Revisiting interhemispheric imbalance in chronic stroke: A tDCS study.

OBJECTIVE: Chronic stroke patients with moderate-severe motor impairment may have an increased reliance on contralesional vs ipsilesional motor areas to control the paretic arm. We hypothesised that increasing contralesional excitability with anodal transcranial direct current stimulation (a-tDCS) would benefit motor performance in patients with moderate-severe impairment.

METHODS: Ten patients with motor impairment at the chronic stage after stroke received a-tDCS, cathodal (c-tDCS) and sham with the target electrode over contralesional motor cortex (M1). Motor performance was quantified from the circularity and size of planar movements made with the paretic arm. Contralateral and ipsilateral corticospinal excitability was inferred using transcranial magnetic stimulation. Corticospinal tract integrity and basal GABA concentration were assessed with magnetic resonance imaging and spectroscopy.

RESULTS: Anodal tDCS increased contralesional corticomotor excitability evident from motor evoked potentials in both wrist extensors (both P<0.043). Cathodal tDCS did not affect corticomotor excitability (P>0.37). The effect of tDCS on motor performance with the paretic limb was negatively associated with ipsilesional GABA concentration after c-tDCS (P=0.001).

CONCLUSIONS: Further investigation of noninvasive brain stimulation protocols that facilitate contralesional M1 is warranted.

SIGNIFICANCE: The inter-hemispheric imbalance model of stroke recovery may not apply to patients with more severe impairment.