A Reappraisal on cortical myoclonus and brief Remarks on myoclonus of different Origins

Abstract

Myoclonus has multiple clinical manifestations and heterogeneous generators and etiologies, encompassing a spectrum of disorders and even physiological events. This paper, developed from a teaching course conducted by the Neurophysiology Commission of the Italian League against Epilepsy, aims to delineate the main types of myoclonus, identify potential underlying neurological disorders, outline diagnostic procedures, elucidate pathophysiological mechanisms, and discuss appropriate treatments. Neurophysiological techniques play a crucial role in accurately classifying myoclonic phenomena, by means of simple methods such as EEG plus polymyography (EEG + Polymyography), evoked potentials, examination of long-loop reflexes, and often more complex protocols to study intra-cortical inhibition-facilitation. In clinical practice, EEG + Polymyography often represents the first step to identify myoclonus, acquire signals for off-line studies and plan the diagnostic work-up.

Keywords: Cortical myoclonus; Cortico-subcortical myoclonus; EEG correlates and network evaluation; EEG plus polymyography; Evoked potentials and reflex responses; High-frequency oscillations; Post-hypoxic myoclonus; Subcortical myoclonus.

Fig. 1

The three panels on the left represent three types of myoclonus. In the top panel: positive myoclonus at rest, which can affect the pair of antagonists, but also occur on a single muscle; in the middle panel: typical positive myoclonus, which occurs during voluntary muscle contraction, with an almost rhythmic pattern; in the bottom panel: negative myoclonus, which interrupts the contraction of a pair of antagonist muscles. The traces in A are the average obtained with the jerk-locked back-averaging (JLBA) technique in a patient with subcortical myoclonus, affected with myoclonus-dystonia syndrome due to DYT11 mutation. There is no correlated EEG event, the rectangle identifies the relatively long duration of the averaged myoclonic jerk. The traces in B represent the JLBA obtained on a cortical myoclonus at rest in a patient with EPC. In this case, JLBA identifies the correlated EEG that precedes the myoclonic jerks by about 14 ms. The traces in C and C1 represent the findings in a patient with cortical myoclonus, affected with progressive myoclonus epilepsy due to CSTB expansion mutation (EPM1). Also in this case the JLBA identifies the cortical transient that precedes action myoclonus. The lower panels are samples of the polygraphic signal. The rhythmicity of the positive action myoclonus in C allows an analysis of cortico-muscular coherence (CMC) with a maximum peak on the contralateral central derivation (C3) and its localization in a coherence map is represented in the panel below.

Fig. 2

Examples of subcortical myoclonus. Brainstem myoclonus characterizes a patient with Creutzfeldt-Jakob disease (CJD), with both positive (A) and positive–negative (B) jerks showing variable temporal association with the typical periodic sharp wave complexes (PSWC). Propriospinal myoclonus is represented in C and D in a patient with a compression of the spinal cord at the lumbar level, starting on the right quadriceps but involving both sides and flowing from proximal to distal muscles. The recordings of jerks considered as psychogenic myoclonus are represented in the panels E and F, showing a scattered muscle activation.

Fig. 3

Example of unilateral giant somatosensory evoked potential (SEP) in a patient with EPC of the left orbicularis and left upper limb muscles (A). On the other side, SEP is normal (B). The star indicates N20 which is bilaterally normal, while the following components are increased on the right sensory cortex only. In C abnormal amplitude ratio between short latency (SLR) and long latency reflex (LLR), indicating unilateral facilitation of the LLR during moderate muscular activation of the left hand. On the other side, always during moderate motor activation, SLR is well evident, while LLR scarcely defined and their amplitude ratio normal (D). The arrow indicates the artefact due to electrical stimulation.

Fig. 4

The origin, main causes, clinical features, and neurophysiological findings are resumed for peripheral, spinal, propriospinal, and reticular myoclonus.

Fig. 5

The origin, main causes, clinical features, and neurophysiological findings are resumed for thalamus basal ganglia (cerebellum), cortical-subcortical (thalamus-cortical), cortical myoclonus, and EPC.