Electric nerve stimulation: analysis of two clinical cases of failed nerve electrolocation by using sheathed needles.

The aim of this work was to explain the reasons of two unsuccessful blocks of sciatic nerve even if anaesthetic solution was injected through insulated needle on elicited twitch. The clinical cases were two outpatients undergoing diagnostic arthroscopy of knee under anaesthetic block of sciatic and femoral nerves. In both patients, the muscular twitch appeared when the ischiatic bone was kept in unexpected touch with needle tip. In spite of the attempt to locate correctly the needle (the touch with bone means that the nerve is not in front of the needle tip), the injection of anaesthetic solution was unsuccessful. In clinical environment, when electroinsulated needles gathered total amount of administered current on the needle tip, it was not possible to elicit a twitch just at the moment of touch of the needle with the bone. Referred events disagree with some experimental works performed out of clinical environment, which found that total amount of administered current through an insulated needle gathers always in front of the tip. Our clinical observations seems to confirm an electrolocation mistake called "electrical shadow". The ability of sheathed needles to work as occasional capacitor due to the alternation of two conductor layers (needle shaft and tissue) and of a dielectric (coating material) can explain some missing electrolocations, as the appearance of electric fields within dielectric needle sheathing.