Repetitive transcranial magnetic stimulation and transcranial direct current stimulation in motor rehabilitation after stroke: An update

AbstractStroke is a leading cause of adult motor disability. The number of stroke survivors is increasing in industrialized countries, and despite available treatments used in rehabilitation, the recovery of motor functions after stroke is often incomplete. Studies in the 1980s showed that non-invasive brain stimulation (mainly repetitive transcranial magnetic stimulation [rTMS] and transcranial direct current stimulation [tDCS]) could modulate cortical excitability and induce plasticity in healthy humans. These findings have opened the way to the therapeutic use of the 2 techniques for stroke. The mechanisms underlying the cortical effect of rTMS and tDCS differ. This paper summarizes data obtained in healthy subjects and gives a general review of the use of rTMS and tDCS in stroke patients with altered motor functions. From 1988 to 2012, approximately 1400 publications were devoted to the study of non-invasive brain stimulation in humans. However, for stroke patients with limb motor deficit, only 141 publications have been devoted to the effects of rTMS and 132 to those of tDCS. The Cochrane review devoted to the effects of rTMS found 19 randomized controlled trials involving 588 patients, and that devoted to tDCS found 18 randomized controlled trials involving 450 patients. Without doubt, rTMS and tDCS contribute to physiological and pathophysiological studies in motor control. However, despite the increasing number of studies devoted to the possible therapeutic use of non-invasive brain stimulation to improve motor recovery after stroke, further studies will be necessary to specify their use in rehabilitation

Modulation of heteronymous reflexes from ankle dorsiflexors to hamstring muscles during human walking

In 16 human subjects, stimulation of the common peroneal nerve (CPN) was applied during walking and standing. The effect of the stimulation was evaluated from the rectified and averaged biceps femoris (BF) electromyographic (EMG) activity. In the swing phase of walking, the CPN stimulation evoked a suppression in the BF EMG in 12 of the subjects. In the early stance phase, the suppression was replaced by facilitation at a similar latency in 9 of the subjects. Of the other 3 subjects, in whom a suppression was observed during swing, a decrease in the suppression was observed in the stance phase in two of them. During a voluntary co-contraction of BF and tibialis anterior while standing, a suppression similar to that observed in the swing phase was observed. The thresholds of the suppression and facilitation were identical, suggesting that afferents of similar diameter were responsible. Cutaneous stimuli, which mimicked the sensation evoked by the CPN stimulation, but without activation of muscle afferents, did not produce similar effects in the BF EMG activity. It is suggested that the observed response and reflex reversal may reflect opening of an excitatory group I pathway in the early stance phase of walking with a concomitant shut-down of heteronymous group I inhibition

Modulation of non-monosynaptic excitation from ankle dorsiflexor afferents to quadriceps motoneurones during human walking

Modulation of non-monosynaptic excitation from ankle dorsiflexors to quadriceps (Q) motoneurones during human treadmill walking was investigated in 25 healthy human subjects. Stimulation of the common peroneal nerve (CPN) evoked a biphasic facilitation in the rectified and averaged (n = 50) Q electromyographic (EMG) activity between 0 and 100 ms after heel strike. Prior to heel strike, the stimulation had no effect on the Q EMG. The latency of both peaks in the response was too long to be explained by a monosynaptic pathway to Q motoneurones. During voluntary tonic co-contraction of Q and tibialis anterior (TA) while standing, only the first of the two peaks was evoked by the CPN stimulation despite a background EMG activity level in the Q and TA muscles corresponding to that observed 30–60 ms after heel strike during walking. Stimulation of cutaneous nerves did not evoke a similar biphasic facilitation in the Q motoneurones, which suggests that muscular afferents mediate the response. The second peak had a higher threshold than the earlier peak. During cooling of the CPN, the latency of the second peak was more prolonged than the latency of the earlier peak. This suggests that afferents of different diameters contributed to the two peaks. It is proposed that afferents from TA assist the contraction of Q during walking via spinal interneurones to stabilize the knee joint and maintain upright posture during walking

Reduction of common motoneuronal drive on the affected side during walking in hemiplegic stroke patients

The objective of this study was to use motor unit coupling in the time and frequency domains to obtain evidence of changes in motoneuronal drive during walking in subjects with stroke. Paired tibialis anterior (TA) EMG activity was sampled during the swing phase of treadmill walking in eight subjects with unilateral stroke. On the unaffected side, short-term synchronization was evident from the presence of a narrow central peak in cumulant densities and from the presence of significant coherence between these signals in the 10-25Hz band. Such indicators of short-term synchrony were either absent or very small on the affected side. Instead, pronounced 10Hz coupling was observed. It is suggested that reduced corticospinal drive to the spinal motoneurones is responsible for the reduced short-term synchrony and coherence in the 10-25Hz frequency band on the affected side in hemiplegic patients during walking.Significance: This is of importance for understanding the mechanisms responsible for reduced gait ability and development of new strategies for gait restoration

Monosynaptic Ia projections from intrinsic hand muscles to forearm motoneurones in humans

Heteronymous Ia excitatory projections from intrinsic hand muscles to human forearm motoneurones (MNs) were investigated. Changes in firing probability of single motor units (MUs) in the flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), flexor digitorum superficialis (FDS), extensor carpi radialis (ECR), extensor carpi ulnaris (ECU) and extensor digitorum communis (EDC) were studied after electrical stimuli were applied to the median and ulnar nerve at wrist level and to the corresponding homonymous nerve at elbow level.Homonymous facilitation, occurring at the same latency as the H reflex, and therefore attributed to monosynaptic Ia EPSPs, was found in all the sampled units. In many MUs an early facilitation was also evoked by heteronymous low-threshold afferents from intrinsic hand muscles. The low threshold (between 0.5 and 0.6 times motor threshold (MT)) and the inability of a pure cutaneous stimulation to reproduce this effect indicate that it is due to stimulation of group I muscle afferents.Evidence for a similar central delay (monosynaptic) in heteronymous as in homonymous pathways was accepted when the difference in latencies of the homonymous and heteronymous peaks did not differ from the estimated supplementary afferent conduction time from wrist to elbow level by more than 0.5 ms (conduction velocity in the fastest Ia afferents between wrist and elbow levels being equal to 69 m s−1).A statistically significant heteronymous monosynaptic Ia excitation from intrinsic hand muscles supplied by both median and ulnar nerves was found in MUs belonging to all forearm motor nuclei tested (although not in ECU MUs after ulnar stimulation). It was, however, more often found in flexors than in extensors, in wrist than in finger muscles and in muscles operating in the radial than in the ulnar side.It is argued that the connections of Ia afferents from intrinsic hand muscles to forearm MNs, which are stronger and more widely distributed than in the cat, might be used to provide a support to the hand during manipulatory movements