Brain Advance Access originally published online on November 4, 2005
Brain 2006 129(1):65-81; doi:10.1093/brain/awh661
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Negative myoclonus induced by cortical electrical stimulation in epileptic patients
1 Department of Neurological Sciences, Bellaria Hospital, University of Bologna, Bologna, and 2 Epilepsy Surgery Center ‘C.Munari’, Niguarda Hospital, Milan, Italy
Correspondence to: Guido Rubboli, MD, Department of Neuroscience, Bellaria Hospital, Via Altura, 3–40139 Bologna, Italy E-mail: guido.rubboli{at}ausl.bo.it
Negative myoclonus (NM) is a motor disorder characterized by a sudden and abrupt interruption of muscular activity. The EMG correlate of NM is a brief (<500 ms) silent period (SP) not preceded by any enhancement of EMG activity (i.e. myoclonus). This study investigated the role of premotor cortex (PMC), primary motor cortex (MI), primary somatosensory area (SI) and supplementary motor area (SMA) in the pathophysiology of cortical NM by means of intracerebral low frequency (1 Hz) electrical stimulation. In three drug-resistant epileptic patients undergoing presurgical evaluation, we delivered single electric pulses (stimulus duration: 3 ms; stimulus intensity ranging from 0.4 to 3 mA) to PMC (2 patients), MI (1 patient), SI and SMA through stereo-EEG electrodes; surface EMG was collected from both deltoids. The results showed that (i) the stimulation of PMC or MI could evoke a motor evoked potential (MEP) either at rest or during contraction, in this latter case followed by an SP; however, in two patients, at the lowest stimulus intensities (0.4 mA), 50% of stimuli could induce a pure SP, i.e. not preceded by an MEP; raising the intensity of stimulation (0.6 mA), the SPs showed an antecedent MEP in >80% of stimuli; (ii) the stimulation of SI at low stimulus intensities (from 0.4 to 0.8 mA) induced in two patients only SPs, never associated with an antecedent MEP, whereas in the third subject the SPs could be inconstantly preceded by an MEP; by incrementing the stimulus intensity (up to 3 mA), in all three patients the SPs tended to be preceded, although not constantly, by an MEP; stimulus intensity affected SP duration (i.e. the higher the intensity, the longer the SP), without influencing the latency of onset of the SPs; (iii) the stimulation of SMA induced only pure SPs, at all stimulus intensities up to 3 mA; as for SI, increment of stimulus intensity was paralleled by an increase in SP duration, without influencing the onset latency of SPs. We conclude that single electric pulse stimulation of PMC, MI, SI and SMA through stereo-EEG electrodes can induce pure SPs, not preceded by an MEP, which clinically appear as NM, suggesting therefore that these cortical areas may be involved in the genesis of this motor phenomenon. However, it must be pointed out that SMA stimulation induced only pure SPs, regardless of the stimulus intensity, whereas occurrence of pure SPs following stimulation of PMC, MI, and SI depended mainly on the intensity of stimulation.
Key Words: negative myoclonus; silent period; supplementary motor area; primary somatosensory cortex; intracerebral electrical stimulation
Abbreviations: DCR = direct cortical response; MEP = motor evoked potential; NM = negative myoclonus; NMA = negative motor area; MI = primary motor area; PMC = premotor cortex; PNP = primary negative potential; SI = primary somatosensory area; SMA = supplementary motor area; SP = silent period; TMS = transcranial magnetic stimulation
Received December 2, 2004. Revised July 18, 2005. Accepted September 19, 2005.