How do you differentiate a tremor of Parkinson's disease (PD) from essential tremor in a patient presenting with tremor?

Differentiating Parkinson's Disease Tremor from Essential Tremor

The most reliable clinical distinction is that Parkinson's disease presents with asymmetric resting tremor that diminishes with voluntary movement, accompanied by bradykinesia and/or rigidity, while essential tremor manifests as symmetric bilateral postural and kinetic tremor without parkinsonian features. 1, 2

Clinical Examination Algorithm

Step 1: Characterize the Tremor Pattern

Tremor timing and distribution:

• PD tremor: Predominantly occurs at rest (4-6 Hz), asymmetric, often begins unilaterally, diminishes with voluntary movement, may exhibit "pill-rolling" quality 1, 3
• ET tremor: Primarily postural and kinetic (bilateral symmetric), affects hands during action, may involve head and voice, rarely affects chin or jaw 1, 3

Critical observation technique:

• Have the patient rest hands completely in lap and observe for tremor 3
• Then have patient hold arms outstretched with hands fully extended 4
• A novel distinguishing maneuver: transition from hands stretched to hands hanging down while arms remain extended—PD tremor intensity increases in 83% of cases with hanging position, while ET tremor decreases in 75% of cases 4

Step 2: Assess for Cardinal Parkinsonian Signs

Bradykinesia assessment (essential for PD diagnosis):

• Test finger tapping, hand opening/closing, and rapid alternating movements 5, 2
• Observe for progressive slowing, decreased amplitude, or hesitations 5
• Assess fine motor tasks (buttoning), gross motor activities (walking/turning), facial expressions, and speech 5
• Bradykinesia is absent in essential tremor 2

Rigidity assessment:

• Passively move patient's limbs through full range of motion while instructing complete relaxation 5, 2
• Test both upper and lower extremities, comparing sides for asymmetry 5
• Use activation maneuvers (have patient open/close opposite hand) to enhance detection of subtle rigidity 5, 2
• Note constant resistance (lead-pipe) or ratchet-like jerky resistance (cogwheel phenomenon) 5
• Rigidity is present in PD but completely absent in ET 2

Step 3: Identify Supporting Clinical Features

Features favoring PD:

• Postural instability (though typically appears later) 5
• Asymmetric symptom onset 1, 3
• Cognitive slowing, speech abnormalities, depression, dysautonomia, sleep disturbances 6
• Reduced arm swing on affected side 3

Features favoring ET:

• Family history of tremor (common) 3
• Tremor improvement with alcohol consumption 3
• Symmetric bilateral involvement from onset 3
• No bradykinesia or rigidity 2

Diagnostic Imaging When Clinical Diagnosis Remains Uncertain

I-123 ioflupane SPECT/CT (DaTscan) is the definitive test:

• Abnormal DaTscan: Shows decreased radiotracer uptake in striatum (usually putamen first), confirms parkinsonian syndrome 7, 5, 1
• Normal DaTscan: Essentially excludes PD and supports diagnosis of ET or drug-induced tremor 7, 5, 2
• This test differentiates true parkinsonian syndromes from ET early in disease course 7, 5

MRI brain without contrast should be obtained first:

• Rules out structural causes, focal lesions, or vascular disease before functional imaging 5
• Often normal in early PD but essential to exclude alternative diagnoses 5

Critical Diagnostic Pitfalls to Avoid

Common errors that lead to misdiagnosis:

• Failure to exclude drug-induced parkinsonism: Review medication history for antipsychotics, antiemetics, and other dopamine-blocking agents 2
• Missing subtle rigidity: Not using activation maneuvers or confusing voluntary muscle contraction with true rigidity 5, 2
• Overlooking atypical parkinsonian syndromes: PSP (vertical gaze palsy), MSA (early severe autonomic dysfunction), or CBD (asymmetric rigidity with alien hand phenomenon) can masquerade as PD 5, 2
• Relying solely on tremor characteristics: Some PD patients have additional postural/kinetic tremor, and some ET patients show asymmetry 3, 8
• Not recognizing disease coexistence: The two conditions may coexist in the same patient over time 8

Diagnostic Confirmation

A diagnosis of PD requires:

• Bradykinesia (essential feature) PLUS at least one of: resting tremor or rigidity 5, 6
• Absence of red flags suggesting atypical parkinsonism 5

Specialist referral is strongly recommended:

• General neurologists or movement disorder specialists should confirm diagnosis, as correctly diagnosing parkinsonian syndromes on clinical features alone is challenging 5
• Specialist involvement prevents missing atypical parkinsonian syndromes with different prognoses and treatment responses 5