Which diagnostic tests are useful for diagnosing essential tremor?

Diagnostic Testing for Essential Tremor

Essential tremor (ET) is primarily a clinical diagnosis, but neurophysiologic testing with accelerometry and surface electromyography (EMG) should be performed when the diagnosis is uncertain, as it demonstrates 97.7% sensitivity and 82.3% specificity for ET. 1

Clinical Diagnosis Comes First

ET diagnosis requires confirmation of characteristic clinical features before considering any testing 2:

• Bilateral action tremor of the hands and forearms
• Absence of other neurological signs (no rigidity, bradykinesia, or gait abnormalities beyond tremor-related issues)
• Isolated head tremor without signs of dystonia
• Duration of at least 3 years is typical

When to Order Neurophysiologic Testing

Accelerometry and Surface EMG (First-Line Diagnostic Test)

Order this test when you cannot clinically distinguish ET from Parkinsonian tremor, dystonic tremor, or other tremor syndromes. 1, 3, 4

The test is positive for ET when ALL six criteria are present 1:

1. Rhythmic burst pattern on postural tremor EMG
2. Tremor frequency ≥4 Hz
3. No rest tremor, or if present, frequency is 1.5 Hz lower than postural tremor
4. No tremor latency from rest to postural position
5. Frequency changes ≤1 Hz after weight loading
6. No amplitude changes with mental concentration

This testing achieves a positive predictive value of 95.1% and negative predictive value of 91.1%, with an odds ratio of 198.43 for ET diagnosis 1.

Advanced Neurophysiologic Measures for Difficult Cases

When standard accelerometry is inconclusive, particularly when distinguishing tremulous Parkinson's disease from advanced ET 5, 4:

• Mean harmonic peak power analysis correctly classifies 94% of patients (higher in Parkinson's disease than ET)
• Waveform asymmetry (autocorrelation decay) correctly classifies 64% of patients

When to Order Brain Imaging

DaTscan (Ioflupane I-123 SPECT/CT) - The Definitive Imaging Test

Order DaTscan when clinical examination and neurophysiology cannot distinguish ET from Parkinsonian syndromes (Parkinson's disease, MSA, PSP, CBD). 6

• A normal DaTscan essentially excludes Parkinsonian syndromes and supports ET diagnosis 6
• DaTscan shows abnormal dopamine transporter uptake in Parkinsonian disorders but remains normal in ET 6
• This test demonstrates abnormality early in Parkinson's disease, often before standard MRI shows changes 6

MRI Brain Without Contrast

Order MRI when you need to exclude structural lesions, vascular disease, or atypical Parkinsonian syndromes (MSA, PSP, CBD) that can mimic ET 6.

MRI is useful for 6:

• Excluding focal lesions or mass effects
• Identifying patterns of atrophy characteristic of PSP, MSA, or CBD (which would argue against ET)
• Detecting vascular disease that might suggest vascular parkinsonism
• Advanced sequences (susceptibility-weighted imaging) may show the "swallow tail" sign absent in Parkinson's disease but present in healthy controls and ET

CT head is NOT recommended as it has limited soft-tissue characterization compared to MRI 6.

Tests That Are NOT Useful

Do not order these for ET diagnosis 6:

• FDG-PET/CT - limited evidence, not indicated for ET evaluation
• Amyloid PET/CT - no role in ET diagnosis
• MR spectroscopy - no supporting evidence
• Functional MRI - no supporting evidence

Specialized Psychophysical Testing (Research/Specialized Centers)

Temporal discrimination testing can help distinguish dystonic tremor from ET when the diagnosis remains unclear after standard testing 7:

• Somatosensory temporal discrimination threshold (TDT): Normal in ET, elevated in dystonic tremor
• Temporal discrimination movement threshold (TDMT): Elevated in ET, normal in dystonic tremor
• Combined testing yields 86.7% positive predictive value for ET and 100% positive predictive value for dystonic tremor 7

Laboratory and Genetic Testing

Routine laboratory tests are NOT diagnostic for ET but may identify secondary causes of tremor 3:

• Thyroid function tests - to exclude hyperthyroidism
• Metabolic panel - to exclude metabolic causes
• Genetic testing - currently not recommended for routine clinical practice, as specific genes are identified in only a minority of cases 3

Common Pitfalls to Avoid

• Do not rely solely on tremor frequency to distinguish ET from Parkinson's disease, as there is significant overlap 5, 4
• Do not assume bilateral tremor excludes Parkinson's disease - tremulous Parkinson's can be bilateral 5
• Do not order imaging in straightforward cases - ET with classic bilateral action tremor, no red flags, and clear family history does not require imaging 6, 2
• Do not confuse enhanced physiologic tremor with ET - neurophysiology can distinguish these 1, 4