Somatosensory evoked magnetic fields from the primary somatosensory cortex (SI) in acute stroke

We recorded somatosensory evoked magnetic fields (SEFs) to median nerve stimulation from 15 patients in the acute stage (1-15 days from the onset of the symptoms) of their first-ever unilateral stroke involving sensorimotor cortical and/or subcortical structures in the territory of the middle cerebral artery (MCA). Neuronal activity corresponding to the peaks of the N20m, P35m and P60m SEF deflections from the contralateral primary somatosensory cortex (SI) was modelled with equivalent current dipoles (ECDs), the locations and strengths of which were compared with those of an age-matched normal population. Four patients with pure motor stroke had symmetric SEFs. In one of the 4 patients with pure sensory stroke, and in 5 of the 7 patients with sensorimotor paresis, the SEFs were markedly attenuated or missing. All except one patient with abnormal SEFs had deficient two-point discrimination ability; especially the attenuation of N20m was more clearly correlated with two-point discrimination than with joint-position or vibration senses. Of the different SEF deflections, P35m and P60m were slightly more sensitive indicators of abnormality than N20m, the former being affected in two patients with symmetric N20m. Three patients with pure sensory stroke and lesions in the opercular cortex had normal SEFs from SI. We conclude that the SEF deflections N20m, P35m and P60m from SI are related to cutaneous sensation, in particular discriminative to touch. The results also demonstrate that basic somatosensory perception can be affected by lesions in the opercular cortex in patients with functionally intact SI.