Motor responses after transcranial electrical stimulation of cerebral hemispheres with a degenerated pyramidal tract.

Motor responses were evoked in the thenar muscles by transcranial electrical cortex stimulation in 5 stroke patients with an isolated lacuna in the internal capsule, in whom wallerian degeneration of the pyramidal tract was demonstrated in vivo. Suprathreshold stimulation of the affected hemisphere elicited bilateral motor responses; whereas, stimulation at identical intensities of the undamaged hemisphere yielded strictly unilateral responses in the contralateral hand, like the responses of all normal control subjects. Focused magnetic brain stimulation was performed in 1 patient and gave identical results. Because muscular excitability to cortical stimulation is preserved in spite of pyramidal tract disruption, other pathways must bypass the lesion. Because of the bilaterality of responses, we suggest polysynaptic corticoreticulospinal connections.