Epileptic Negative Myoclonus with Lateralized Periodic Discharges: Insights into Pathophysiology
Persen Sukpornchairak1, Joshua Edmondson1, Neel Fotedar1
1Neurology, Case Western Reserve University, University Hospitals Cleveland Medical Center
Objective:
To report two cases of epileptic negative myoclonus (ENM) associated with lateralized periodic discharges (LPDs).
Background:
Negative myoclonus is a jerky movement characterized by a sudden, brief, and involuntary interruption of sustained muscle activity, known as silent period (SP). ENM is defined by the SP being time-locked to an epileptiform discharge like a spike. ENM has been described in chronic epilepsy syndromes but not in acute neurological conditions with LPDs.
Design/Methods:
We analyzed the EEG and surface-EMG recordings of two patients with ENM associated with LPDs in the setting of acute pathologies (subdural hematoma and viral encephalitis).
Results:
Both patients had contralateral facial clonic seizures on presentation. The facial clonic contractions were time-locked to LPDs with superimposed fast activity (LPD+F) at >2Hz frequency. As the seizures subsided by day five-six, the LPDs became blunt with a reduced frequency of 0.5-1Hz. With sustained muscle contraction, brief type I SPs were noted on sEMG electrodes of the contralateral arm. These SPs were time-locked to the LPDs. The maximum initial negativity of the LPDs localized to the parieto-central region. The LPD-SP latency varied from 20-25ms to 50-60ms.
Conclusions:
Our results indicate that ENM can occur in acute epileptogenic neurological insults like subdural hematoma and encephalitis with the SPs being time-locked to LPDs. The most likely generator is post-central (S1) as indicated by the localization of the LPDs and SPs being type I only. ENM tends to develop late in the course as the LPDs become less frequent and the superimposed fast activity disappears (“improved epileptogenicity”). Cortical stimulation studies have similarly shown that “low-intensity” stimulation of the peri-rolandic cortex generates inhibition in deeper cortical layers which can interrupt sustained muscle contraction. “High-intensity” stimulation produces high-frequency oscillations which activate the pyramidal tract neurons (descending I-waves), hence producing a positive twitch.