Intensity of the stimulating current may not be a reliable indicator of intraneural needle placement

The current intensity at which a motor response is elicited with an intraneural needle placement has been inadequately studied. We hypothesized that electrical current delivered through an intraneurally placed needle does not always result in an evoked motor response. Our secondary objective was to determine the relationship between electrical current intensity and needle-to-nerve distance.

Twenty pigs were given general anesthesia and the sciatic nerves (SN) were exposed bilaterally. Electrical nerve stimulation was applied 2 cm, 1cm, 0.5 cm, 0.2 cm, and 0.1cm away from the SN, transepineurally, and intraneurally (in the subepineurium). Stimulation was started at 2.0 mA and decreased to the minimal current at which visible motor response was obtained. Two blinded observers agreed on the intensity and type of motor response. Specific response of SN was defined as a distal motor response (hoof twitch); nonspecific response was defined as a local muscle twitch (no hoof response).

At a distance of 0.5 cm to 2 cm away from the SN, only nonspecific muscle responses were observed. Specific SN responses were obtained starting at 0.1 cm away from the nerve and transepineurally with currents of 0.92 +/- 0.33 mA (median 1.00 mA; range 0.24-1.48 mA) and 0.39 +/- 0.33 mA (median 0.3 mA; range 0.15-1.4 mA), respectively. With the needle tip positioned intraneurally, specific motor response could be obtained at 0.56 +/- 0.54 mA (median 0.3 mA; range 0.08-1.80 mA). Five (12.5%) intraneurally positioned needles only elicited a specific motor response at 0.8-1.8 mA.

Specific response to nerve stimulation with currents <0.2 mA occurred only when the needle tip was positioned intraneurally. However, motor response could be absent with intraneural needle placement at a current intensity of up to 1.7 mA.