Magnetoneurographic registration of propagating magnetic fields in the lumbar spine after stimulation of the posterior tibial nerve.

Stimulation of the posterior tibial nerve has been associated with different somatosensory evoked potentials (SEP) recorded along the spine and thorax. The aim of this study was to register and describe the magnetic fields corresponding to different components of spinal SEP after stimulation of tibial nerves. In nine healthy subjects, right and left posterior tibial nerves were transcutaneously electrostimulated at the ankles. Neuromagnetic fields were registered over a circular 800 cm(2) area of the lumbosacral spine using a 61-channel biomagnetometer. Magnetic field maps were constructed and examined visually for dipolar patterns. Equivalent current dipoles (ECD) were calculated for each somatosensory evoked field (SEF) using a least-squares fit in a spherical model. In seven subjects dipolar SEF were detected over the lower back at two separate latencies and locations and propagating ECD could be localized. Both the first and second components found agreed anatomically and functionally with respect to propagation in the underlying nerve fibers. It was possible to record and identify SEF which correspond to the SEP described in the literature. Dipole localization based on an equivalent current dipole model allowed a basic evaluation of the plausibility of the measurements with respect to the processes being examined.