Cathodal Genesis of Ipsilateral Hand (Crossover) Motor Evoked Potentials: Evidence from a Patient with Previous Stroke.

A case is described where baseline transcranial electrical motor evoked potentials (TcMEP) and somatosensory evoked potentials (SSEP) results were unilaterally absent in a patient with previous hemispheric stroke undergoing a right-sided carotid endarterectomy. SSEP data confirmed right cortical pathology and excluded a technical rationale for absent motor evoked responses. Attempts at generating left-hand (contralateral) TcMEP from right cortical anodal stimulation failed despite high stimulus intensities. However, TcMEP responses from anodal stimulation of the right cortex were recorded from the right-hand (ipsilateral) which were attributed to "crossover." Ipsilateral TcMEP onset latencies derived from the stimulus-response data supports the idea that crossover is a product of cathodal stimulation initially acting on pericortical motor pathways.