The role of the intact hemisphere in recovery of midline muscles after recent monohemispheric stroke

Transcranial magnetic stimulation (TMS) of the motor cortex was used to stu dy basic mechanisms of motor reorganization after major hemispheric stroke in humans. We sought to clarify the possible role of the intact hemisphere in motor recovery of the lingual muscles, and to evaluate the compensatory use of preexisting uncrossed motor pathways projecting to these midline mus cles. TMS and bilateral surface recordings from the lingual muscles were ca rried out in six selected stroke patients who presented with a unilateral l ingual paralysis after a limited monohemispheric ischemia. The first examin ation was performed during the symptomatic stage (t(1)) and was repeated af ter complete recovery of lingual function had been established (t(2)) The c ortical motor output patterns were analyzed and compared with the data from 40 healthy controls. In the controls TMS of either hemisphere invariably p roduced contralateral and ipsilateral compound muscle action potentials (CM APs), elicited through crossed and uncrossed central motor pathways, respec tively. In most individuals an asymmetric cortical motor output pattern was found, as significantly greater mean CMAPs of shorter onset latencies were recorded from the contralateral lingual muscles than from the ipsilateral responses. In the six patients with a unilateral lingual paralysis a simila r pattern was found on initial examination by stimulating the intact hemisp here, whereas TMS of the affected hemisphere failed to elicit any CMAP bila terally. At t(2) all patients had regained normal lingual function. Only on e patient showed evidence of a complete recovery of the primarily affected hemisphere, as TMS now elicited normal CMAPs bilaterally. In the remaining five patients the unilateral interruption of the corticonuclear pathways pe rsisted in spite of complete functional recovery. In these subjects the rec overy of symmetric lingual movements must be attributed to the intact hemis phere. From this it is concluded that recovery of a unilateral lingual para lysis after restricted monohemispheric lesions is possible without recovery of the cortical motor projections from the affected hemisphere. in these c ases the intact hemisphere is responsible for restoration of normal lingual movements, most likely by potentiating the effect of preexisting uncrossed motor pathways.