Mapping of motor cortical reorganization after stroke. A brain stimulation study with focal magnetic pulses.

Abstract

Focal transcranial magnetic stimulation (TCS) is used for noninvasive and painless mapping of the somatotopical organization of the motor cortex. TCS mapping of motor cortical output to the abductor digiti minimi (ADM) muscle was followed up in monohemispheric stroke patients by evaluating motor evoked potentials (MEPs). This approach allowed noninvasive investigation of the functional reorganization of hand motor areas. Motor maps were constructed for 15 subacute stroke patients about 2 months from the ictus by recording MEPs from the ADM muscle via focal TCS in the affected hemisphere (AH) and unaffected hemisphere (UH) at the beginning of (T1) and after 8 to 10 weeks of neurorehabilitation (T2). Barthel Index and Canadian Neurological Scale scores were evaluated as well. An age-sex matched group of 15 healthy control subjects was enrolled to establish normative data. MEP excitability threshold was significantly higher in the AH of stroke patients than in normal subjects and in the UH (P < .001); excitability threshold was not significantly different between normal subjects and UH. In the AH, MEPs were significantly (P < .001) delayed in latency both in T1 and T2, with a significant decrease of the extenuation of motor output area to the ADM muscle (P < .05) in T1 versus control group and UH. This area was significantly enlarged (P < .05) in T2. Amplitude of MEPs from the AH, both at rest and during voluntary contraction, was significantly lower than normal in T1 (P < .001); it increased in T2 (P < .01) during relaxation but was still smaller than normal during contraction (P < .001). In combination with these findings, an improvement of Barthel Index and Canadian Neurological Scale scores (P < .001) was observed between T1 and T2 (P < .001). Central conduction time was prolonged in stroke patients both in T1 and T2. Changes in the shape of motor maps in the AH during follow-up in T2 were either isolated (therefore increasing the interhemispheric asymmetry) or also were "mirrored" on the UH. Our neurophysiological data are consistent with the presence of a rearrangement of the motor cortical output area and correlate well with an improvement of motor performances. These findings confirm the existence in adults of a "plasticity" in the central nervous system that is still operating between 2 and 4 months from the acute ictal episode. The observed neurophysiological modifications are significantly correlated with clinical improvement of disability and clinical scores.