Percutaneous magnetic coil stimulation of the phrenic nerve roots and trunk

We describe a technique of percutaneous magnetic coil (MC) stimulation of the phrenic nerve trunk on one side of the neck and phrenic roots over the upper cervical vertebral column in 10 normal subjects and 2 patients. We were able to obtain compound muscle action potentials (CMAPs) from the diaphragm at two sites (xiphoid process and 7th intercostal space) after stimulation of the phrenic nerve trunk and roots. We noted that the onset latencies after phrenic root stimulation remained fixed despite increasing the stimulus intensity from 50% to 100% and on moving the MC vertically or laterally, suggesting that stimulation of the fastest conducting fibers was occurring at a fixed site, most likely at the intervertebral foramina. Absent responses unilaterally in one and prolonged latencies to diaphragmatic CMAPs in another patient confirmed phrenic neuropathy in these patients.