Electrophysiologic Assessments of Involuntary Movements: Tremor and Myoclonus

Abstract

Tremor is defined as a rhythmical, involuntary oscillatory movement of a body part. Although neurological examination reveals information regarding its frequency, regularity, amplitude, and activation conditions, the electrophysiological investigations help in confirming the tremor, in differentiating it from other hyperkinetic disorders like myoclonus, and may provide etiological clues. Accelerometer with surface electromyogram (EMG) can be used to document the dominant frequency of a tremor, which may be useful as certain frequencies are more characteristic of specific etiologies than others hyperkinetic disorders. It may show rhythmic bursts, duration and activation pattern (alternating or synchronous). Myoclonus is a quick, involuntary movement. Electrophysiological studies may helpful in the evaluation of myoclonus, not only for confirming the clinical diagnosis but also for understanding the underlying physiological mechanisms. Electroencephalogram (EEG)-EMG correlates can give us important information about myoclonus. Jerk-locked back-averaging and evoked potentials with recording of the long-latency, long-loop reflexes are currently available to study the pathophysiology of myoclonus.

How can neurophysiological studies help with movement disorders characterization in clinical practice? A review

“…These features help to classify the tremor into one of the commonly recognizable syndromes 13 . Electromyography (EMG) with surface electrodes can be placed on antagonist muscles, such as extensor carpi radialis (ECR) and flexor carpi ulnaris (FCU), together with an accelerometer, in order to evaluate tremor frequency, synchronous or alternating activity of antagonist muscles, burst duration, and frequency/amplitude variations induced by weight-bearing or posture change 6 .…”

“…Reticular myoclonus is a specific type of brainstem myoclonus. It has a distinctive muscle activation order, ascending in the brainstem and descending in the spinal cord 6 . Its burst has a duration of 20-70 ms and happens on rest, spontaneously, and propagates in high speed (approximately 40 m/s).…”

“…Its burst has a duration of 20-70 ms and happens on rest, spontaneously, and propagates in high speed (approximately 40 m/s). Its fast propagation reflects stimuli conduction through the reticulospinal tract 6,47,61 . The main etiology is post-hypoxic (Lance-Adams syndrome) 62 .…”

“…Hyperekplexia is an exacerbated startle response, with a burst duration of >75 ms, slow propagation speed, and bidirectional spreading. It is mostly evoked by sudden noise or light 6 . In essential myoclonus, the myoclonus is the most prominent or only clinical finding 63 .…”

“…They are also potential tools to clarify the origin of abnormal movements and to differentiate between organic and functional disorders, previously also referred as psychogenic disorders. Diverse techniques, such as bereitschaftspotential, back-averaging electroencephalogramelectromyography (EEG-EMG), and special studies using different paradigms for tremor can be applied for this function 5,6,7,8 .…”