Monopolar needle stimulation: safety considerations

Stimulation of nerves for conduction studies can be performed with a monopolar needle used routinely for electromyographic studies. Although invasive, this technique allows precise localization of stimulation and access to deeply situated nerve trunks, thus providing otherwise unobtainable information regarding peripheral nerve function. The electric current at the needle tip was measured during supramaximal stimulation of the median and peroneal nerves at distal and proximal sites. After measuring the area of uninsulated needle tip, the local current density was calculated. The maximum current during the stimulation of 0.05msec duration was 0.80mA and the resulting current density was 4.1mA/mm2. It is calculated that this current could cause a local maximum temperature rise of 2.5C in the tissue within 0.5mm of the needle tip. This current flow would not be expected to cause either direct electric or heat damage to the tissue.