Stimulation of corticospinal pathways at the level of the pyramidal decussation in neurological disorders

A newly developed technique of brainstem stimulation was applied in 14 normal subjects and 23 patients with various neurological disorders. The electromyographic (EMG) responses of limb muscles following cortical, brainstem and cervical stimulation were recorded. The cortical-brainstem conduction time and brainstem-cervical conduction time were then calculated from the difference in latency between the two sites of stimulation. From the regression lines of the relationship between the normal conduction times in the first dorsal interosseous muscle and the length of the descending tracts, the site of activation by brainstem stimulation was estimated to lie near the cervical-medullary junction. The most distal lesion causing prolongation of cortical-brainstem conduction time was a small cerebral infarction in the lower pons. Herniation of the third cervical spinal disc was the most rostral lesion resulting in delayed brainstem-cervical conduction time and normal cortical-brainstem conduction time. These observations suggest that activation occurs at the level of the cervical-medullary junction where the pyramidal decussation lies. The conduction velocities of the activated tracts estimated from the regression lines for normal individuals were 57-92 m/s. In patients with supratentorial lesions, the threshold for brainstem stimulation was abnormally high. The abnormal findings in this test were correlated significantly with the clinical pyramidal signs. This suggests that the EMG responses elicited by brainstem stimulation are mediated mainly by the corticospinal tract. We conclude that the brainstem stimulation technique would be clinically useful for localization of lesions in the corticospinal tract; the primary lesion can be localized whether above or below the pyramidal decussation.