Magnetic and electrical stimulation of cervical motor roots: technique, site and mechanisms of excitation.

Cervical motor roots and the brachial plexus were excited transcutaneously with magnetic (MagStim) and electrical stimulation (ElStim) applied dorsally over the spine and over the supraclavicular fossa (Erb's point). The compound muscle action potentials (CMAPs) from the abductor digiti minimi (ADM) and the biceps muscles (BICEPS) could be evoked with either stimulating technique in all 52 subjects tested. With MagStim over the spinous process C7, greater CMAPs were obtained from ADM (p less than or equal to 0.0001, paired t test) and BICEPS (p less than or equal to 0.005) when the inducing current in the coil as viewed from behind was clockwise for the right arm and vice versa. ElStim with the cathode over C7/T1 and the anode directed cranially provided greater CMAPs from the ADM (p less than or equal to 0.0001) and smaller CMAPs from the BICEPS (p less than or equal to 0.01) than with the inverse polarity. MagStim of the cervical roots provided CMAPs which were smaller from ADM (p less than or equal to 0.0001), and greater from BICEPS (p less than or equal to 0.0001), than ElStim (cathode C7/T1), whereas latencies did not differ significantly (p less than or equal to 0.3). When comparing ElStim and MagStim applied over Erb's point, the former yielded greater CMAPs and 0.5 ms longer latencies from both the ADM and BICEPS (p less than or equal to 0.001). From these data and additional studies in four patients, including direct intraoperative root stimulation in one of them, it is concluded that ElStim and MagStim over the spine excite the motor roots at a similar site, that is, within a few cm outside the intervertebral foramina. The site of stimulation is difficult to predict and depends on the placement of the stimulating devices and the intensities used. In contrast, MagStim of the brachial plexus over Erb's point occurs on average about 3.5 cm distal to the site of ElmStim.