Giant early components of somatosensory evoked potentials to tibial nerve stimulation in cortical myoclonus

Abstract

Enlarged cortical components of somatosensory evoked potentials (giant SEPs) recorded by electroencephalography (EEG) and abnormal somatosensory evoked magnetic fields (SEFs) recorded by magnetoencephalography (MEG) are observed in the majority of patients with cortical myoclonus (CM). Studies on simultaneous recordings of SEPs and SEFs showed that generator mechanism of giant SEPs involves both primary sensory and motor cortices. However the generator sources of giant SEPs have not been fully understood as only one report describes clearly giant SEPs following lower limb stimulation. In our study we performed a combined EEG-MEG recording on responses elicited by electric median and tibial nerve stimulation in a patient who developed consequently to methyl bromide intoxication CM with giant SEPs to median and tibial nerve stimuli. SEPs wave shapes were identified on the basis of polarity-latency components (e.g. P15-N20-P25) as defined by earlier studies and guidelines. At EEG recording, the SEP giant component did not appear in the latency range of the first cortical component for median nerve SEP (N20), but appeared instead in the range of the P37 tibial nerve SEP, which is currently identified as the first cortical component elicited by tibial nerve stimuli. Our MEG and EEG SEPs recordings also showed that components in the latency range of P37 were preceded by other cortical components. These findings suggest that lower limb P37 does not correspond to upper limb N20. MEG results confirmed that giant SEFs are the second component from both tibial (N43m-P43m) and median (N27m-P27m) nerve stimulation. MEG dipolar sources of these giant components were located in the primary sensory and motor area.

Keywords: Cortical myoclonus; Magnetoencephalography; Motor cortex; Somatosensory evoked field; Somatosensory evoked potential; Tibial nerve stimulation.

Fig. 1

Median nerve SEP a) Mean SEPs for the patient (blue line) and for a representative subject of the control group (red line). SEPs from the central (C3, C4) and frontal (F3, F4) electrode following the right and left median nerve stimulation respectively are shown. b) Flux map of the patient and a representative control subject for the three components at 20 ms, 25 ms and 33 ms from the stimulus onset. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Tibial nerve SEP a) Mean SEPs for the patient (blue line) and for a representative subject of the control group (red line). SEPs from the centro-frontal channel are shown. b) Flux map of the patient and a representative control subject for the three components at 30 ms, 37 ms and 45 ms from the stimulus onset. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Median and tibial nerve SEF a) Mean SEFs from two magnetic channels following the left and right median nerve stimulation showing four components: N22m-P22m (red line), N27m-P27m (blue line), N33m-P33m (green line), and N45m-P45m (yellow line). b) MEG field topography on patient structural MRI at 22 ms, 27 ms, 33 ms and 45 ms. c) Mean SEFs from four magnetic channels following the left and right tibial nerve stimulation showing six components: N34m-P34m (red line), N43m-P43m (blue line), N46m-P46m (green line), N50m-P50m (pink line), N67m-P67m (yellow line), N79m-P79m (cyan line). d) MEG field topography on patient structural MRI at 34 ms, 43 ms, 46 ms, 50 ms, 67 ms and 79 ms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)