Entrainment is the single most useful bedside feature of functional tremor. When the patient performs a voluntary rhythmic movement at an externally set frequency with the unaffected limb, a functional tremor either stops, becomes irregular, or shifts to the imposed frequency, because both movements are generated by the same voluntary motor system and cannot be run independently. Organic tremors such as essential or parkinsonian tremor keep their own frequency regardless of what the other hand does.
Two related manoeuvres depend on the same principle. Ballistic movements of the unaffected limb transiently suppress a functional tremor, and the whack-a-mole sign describes the appearance of tremor elsewhere when the affected limb is restrained. Weight loading is a useful confirmatory test with the opposite direction of effect from organic tremor: functional tremor typically increases in amplitude when loaded because the patient unconsciously increases drive, whereas essential tremor amplitude usually falls. Electrophysiologic laboratory-supported criteria add coherence analysis and tonic coactivation of antagonists at tremor onset.
Entrainment testing is a bedside manoeuvre expected of every neurology resident, placing this item at the Core level of RITE content. Note that abrupt onset after physical injury, variability and a mixed rest-postural-action distribution are all suggestive but none of them is by itself diagnostic.
Incorrect Answers
- A. Tremor occurring at rest, with posture and with action is suggestive but also occurs in severe essential tremor, dystonic tremor and Holmes tremor.
- B. Abrupt onset after a physical precipitant is common in functional disorders but also occurs with stroke, trauma and drug exposure, so it is not a rule-in sign.
- C. The weight-loading response is confirmatory rather than definitive; loading can increase amplitude in some organic tremors and the manoeuvre is less reproducible than entrainment.
- D. Absence of parkinsonian or dystonic signs simply argues against those specific diagnoses and is a negative rather than a positive feature.
Testing Pearls
- Entrainment, distractibility and suppression by contralateral ballistic movement are the key signs of functional tremor.
- Functional tremor amplitude typically increases with weight loading, unlike essential tremor.
- Variable frequency and variable amplitude within one examination favour a functional cause.
- Tonic coactivation of antagonists at tremor onset is a supportive electrophysiologic feature.
- The whack-a-mole sign describes tremor moving to another limb when the affected limb is held.
References
- Jankovic J, Mazziotta JC, Newman NJ, Pomeroy SL, editors. Bradley and Daroff's Neurology in Clinical Practice. 8th ed. Philadelphia, PA: Elsevier; 2022. p. 2082.
- Schwingenschuh P, Saifee TA, Katschnig-Winter P, Macerollo A, Koegl-Wallner M, Culea V, et al. Validation of "laboratory-supported" criteria for functional (psychogenic) tremor. Mov Disord. 2016;31(4):555-562.
- Espay AJ, Aybek S, Carson A, Edwards MJ, Goldstein LH, Hallett M, et al. Current concepts in diagnosis and treatment of functional neurological disorders. JAMA Neurol. 2018;75(9):1132-1141.
