Abstract
[Purpose] The magnetic field evoked from the cervical spinal nerve after median nerve stimulation at the elbow was measured by magnetoneurography (MNG), and the conduction of nerve activity in the upper cervical spinal cord could not be confirmed. In this study, we measured the evoked magnetic field of the upper cervical spinal cord after the axillary median nerve stimulation, and visualized the reconstructed current in healthy subjects. [Material & Method] The subjects were five healthy adults without limb neurological symptoms, five nerves, aged 26–60 years (average 41 years), four males and one female. The median nerve was stimulated between the biceps and triceps muscles 15–17 cm proximal from the medial epicondyle of the humerus, and the evoked magnetic field of the upper cervical nerve was measured by using MNG. We monitored the compound muscle action potential (CMAP) of the abductor digiti minimi muscle to avoid current spread to the ulnar nerve. The stimulus intensity was set at the supramaximal level of CMAP of the abductor pollicis brevis muscle (6.6–9.4 mA, average 7.68 mA), with a stimulus frequency of 5 Hz and a stimulus duration of 0.3 ms. The evoked magnetic fields were recorded by the MNG system using a 40-kHz sampling rate and 100–5000-Hz bandpass filter, and 4000 responses were averaged. [Result] In all five cases, the magnetic field data derived from nerve activity in the upper cervical spinal cord was observed. The intensity of magnetic fields evoked by median nerve stimulation at axillar were about 1.5–2 times stronger than at the elbow. In particular, the upward propagation of inward reconstructed current, which is thought to correspond to the depolarization point on the nerve membrane, was observed up to the C2 level in three of the five cases and up to the brainstem area in the remaining two cases. The average conduction velocity estimated from the reconstructed current was 72.8 m/sec (65–78 m/sec), and visualization of nerve activity was confirmed by superimposing simple cervical spine X-ray images. [Conclusion] Visualization of the evoked magnetic fields in the cervical spinal cord has already been reported by Sumiya et al. However, those in the upper cervical spinal cord could not be obtained by the conventional method due to the influence of temporal dispersion, but the axillary stimulation made recording and visualization of nerve activity in the upper cervical spinal cord possibility. In the future, MNG is expected to be a useful test for functional image evaluation of upper cervical spinal cord diseases.
