Neurophysiologie Clinique/Clinical Neurophysiology
Original article / Article originalNeurophysiological classification of myoclonusClassification neurophysiologique des myoclonies
Introduction
Myoclonus can be diagnosed and classified mainly based on clinical features, namely into cortical, brainstem, spinal, and unclassified. Neurophysiological studies help confirming the clinical diagnosis as well as the classification, and understanding the underlying physiological mechanisms [1]. Myoclonus can be classified from various viewpoints depending on which physiological parameter is focused upon, and which physiological test is adopted. In other words, the most appropriate test has to be chosen to study each parameter.
Section snippets
Classification based on the electromyographic (EMG) correlates
Recording of muscle activities associated with myoclonus (EMG correlates of myoclonus) by using surface electrodes provides the most essential information on any kind of myoclonus.
Classification based on the electroencephalographic (EEG) correlates
Demonstration of EEG spikes or spike-and-waves in association with myoclonus usually suggests its cortical origin, but absence of EEG spikes in association with myoclonus does not entirely exclude the possibility of cortical origin. Myoclonus of hand is associated with EEG spike or spike-and-wave, which is localized to the hand area of the contralateral sensori-motor cortex (approximately at C3 or C4 of the International 10–20 System), and in case of foot myoclonus to the midline central region
Classification based on the jerk-locked back averaging
Back averaging of EEGs time-locked to the onset of myoclonus EMG discharges might disclose a cortical spike which may not be detected by the conventional EEG-EMG recording. However, failure of back averaging to demonstrate the spike does not necessarily means subcortical origin of the myoclonus, because back averaging of the simultaneously recorded magnetoencephalogram (MEG) sometimes detects the spike that is not detectable on EEG even by back averaging. This is due to the different
Classification based on the cortico-muscular coherence
As described above, cortical myoclonus is often composed of rhythmic oscillations of muscle discharges, and accompanied by rhythmic oscillations of EEG. In this case, demonstration of increased coherence between cortical and muscle activities for certain frequency bands suggests the important role of cortical driving in the myoclonus generation [4]. This technique is especially useful for the study of rhythmic cortical myoclonus, and in contrast with jerk-locked back averaging, it takes much
Classification based on sensory evoked responses and long-loop reflexes
Most patients with cortical myoclonus show extremely enhanced cortical components of somatosensory evoked potentials (SEPs). Demonstration of giant SEP suggests cortical origin of the myoclonus. When recording sensory evoked responses, it is worthwhile recording EMGs simultaneously from the extremity which is stimulated as well as from other extremities to record long-loop EMG reflexes. If an EMG discharge is evoked in the hand at 40–45 ms after the median nerve stimulation at wrist, or
Paired stimulation evoked response/long-loop reflex and jerk-locked evoked responses
These tests form a special test battery to evaluate the excitability change of the primary somatosensory cortex following the cortical response/C reflex to the initial impulse arrival in comparison with that following the spontaneous myoclonus [2]. In some cases of cortical reflex myoclonus, especially those manifesting rhythmic myoclonus, the increased excitability of the somatosensory cortex is demonstrated at a certain time after the first stimulus as well as the spontaneous myoclonus.
Transcranial magnetic stimulation (TMS)
This test is applied to study the change of motor cortex excitability following the impulse arrival due to external stimuli as well as following a spontaneous myoclonus. In contrast to the paired stimulation evoked response/long-loop reflex and jerk-locked evoked responses described above, TMS can evaluate the excitability state of the primary motor cortex directly [8].
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