Table 3.

Electrophysiological characteristics of psychogenic temor and jerks

Psychogenic tremor Frequency tremor: usually low, not >11 Hz Inability to maintain the same frequency for any length of time Dissipated or variable tremor frequency on power spectrum Change of amplitude and frequency of tremor with distraction Presence of coactivation sign Duration of tremor bursts is usually long (> 70 ms) Absence of frequency dissociation Increase in tremor amplitude with peripheral loading Entrainment of tremor with the frequency of voluntary movement performed with unaffected limb. High coherence is present between the frequency spectra of voluntary activity and the “involuntary tremor” Psychogenic jerks Jerks with bursts of EMG of long duration (usually > 70 ms) Well organized triphasic pattern of activation of agonist and antagonist muscles. Stimulus-evoked jerks or jumps with variable latencies (usually > 100 ms) and variable muscle recruitment Apparent startle response may not be preceded by contraction of orbicularis occuli or a cranio-caudal progression of activation of muscles A Breitschaftspotential (premovement potential) may precede psychogenic jerks Normal electroencephalogram Absence of giant somatosensory evoked potentials No enhancement of long-loop reflexes