MOTOR EVOKED-POTENTIALS IN UNILATERAL LINGUAL PARALYSIS AFTER MONOHEMISPHERIC ISCHEMIA

Objectives-The occurrence of a lingual paralysis after unilateral uppe r motor neuron lesions is an infrequent clinical phenomenon, and the u nderlying pathophysiological mechanisms are poorly understood. We stud ied the cortical motor representations of ipsilateral and contralatera l Lingual muscles in healthy controls and in a selected group of strok e patients, to clarify the variable occurrence of a lingual paralysis after recent monohemispheric ischaemia. Methods-A special bipolar surf ace electrode was used to record the ipsilateral and contralateral com pound muscle action potentials (CMAPs) from the Lingual. muscles after transcranial magnetic stimulation (TMS) of the human motor cortex and peripheral electrical stimulation (PES) of the hypoglossal nerve medi al to the angle of the jaw. Four patients with a Lingual paralysis (gr oup 1) and four patients with symmetric lingual movements (group 2) af ter monohemispheric first ever stroke were studied and compared with 4 0 healthy controls. Results-In controls, TMS of either hemisphere inva riably produces CAMPs in the ipsilateral and contralateral Lingual mus cles, elicited through crossed and uncrossed central motor pathways, r espectively. In the 40 healthy controls, TMS of either hemisphere elic ited CMAPs of significantly greater amplitudes and shorter onset laten cies from the contralateral muscles compared with the ipsilateral resp onses (p < 0.0001). In the patient groups, TMS of the affected hemisph ere failed to evoke any CMAP from either lingual side; TMS of the unse vered hemisphere always produced normal ipsilateral and contralateral responses, irrespective of whether the ipsilateral muscles were paraly sed or not. Conclusions-Bilateral crossed and uncrossed corticonuclear projections are invariably existent in humans. After unilateral inter ruption of these pathways, some people do exhibit a lingual paralysis whereas others do not. The development of a central lingual paralysis is most likely dependent on the ability of the unsevered hemisphere to utilise the pre-existent uncrossed motor projections. The variable av ailability of these pathways among individual subjects is in good agre ement with the inconstant occurrence of a lingual paralysis after rest ricted monohemispheric lesions.