Peripheral nerve stimulation (PNS) has long been an important technique in regional anesthesia, providing a functional method for identifying peripheral nerves through elicited motor responses. Introduced in the 1970s, PNS improved upon landmark-based and paresthesia-guided approaches by offering a more objective and reproducible means of nerve localization. By delivering a low-intensity electrical current through an insulated needle, clinicians can provoke contraction of muscles innervated by the target nerve, thereby estimating the proximity of the needle tip. Traditionally, a motor response obtained at a current between 0.2 and 0.5 milliamps (mA) has been considered indicative of close needle-to-nerve positioning and suitable conditions for local anesthetic injection (1). However, more recent changes to regional anesthesia, such as ultrasound-guided techniques and the movement towards motor-sparking blocks, drive discussion on the current role of nerve stimulation.
The physiological basis of PNS lies in the depolarization of motor fibers within mixed peripheral nerves. As the needle advances toward the nerve, progressively lower current intensities are required to elicit a response, reflecting decreasing distance between the needle tip and the nerve. However, this relationship is not always consistent in clinical practice. Electrical current may be dispersed or redirected by surrounding tissues such as fascia, fat, or fluid, leading to false-negative responses even when the needle is in close proximity to the nerve. Conversely, false-positive responses may occur when adjacent structures are stimulated despite suboptimal needle positioning, potentially resulting in ineffective blockade (2). These limitations highlight the imperfect sensitivity and specificity of PNS when used as a sole localization technique.
The introduction of ultrasound-guided techniques has significantly altered the role of nerve stimulation in regional anesthesia. Ultrasound enables direct visualization of nerves, adjacent anatomical structures, and needle trajectory in real time, improving both the accuracy and safety of nerve blocks. Multiple systematic reviews and meta-analyses have demonstrated that ultrasound guidance is associated with higher block success rates, faster onset times, and reduced complication rates compared with nerve stimulation alone. For example, ultrasound-guided techniques have been shown to reduce vascular puncture and procedural pain while improving overall block quality (3). As a result, ultrasound has become the primary modality for nerve localization in modern practice.
Despite this shift, PNS remains clinically relevant as an adjunct to ultrasound. The combined use of ultrasound and nerve stimulation, often referred to as dual guidance, provides complementary information for regional anesthesia. While ultrasound offers anatomical visualization, PNS provides functional confirmation of nerve proximity. Evidence suggests that the addition of PNS does not significantly improve success rates for superficial or easily visualized nerves, but it may be beneficial in cases involving deep or poorly visualized structures. More importantly, PNS may enhance safety by acting as a warning system for needle–nerve contact. A motor response elicited at very low current intensity (<0.2 mA) is highly specific for intraneural or near-contact needle placement, alerting the clinician to reposition the needle to avoid nerve injury (1).
In addition to its clinical applications, PNS plays a valuable role in education and training. For novice practitioners, correlating ultrasound images with evoked motor responses reinforces understanding of anatomical relationships and improves procedural skills. This combined feedback can accelerate the learning process and minimize technical errors during the initial stages of regional anesthesia practice.
The role of nerve stimulators in regional anesthesia has evolved with advances in imaging technology, particularly ultrasound-guided blocks. While ultrasound has largely replaced PNS as the primary method of nerve localization, nerve stimulation continues to serve as a valuable adjunct that enhances safety, supports difficult blocks, and facilitates education. A multimodal approach that integrates both techniques offers the most balanced strategy for optimizing outcomes in contemporary regional anesthetic practice.
References
1. Gadsden JC. The role of peripheral nerve stimulation in the era of ultrasound-guided regional anaesthesia. Anaesthesia. 2021;76 Suppl 1:65-73. doi:10.1111/anae.15257
2. Perlas A, Niazi A, McCartney C, Chan V, Xu D, Abbas S. The sensitivity of motor response to nerve stimulation and paresthesia for nerve localization as evaluated by ultrasound. Reg Anesth Pain Med. 2006;31(5):445-450. doi:10.1016/j.rapm.2006.05.017
3. Abrahams MS, Aziz MF, Fu RF, Horn JL. Ultrasound guidance compared with electrical neurostimulation for peripheral nerve block: a systematic review and meta-analysis of randomized controlled trials. Br J Anaesth. 2009;102(3):408-417. doi:10.1093/bja/aen384
