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 study 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 muscles. TMS and bilateral surface recordings from the lingual muscles were carried out in six selected stroke patients who presented with a unilateral lingual paralysis after a limited monohemispheric ischemia. The first examination was performed during the symptomatic stage (t1) and was repeated after complete recovery of lingual function had been established (t2). The cortical motor output patterns were analyzed and compared with the data from 40 healthy controls. In the controls TMS of either hemisphere invariably produced contralateral and ipsilateral compound muscle action potentials (CMAPs), elicited through crossed and uncrossed central motor pathways, respectively. 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 ipsitateral responses. In the six patients with a unilateral lingual paralysis a similar pattern was found on initial examination by stimulating the intact hemisphere, whereas TMS of the affected hemisphere failed to elicit any CMAP bilaterally. At t2 all patients had regained normal lingual function. Only one 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 persisted in spite of complete functional recovery. In these subjects the recovery of symmetric lingual movements must be attributed to the intact hemisphere. From this it is concluded that recovery of a unilateral lingual paralysis after restricted monohemispheric lesions is possible without recovery of the cortical motor projections from the affected hemisphere. In these cases the intact hemisphere is responsible for restoration of normal lingual movements, most likely by potentiating the effect of preexisting uncrossed motor pathways.