The electrophysiological study of myoclonic movements touches on the interests of clinical electrophysiologists, epileptologists, movement disorders specialists, and sleep medicine specialists alike. As early as 1935, Gibbs and colleagues1 had described patients with spike-and-wave discharges in the electroencephalogram (EEG), with muscle jerking at the same rate as the EEG spikes. Grinker and colleagues2 are credited with the first description of polyspike discharges in the EEG, with close association to myoclonic jerking in patients with progressive myoclonic epilepsy. In 1946, Dawson3 produced a detailed description of the relationship between EEG spikes and muscle jerks in patients with myoclonus, reporting also, in some of his patients, the possibility of inducing myoclonic jerks by tendon tapping. One year later, Dawson himself demonstrated not only the first recording of somatosensory-evoked potentials (SEPs) from the scalp in humans,4 but also that the SEPs in patients with myoclonus could be grossly exaggerated in amplitude.5
Electrophysiological studies aid in making the diagnosis and provide insight into the pathophysiology of myoclonus.6–10 The field has grown steadily as technological advances and new experimental techniques have emerged over the last decades. The electrophysiological assessment of myoclonus serves as a prototypical example of the valuable insights to be gained in the understanding of movement disorder pathophysiology by the application of clinical and experimental electrophysiological methods.
Electromyographic (EMG) activity is a direct measure of α motor neuron activity, thus providing information on the nature of the central nervous system events that generate involuntary movements of myoclonus. Multichannel surface EMG is a powerful tool to evaluate the quick involuntary movements of myoclonus and distinguish them from other movement disorders. There is no predefined protocol to conduct a polygraphic EMG study in a patient with myoclonus, but rather, the study design should be guided by the particular patient's clinical presentation and by the questions at hand.6 When possible, polygraphic EMG should be recorded simultaneously with EEG in the same session. The selection of muscles to record should follow a general logic. Generally, it is recommended to sample agonist/antagonist muscle pairs acting on a given joint. It is also desirable to perform bilateral recordings of homologous muscles. Proximal and distal muscle groups in a given limb give additional information on the axial versus distal character of the involuntary muscle activation. Often, there is a desire to study the order of muscle activation and the velocity of the “wave” of activation in myoclonic movements, thus reflecting on the pathway of spread.11–13 In those cases, muscles representing multiple levels of the neuroaxis from upper brain stem motor nuclei to lower spinal cord levels myotomes are recorded simultaneously. Polygraphic recordings are often conducted under conditions of rest, posture, actions, and stimuli that elicit the involuntary movements. Nowadays, most studies are collected and stored in digital media for off-line analysis.
Valuable information is contained in polygraphic EMG recordings ...