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Transcutaneous magnetic and electrical stimulation over the cervical spine: excitation of plexus roots-rather than spinal roots.

Percutaneous magnetic stimulation (MagStim) and electrical stimulation (ElStim) over the cervical spine were performed in 65 subjects. Compound muscle action potentials (CMAPs) from the abductor digiti minimi (ADM) and the biceps muscles (BICEPS) could be recorded in all subjects tested with both, ElStim and MagStim. Maximal stimulation as determined from the amplitudes of CMAPs was possible in most cases with ElStim but not with MagStim. Latencies after MagStim (centered over C7) and ElStim (cathode over C7/T1, anode positioned 6 cm cephalad in the midline) were not statistically significantly different (P = 0.3). However, in the individual case, it was difficult to predict the precise site at which the motor roots would be activated, since latencies of CMAPs to ElStim and MagStim could differ by up to 1.2 msec. In addition, the excitation site migrated distally when the stimulation intensity was increased with the aim to obtain CMAPs of nearly maximal amplitudes. The results of studying brachial plexus stimulation as well in 22 of the subjects led us to the conclusion that excitation of the motor roots occurs on the average 7 cm (range: 1.4-8.2 cm) proximally to Erb's point. This was confirmed by the finding that when the C7 motor root was stimulated directly intraoperatively within the intervertebral foramen, the excitation site was calculated to be 7.7 cm distal to the foramen and 8.4 cm proximal to Erb's point.