Interhemispheric differences of sensory hand areas after monohemispheric stroke: MEG/MRI integrative study

Seventeen clinically stabilized monohemispheric stroke patients were studied in order to investigate the chronic topographical modifications induced on primary sensory cortical hand areas by a monohemispheric stroke within the middle cerebral artery territory. Magnetoencephalographic (MEG) localization of the cortical areas activated following electrical separate stimulation of the median nerve, thumb, and little fingers was integrated with magnetic resonance imaging. Spatial localization of Equivalent Current Dipoles (ECDs) of the short-latency cortical responses generated in primary sensory cortices, "hand area" (distance between 1st and 5th digits ECDs), interhemispheric differences of such parameters, as well as of somatosensory-evoked fields waveshapes were investigated and compared with a control population. Lesions involving the cortico-subcortical areas receiving sensory input from the hand induced excessive asymmetry of MEG spatial parameters and response morphology between the unaffected (UH) and the affected hemisphere (AH). "Hand area" was significantly larger on AH in 20% of cases after a subcortical, and in 13% after a cortical, lesion. Responses from AH were excessively delayed in 20% ECDs. Interhemispheric ECDs strength differences were larger than normal in 25% of cases after both types of lesions; the strength in the AH being enlarged after all cortical, and only 24% of subcortical strokes. In a significant percentage of monohemispheric strokes, excessive interhemispheric differences were found between AH and UH, suggesting that brain areas outside the normal boundaries and usually not reached by a dense sensory input from the opposite hand and fingers may act as somatosensory "hand" centers. Correlation analysis between clinical outcome and cortical reorganization in the AH suggests that this mechanism is linked with hand sensorimotor recovery.