Magnetomyographic evaluation of motor unit size that is robust to changes in distance to sensor

In clinical practice, motor unit (MU) size is evaluated using needle electromyography to diagnose the cause of muscle weakness, whether myogenic or neurogenic. However, needle electromyography is influenced by the conductance of the muscle tissue and the distance from the MU to the electrode. In contrast, the magnetic field generated by a skeletal muscle is not distorted by subcutaneous tissues because their magnetic permeability is considered equal to that of free space. Therefore, we hypothesized that MU amplitude can be measured via magnetic field recordings. We tested this hypothesis by recording MU activity in the abductor pollicis brevis muscle. We then evaluated the MU size difference between healthy individuals and patients with spinal muscular atrophy and spinal-bulbar muscular atrophy. Furthermore, we assessed whether our method could consistently evaluate the MU size regardless of the sensor-muscle distance. Myomagnetic fields of single MUs evoked by electrical stimulation of the median nerve were measured. We used a biomagnetometer equipped with 132-channel superconducting quantum interference device sensors orientated upwards and arrayed on a quasi-planar surface. We applied a spatial filtering method that can estimate the current distribution from the magnetic field even for a conductor with unknown configuration or conductivity distribution and that does not require an a priori assumption of how many source currents are present. We visualized the electrical activity of 12 MUs of the abductor pollicis brevis muscle from eight healthy individuals and of two MUs from two patients with spinal muscular atrophy and spinal-bulbar muscular atrophy. Four current patterns were identified in the MU electrical activity. In all MUs, current towards the innervation zone was observed just after the start of activities. We called this current 'initial muscle-directing current'. At the same time, currents directed proximally and distally from the middle of the muscle were observed in most MUs. Initial muscle-directing current was more than 3 or 10 times larger in patients with spinal muscular atrophy and spinal-bulbar muscular atrophy than in healthy individuals. Initial muscle-directing current was estimated to weaken by 5.5% for every 5-mm increase in distance from the sensor array. Initial muscle-directing current is considered to reflect the activity near the neuromuscular junction and can be an index of MU size. The results confirmed our hypothesis that MU amplitude can be evaluated using magnetic measurements. This novel and non-invasive magnetomyography method can evaluate MU size with little influence of distance and has the potential to supersede needle electromyography.