A 77-year-old man with prior ischemic strokes in the territory of the right middle cerebral artery and mild residual left arm weakness was found at home with worsened left-sided hemiparesis and neglect. Cognitively, he also presented executive dysfunction and a deficit of anterograde memory. On examination, in addition to a slow drift of the left arm, the patient repeatedly displayed sudden, brief periods of left-sided wrist drop, giving the appearance of unilateral asterixis (Video 1). Brain MRI showed the known previous cerebral infarctions, but no acute lesion was visible. Routine video-EEG was unremarkable. Video-EEG with simultaneous EMG recordings of the left extensor carpi radialis showed that the episodes of wrist drop were caused by transient periods of EMG silence lasting about 100 ms, without a preceding jerk (negative myoclonus). Back-averaging of the EEG traces locked to the onset of the EMG silent periods revealed a focal cortical epileptiform discharge in the right central region immediately preceding the EMG silences (Fig. 1). Later during his hospital stay, the patient was observed to suffer a bilateral tonic-clonic seizure. Despite multiple trials of antiseizure medications, the negative myoclonus persisted, although bilateral tonic-clonic seizures did not recur. Four years later, the neurological examination was unchanged.
Fig. 1.
Left: EEG (top) and EMG (bottom) recordings around a single occurrence of wrist drop. No epileptiform discharge is visible prior to the EMG silence. Right: averaged recordings to 85 occurrences of wrist drop. An epileptiform discharge clearly precedes the EMG silence. Far right: voltage topographies of the averaged EEG at the IED's negative (top) and positive (bottom) peaks. Standardized 25-electrode array of the International Federation for Clinical Neurophysiology (Seeck et al., 2017). ECR: extensor carpi radialis; IED: interictal epileptiform discharge; SEM: standard error of the mean.
Epileptic negative myoclonus is thought to be the direct consequence of epileptiform activity in negative motor cortical areas (Guerrini et al., 1993). In epileptic myoclonus, the brief epileptiform discharges do have a clinically noticeable effect, blurring the distinction between what is considered ictal and interictal activity. The pathophysiology of epileptic negative myoclonus is incompletely understood. Direct electrical stimulation through intracranial electrodes has implicated the primary and premotor cortices, supplementary motor area, and primary somatosensory cortex, presumably through the recruitment of inhibitory interneurons (Rubboli et al., 2006). Given that the typical latency of motor evoked potentials to forearm and hand muscles is 20–25 ms (Rossini et al., 2015), it is unlikely that the initial, surface-negative peak of the epileptiform discharge, with its 74-ms latency to the EMG silence onset, is directly responsible for the observed motor inhibition. The 31-ms latency of the later, surface-positive peak makes it more plausible, adding a few milliseconds to allow for synaptic transmission from neighboring cortical areas to primary motor cortex (and accounting for the uncertainty in determining the true onset of EMG silent periods). Interestingly, latencies from the cortical stimulation pulse to the EMG silence in Rubboli et al. (2006) were similar to what we report or even longer, especially upon stimulation of the supplementary motor area and primary somatosensory cortex.
When epileptic negative myoclonus is quasi-continuous, as in our patient, it could be conceived of as an epilepsia partialis continua. Clinically, it resembles unilateral asterixis, which has been described as a rare consequence of stroke involving the thalamus. In unilateral asterixis, however, the pathophysiology differs from that of epileptic negative myoclonus, as it features excessive inhibition of sensorimotor cortex as a result of the thalamic lesion (Inoue et al., 2012). When individual epileptiform discharges are too low-amplitude to be noticed on routine EEG, back-averaging of EEG activity time-locked to the onset of the motor phenomenon may reveal a cortical discharge (Latorre et al., 2025). PET scanning may assist in establishing the epileptic nature of continuous focal myoclonus (including negative myoclonus), but must be interpreted cautiously, since both focal hypermetabolism and hypometabolism have been reported (Hajek et al., 1991; Yu et al., 2009).
Ethical statement
The patient provided written informed consent for the publication of the manuscript and video.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The following are the supplementary data related to this article.
The patient was asked to raise both arms in front of him. In addition to the slow drift of the left arm, quasi-continuous brief episodes of sudden left-sided wrist drop are noticeable.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.cnp.2026.07.003.
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Supplementary Materials
The patient was asked to raise both arms in front of him. In addition to the slow drift of the left arm, quasi-continuous brief episodes of sudden left-sided wrist drop are noticeable.