Long-term reorganization of motor cortex outputs after arm amputation

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Abstract
Objective: To investigate the reorganization of the corticospinal system long after arm amputation at different levels. Methods: Focal transcranial magnetic stimulation (TMS) was performed in 15 patients 21 to 65 years after arm amputation at the level of the forearm, upper arm, or shoulder. Cortically elicited electromyographic responses were investigated in muscles immediately proximal to the stump. TMS was performed on a skull surface grid overlying the motor cortex. The response threshold, number of effective stimulation sites, and the sum of the amplitudes elicited at these sites were evaluated for slightly contracted muscles. Results: Seven of eight patients with forearm amputation had larger stimulation effects in the biceps supplied by the motor cortex contralateral to amputation, as indicated by variable patterns of lowered response thresholds, increased response amplitudes, or increased numbers of effective stimulation sites. In seven patients with a more proximal amputation, the motor responses were investigated in the deltoid and trapezoid muscle. In only two of them, the motor cortex contralateral to amputation showed an increased excitability. Three patients presented with a higher excitability of the motor cortex contralateral to the intact arm and two with a balanced type of excitability. Conclusion: Reorganization of the motor system can be present more than 20 years after amputation. Furthermore, differential patterns of reorganized corticospinal output were found for different stump muscles, which might be due to varying amounts of ipsilateral corticospinal projections.