Induction of cortical plastic changes in wrist muscles by paired associative stimulation in healthy subjects and post-stroke patients.

It has been shown on hand muscles in normal subjects that paired associative stimulation (PAS) combining peripheral nerve stimulation and transcranial magnetic stimulation (TMS) induces lasting changes in cortical motor excitability (Stefan et al., Brain 123 (Pt3):572-584, 2000). Because the motor recovery of distal upper limb and particularly wrist extension in post-stroke patients is one of the major rehabilitation challenge, we investigate here the effect of one session of paired associative stimulation on the excitability of the corticospinal projection to extensor carpi radialis (ECR) muscle (motor evoked potential size) before and after PAS in 17 healthy subjects and in two patients 5 months after stroke. The time course, the topographical specificity, changes in rest motor threshold (RMT), short intracortical inhibition and intracortical facilitation (SICI and ICF), the respective role of cutaneous and muscular afferents and the effect of a prolonged peripheral stimulation alone were also studied in normal subjects. Using a protocol derived from that of Ridding et al. J Physiol 537:623-631 (2001), PAS was able to induce lasting changes in the excitability of corticospinal projection to wrist muscles in healthy subjects and in the two post-stroke patients studied. Electrophysiological features of these plastic changes were similar to those previously observed in hand muscles: rapid evolution, 30-60 min duration, reversibility, relative topographical specificity and associative dependence suggesting an LTP-like mechanism. A contribution of cutaneous afferents in inducing PAS effects was also demonstrated. The decrease in ECR RMT after PAS observed in patients and in healthy subjects was an unexpected result because it has not been previously reported in the hand muscles of healthy subjects. However, it has been observed in dystonic patients (Quartarone et al., Brain 126:2586-2596, 2003). This suggests that other mechanisms like changes in membrane excitability could be involved in ECR facilitation after PAS. Further studies performed on patients using daily repeated PAS protocols and showing a functional improvement in hand motor function will be necessary to confirm that this technique could be relevant in motor rehabilitation, at least for some selected patients.