Magnetoelectric stimulation of human motor cortex: normal values and potential safety issues in spinal cord injury

Upon magnetoelectric stimulation (MES) of the motor cortex, maximal amplitude and minimal latency of compound muscle action potentials were recorded for four limbs in 20 able-bodied volunteers after ten sequential stimulations. The ratio of maximal cortical to maximal peripheral compound muscle action potential was calculated. Standard statistical parameters were calculated for values from each limb. A paired t-test revealed a statistically significant difference in lower extremity side-to-side amplitude. Regression analysis of patient height vs upper extremity and lower extremity latency demonstrated a strong correlation: r = .720 and r = .601, respectively. A pendulum model demonstrated that a miniscule amount of energy is imparted onto metallic fixation devices by the magnetic coil, but no significant paraspinal activity could be measured in any of five volunteers upon stimulation of the motor cortex. This study has established a normal MES data base that might be useful in evaluating spinal cord injured patients. It has also demonstrated that initial safety concerns about the use of MES in this population are likely unfounded.