Long-term reorganization of motor cortex outputs after arm amputation

Objective: To investigate the reorganization of the corticospinal system lo ng after arm amputation at different levels. Methods: Focal transcranial ma gnetic stimulation (TMS) was performed in 15 patients 21 to 65 years after arm amputation at the level of the forearm, upper arm, or shoulder. Cortica lly elicited electromyographic responses were investigated in muscles immed iately proximal to the stump. TMS was performed on a skull surface grid ove rlying the motor cortex. The response threshold, number of effective stimul ation sites, and the sum of the amplitudes elicited at these sites were eva luated for slightly contracted muscles. Results: Seven of eight patients wi th 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 delt oid and trapezoid muscle. In only two of them, the motor cortex contralater al 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: Reorganizati on 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 o f ipsilateral corticospinal projections.