Somatosensory evoked magnetic fields following electric tongue stimulation using pin electrodes

Quantitative evaluation of the sensory disturbance of the tongue is important clinically. However, because the conventional electrophysiological approach to the peripheral nerve cannot be used in the mandible owing to the deep route of the lingual nerve, we applied evoked potentials in the central nervous system. Somatosensory evoked magnetic fields (SEFs) following electric stimulation were recorded in 10 healthy subjects by means of pin electrodes placed on the tongue mucosa. Three or four components (P25m, P40m, P60m, and P80m) were identified over the contralateral hemisphere with unilateral stimulation. Because none of the components were consistently detected in all subjects, we evaluated the root mean square (RMS) of 18 channels over the contralateral hemisphere. To estimate the activated cortical response, we calculated the difference in mean RMS amplitude between 10 and 150 ms and that of the baseline period (aRMS=RMS[10, 150]-RMS[-50, -5]). The aRMS values for right-sided and left-sided stimulation were 10.18+/-7.92 and 10.99+/-8.98 fT/cm, respectively, and the mean laterality index, expressed by [(left-right)/(left+right)] was 0.025+/-0.104. This parameter can be useful for evaluating patients with unilateral sensory abnormality of the tongue.