Laboratory-supported criteria for functional tremor rest on demonstrating that the tremor is produced by the voluntary motor system. The defining electrophysiologic features are coherence between the tremor and a voluntary rhythmic task performed by another body part, frequency entrainment or complete suppression during that task, a variable and non-stationary tremor frequency, and tonic coactivation of antagonist muscles for approximately 300 milliseconds before tremor onset. A validated multi-parameter test battery using these variables distinguishes functional from organic tremor with high accuracy.
Physiologically, a voluntary rhythm generator can drive only one frequency at a time, so attempting a second externally paced rhythm forces the tremor to entrain, become erratic or stop. Coactivation reflects the fact that the limb is stiffened voluntarily before the oscillation starts, which does not occur in parkinsonian or essential tremor. This case also illustrates the value of a normal dopamine transporter scan: normal presynaptic dopaminergic imaging with a levodopa-unresponsive rest tremor should prompt consideration of functional parkinsonism rather than early Parkinson disease.
This item maps to the Diagnostic procedures axis of the ABPN blueprint, because the competency assessed is selection and interpretation of a confirmatory neurophysiologic study when the bedside examination is inconclusive. Laboratory support is particularly valuable when a wrong diagnosis has been carried for years and the patient needs objective evidence to accept a new explanation.
Incorrect Answers
- A. An invariant 4 to 6 Hz alternating rest tremor that is unaffected by every task is the expected pattern of parkinsonian tremor and argues against a functional cause.
- B. A time-locked cortical correlate on jerk-locked back averaging at short latency indicates cortical myoclonus, not functional tremor.
- C. Rhythmic 8 to 12 Hz activity abolished by eye closure describes the occipital alpha rhythm, an electroencephalographic artefact of no relevance to tremor classification.
- E. Irregular slow proximal jerking with long burst durations suggests a symptomatic myoclonus or Holmes tremor rather than providing functional laboratory support.
Testing Pearls
- Laboratory-supported functional tremor requires entrainment or coherence with a voluntary contralateral rhythm.
- Tonic antagonist coactivation for roughly 300 ms before tremor onset supports a functional origin.
- Non-stationary, variable tremor frequency across recordings favours functional tremor.
- A normal dopamine transporter scan with levodopa-unresponsive rest tremor suggests functional parkinsonism.
- Accelerometry with surface electromyography is the practical laboratory tool when bedside testing is equivocal.
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.
