Electrophysiologic evaluation of psychogenic movement disorders.

Psychogenic movement disorders (PMD) are a group of disorders which are in the border zone between neurology and psychiatry. All necessary laboratory investigations should be done to rule out an underlying organic disorder. While clinical acumen of a trained movement disorder specialist may be sufficient to diagnose most PMD, there are clinical situations where electrophysiological tests are required either to rule out an organic movement disorder or even diagnose a PMD. Current electrophysiological test are most useful for tremor, followed by jerks and least for spasms or dystonia. Commonly used electrophysiologic tests include multichannel surface electromyography (EMG), accelerometry, electroencephalography time locked with EMG, premovement potential (Bereitschaftspotential), and somatosensory evoked potentials. Psychogenic tremor is a low frequency tremor with variable frequency and duration of EMG bursts, entrainable, has a high coherence with voluntary movements, and presence of coactivation sign. Patients with psychogenic jerks have well organized triphasic pattern of activation of agonist and antagonist muscles. The jerks are associated with EMG bursts of long duration (usually > 70 ms), long and variable latencies in stimulus induced jerks, absence of craniocaudal pattern of muscle recruitment in apparent startle response, and often a Breitschaftspotential (premovement potential) precedes the jerk. Electrophysiological characterization of psychogenic dystonia is difficult and the tests are usually performed to rule out organic dystonia with characteristic findings. Finally, caution should be exerted in interpreting the electrophysiological tests as both false positive and false negative diagnosis of PMD may still occur.