Visualization of electrophysiological activity in patients with carpal tunnel syndrome using magnetoneurography

Abstract

Carpal tunnel syndrome (CTS) is mainly diagnosed via electrophysiological examination. However, electrophysiological examinations are affected by the surrounding bone and soft tissue. Because magnetic fields are not affected by the surrounding tissue and theoretically have high spatial resolution, we developed magnetoneurography (MNG) to measure neuromagnetic fields from peripheral nerves. In this study, we used MNG to evaluate electrophysiological activity in CTS patients. Using a SQUID magnetometer, the neuromagnetic fields of 10 CTS hands were measured at the surface of the carpal tunnel in response to stimulation of the median nerve at the elbow joint. Current sources were estimated using spatial filtering techniques and were superimposed on X-ray images of the hand. We set virtual electrodes along the median nerve and calculated the current waveform. Because noise reduction methods enabled us to clarify magnetic fields that are small and buried in noise, neuromagnetic fields propagating in the proximal to distal direction were successfully measured in all hands. The evoked currents reconstructed using the spatial filter technique and converted to a psuedocolour map were superimposed on an X-ray images of the hands. The nerve action currents calculated from MNG were attenuated at the carpal tunnel in all hands. MNG visualizes propagating median nerve excitation with high spatial and temporal resolution, enabling quantitative evaluation of conduction block in CTS patients. MNG is expected to contribute to the clinical diagnosis and treatment of CTS.