Cortical myoclonus

The following sections about cortical myoclonus was excerpted from Eplilesy textbook on 2013/02/16
Cortical myoclonus reflects impulses that originate in the sensorimotor cortex and travel down the brainstem. Cortical myoclonus is typically seen in progressive myoclonus epilepsy. Muscles involved tend to be distal more than proximal and flexor more than extensor, and to involve more the face and upper extremities than the rest of the body. Cortical myoclonus is more commonly encountered in a multifocal form, presenting with multifocal spike discharges. If myoclonus is triggered by stimuli, the term cortical reflex myoclonus is used. If myoclonus occurs periodically, the term epilepsia partialis continua is used. The neurons in the sensorimotor cortex may be primarily hyperexcitable, or may be driven by abnormal inputs from the neurons of other brain parts. Therefore, cortical myoclonus occasionally is called fragmented epileptic convulsion.
In patients with cortical reflex myoclonus, the cortical components of median-nerve SEP showed abnormally large amplitude. Usually, the initial peaks (N20/P22) are not large, and the following components become higher. This giant SEP is thought to indicate hyperexcitability of the sensorimotor cortex. Abnormally large evoked potentials were also reported by photic stimulation.
When the peripheral nerve is stimulated, the stimulus goes up the spino-thalamo-cortical tract and, after excitation of the pyramidal neuron, it goes down the cortico-spinal tract, resulting in muscle contraction (long-loop reflex). In normal subjects, long-loop reflex can be recorded only when subjects maintain muscle contractions. In patients with cortical reflex myoclonus, however, this reflex can be recorded even while resting (C-reflex). The latency of C-reflex for median nerve stimulation is about 40 to 45 msec, which is almost double of the latency of N20 to the median nerve stimulation. When the C-reflex is recorded from the contralateral limbs to the stimuli, the latency delay is about 10 msec to the ipsilateral limbs, which corresponds to the traveling time of the transcallosal pathway. This stimulation-locked muscle contraction is believed to share the same underlying mechanism with cortical reflex myoclonus.
Some EEG correlates are time-locked to cortical myoclonus. However, because of the relatively smaller amplitude of the EEG spikes in comparison with the background activities, the physiologic correlates of myoclonus can only be detected by using jerk-locked (EEG or magnetoencephalograhic [MEG]) averaging (JLA of jerk-locked magentic field [JLF]) or coherence analysis method. In JLA, EEGs are averaged with respect to the EMG onset, to reduce the non–time locked background EEG activities. Positive peak of the EEG spikes is 15 to 20 msec prior to the myoclonus for the upper limbs, and 25 to 40 msec for the lower limbs. Spikes are located around the contralateral primary motor cortex.
As such, cortical reflex myoclonus is caused by hyperexcitability of the primary sensorimotor cortex. However, because giant SEPs are not always present in patients with cortical reflex myoclonus (as in dentatorubral-pallidoluysian atrophy [DRPLA]), some other pathophysiologic mechanisms may exist.
In Lennox-Gastaut syndrome (LGS), myoclonus is rare and disclosed only in those cases with a cortical lesion affecting the rolandic area; thus, myoclonus appears to be produced by a secondary generalization of focal cortical myoclonus. They also present with arrhythmic, distal small focal jerks, leading to the individual tiny finger movements unaccompanied by premyoclonic potentials on JLA that Wilkins et al. proposed to call minipolymyoclonus. Brown et al. indicated that the major role of facilitation of inter- and intra-hemispheric spread of cortical myoclonic activity is through trans-callosal or intrahemispheric corticocortical pathways in producing generalized or bilateral myoclonus. Therefore, bilateral jerks may not be synchronous in patients with cortical myoclonus.