T112. Visualization of electrical activities in the carpal tunnel area by magnetoneurography of median nerve

Abstract

Introduction In recent years, diagnosis of the carpal tunnel syndrome is mainly performed by electrophysiological examination. In particular, the utility and reliability of inching method were demonstrated by some reports. However, electrophysiological examination is affected by the surrounding tissues such as bone and soft tissue, and the inching method is not technically easy and requires many electrodes. On the other hand, the magnetic field is not affected by the surrounding tissue. We have developed the magnetometer for the spinal cord and the spinal nerve activity and succeeded in measuring electrophysiological activity in the spinal cord and spinal nerve roots. Now, we are working on magnetoneurography (MNG) which is measurement of magnetic fields generated by the peripheral nerve. In this study, we aimed to evaluate spatiotemporal spread of digital nerve-evoked currents in the median nerve at the carpal tunnel area using MNG. Methods Using a newly developed superconducting quantum interference device (SQUID) magnetometer, neuromagnetic fields of 9 healthy volunteers’ hands were measured at the surface of the carpal tunnel area after stimulation of the digital nerves at the index or middle finger. Current sources were estimated using spatial filter techniques and were superimposed on X-ray images of the hand. We set a virtual electrode in the carpal tunnel along the median nerve. We reconstructed the current waveform from the magnetic field and calculated the nerve conduction velocity from the peak latency. We also measured sensory nerve action potential just above the carpal tunnel, 3 cm distal and 3 cm proximal to the carpal tunnel in response to stimulation of the digital nerves of the index finger. The conduction velocity was calculated from the peak latency. Results Neuromagnetic fields propagating from the finger to the wrist were successfully measured in all subjects. Distribution of action currents calculated from MNG showed the axonal activity pattern showing the intra-axonal current and the depolarization/ repolarization current. Nerve conduction velocity estimated from the MNG was 55.3 m/s and corresponded with the sensory nerve conduction velocity. Conclusion We could visualize action current at any point in carpal tunnel with high resolution using MNG. Moreover, MNG is not affected by the surrounding bone and soft tissues and can be fused with morphological images such as X-rays. MNG is expected to contribute to the clinical diagnosis and treatment of carpal tunnel syndrome.