after 40-50% of dopaminergic neurons are lost 2, 4