When the stimulating needle was in contact with the nerve or was intraneural, lower minimal stimulation currents were observed when compared to when the needle was in the control position.  The image (figure 1 from the article) shows control needle tip position, 1 mm distant to the nerve. Exposed median nerve; left, caudal; right, apical; down, lateral.  Distance between needle tip and median nerve is depicted by dotted lines. (Image source: Anesthesia & Analgesia)

When the stimulating needle was in contact with the nerve or was intraneural, lower minimal stimulation currents were observed when compared to when the needle was in the control position. The image (figure 1 from the article) shows control needle tip position, 1 mm distant to the nerve. Exposed median nerve; left, caudal; right, apical; down, lateral. Distance between needle tip and median nerve is depicted by dotted lines. (Image source: Anesthesia & Analgesia)

You’re performing a regional anesthetic and you are using nerve stimulation to determine whether you are close to the nerve.  You obtain a motor response to stimulation, yet the stimulation current is minimal.  Is the needle in the nerve or simply in contact with the nerve?

Dr. Thomas Wiesmann, MD, Department of Anesthesiology and Intensive Care Therapy, Philipps University Marburg, University Hospital Giessen-Marburg, Campus Marburg, Marburg, Germany, and colleagues from St. Luke’s and Roosevelt Hospital Center, University Hospital of Columbia University, College of Physicians and Surgeons, New York City, New York, and Institute of Medical Informatics, Biostatistics and Epidemiology (IBE), Ludwig-Maximilians-University (LMU), Munich, Germany, used six anesthetized pigs to examine the relationship of pulse duration and distance from the nerve using different brachial plexus nerves.  Their results are in the article “Minimal Current Intensity to Elicit an Evoked Motor Response Cannot Discern Between Needle-Nerve Contact and Intraneural Needle Insertion” that appeared in this month’s issue of Anesthesia & Analgesia.

The tip of the needle was placed under direct vision, either 1 mm away from the target nerve (the control position), in contact with the nerve, or intraneurally.  Three pulse duration settings (0.1, 0.3, and 1.0 milliseconds) were used.  Radial, median, ulnar, musculocutaneous, and caudal pectoral nerves were tested. The minimal current intensity was the lowest intensity that elicited an evoked motor response

The minimal current intensity decreased with increasing pulse duration. At all pulse durations, the minimal current intensity was less than 0.2 mA when the stimulating needle in direct contact with the nerve or intraneural. However, when the needle was placed 1 mm away, the minimal current intensity was uniformly higher at every pulse duration. The authors developed confidence intervals for each needle tip location and pulse duration. Based on the confidence intervals, intraneural injections can be avoided if the minimal current that evokes a motor response is greater than 0.2 mA.

The open approach used in the study might yield different impedence results from the typical clinical setting. Although it seems likely that mammalian nerves are very similar, it is possible that human nerves might produce different results from pig nerves. Finally, it is unclear if impedance might be different for different nerves, e.g., radial vs. median nerves.

Nonetheless, whether the needle is in the nerve or making contact with the nerve, if threshold currents to elicit motor responses is less than 0.2 mA, the needle should probably be withdrawn.