Magnetic stimulation of the lumbosacral vertebral column in children: normal values and possible sites of stimulation.

We describe a technique of lumbosacral magnetic stimulation (MS) using an 8-shaped coil in 44 normal children and 7 normal adults on the lower limb muscles. In most child subjects and some adult subjects, we observed two peaks in amplitude of the motor evoked potential (MEP) following MS when the coil was moved caudally from Th8 to S2. The rostral peaks of MEP were elicited at L1/2, and the caudal peaks were elicited below L3. Rostral peak amplitudes were considerably higher in younger children than in adolescents and adults. MEP latencies tended to decrease as the coil was moved from Th11 to L4/5, and were almost constant at caudal to L4/5 in most child subjects, especially in younger children. Comparison of MEP latencies with the peripheral motor conduction times by the F technique suggests that the proximal sites of excitation of the nerve root are 0.0-0.6 ms conduction times distal to the motoneuron, while the distal sites of excitation are 2.1-2.9 ms conduction times distal to the motoneuron. We conclude that performing MS using the 8-shaped coil enables us to excite the nerve roots near the motoneuron as well as those near the intervertebral foramen, and that MEPs are elicited more easily in younger children than in adolescents and adults.