SY2.2. Evaluation of myoclonus

Abstract

Myoclonus is a shock-like jerky movement observed in the extremities or the trunk without consciousness loss. This involuntary movement is generated by abnormal neuronal discharges in the central nerve system caused by many kinds of etiologies. Based on the physiological generator mechanism, myoclonus is classified mainly into the three groups; cortical myoclonus, subcortical myoclonus and spinal myoclonus. Cortical myoclonus includes spontaneous cortical myoclonus and cortical reflex myoclonus. In cortical myoclonus, an EEG spike precedes a muscle jerk. Backward EEG averaging time-locked to the onset of myoclonus (jerk-locked back averaging: JLA) often detects cortical spike preceding the myoclonus, which is unable to be shown by EEG-EMG polygraph recordings. The duration of EMG jerks is usually shorter than 50-100ms. Somatosensory evoked potentials are often enlarged (giant SEP) in cortical reflex myoclonus. Enhanced long loop reflex is also often seen in the polygraph in the cortical reflex myoclonus (so called “C” reflex). The disinhibition of the sensory cortex can be revealed by SEP recovery curve studied by paired pulse stimulation. The hyperexcitability of the motor cortex is shown by the paired pulse transcranial magnetic stimulation (TMS) technique. The short interval intra-cortical inhibition (SICI) was abnormally reduced. These physiological features indicate the cortical hyper- excitability. Subcortical myoclonus is supposed to be generated at the basal ganglia or brainstem. The duration of muscle jerk is longer than 50-100ms. In some subcortical myoclonus (reticular myoclonus), the latency of long loop reflexes is shorter than those of enhanced C-reflexes. JLA reveals no cortical spikes, and no giant SEPs are seen. Spinal myoclonus includes “segmental spinal myoclonus” which is generated by activation of certain spinal segments, and “propriospinal myoclonus” which originates from a certain spinal segment and spreads upward/downward slowly though the propriospinal tract. Simultaneous EMG recordings of many muscles are useful to estimate the spinal conduction between different spinal cord levels. Electrophysiological studies are useful to evaluate myoclonus and to suggest the generator mechanisms for myoclonus. Here, I will introduce some neurophysiological methods for analyses of myoclonus.