Magnetoneurography of evoked compound action currents in human cervical nerve roots

Objective: A measurement protocol for magnetoneurography (MNG) is established which allows the non-invasive localization and tracing of evoked compound action currents propagating along cervical nerve roots in man. Methods: Inside a magnetically shielded room either both median or both ulnar nerves of healthy subjects were conventionally electrostimulated in alternation. Evoked magnetic responses were recorded using a multichannel SQUID-detector with a planar measuring area centered over the neck. Simultaneously, electric surface potentials were recorded using cervical bipolar electrode montages. Results: Upon median (ulnar) nerve stimulation somatosensory evoked magnetic fields up to 20 fT (10 fT) amplitude were detected propagating over the cervical transforaminal root entry zone, with corresponding electrical surface potentials of 1.5 μV (0.5 μV). Furthermore, the signal-to-noise ratio of the spatiotemporal magnetic field mappings in median nerve stimulation experiments allowed dipolar source reconstructions and tracing of the propagation of the compound action currents along nerve root fibers. Conclusion: Magnetoneurography allows tracing of the propagation of evoked compound action currents along cervical roots in healthy subjects with millisecond temporal and high spatial resolution. Thus, MNG offers a sensitivity appropriate to serve as a clinical diagnostic tool for localizing focal neuropathies of cervical nerve roots.