Nerve Stimulator-Guided Interscalene Block

Interscalene blocks performed using a nerve stimulator are among the most technically challenging regional anesthetic techniques. The anatomy of the brachial plexus in the cervical region is highly variable and often obscured by redundant soft tissue, making landmark identification difficult. Reported success rates range from 75–80%, and the potential for complications is significant. However, the block offers substantial clinical benefits, particularly for upper extremity procedures—most notably shoulder surgery—by reducing perioperative pain, opioid consumption, and overall hospital stay, contributing to improved patient outcomes and decreased healthcare costs.

Proficiency in this technique relies heavily on accurate interpretation of elicited motor responses (twitches), as well as strict adherence to fundamental injection principles. Several variations of the interscalene approach exist, with some practitioners favoring a lower approach to improve anesthetic coverage of the ulnar nerve. Evidence supports the use of regional anesthesia for a wide range of shoulder procedures and manipulations under anesthesia, with patients often reporting improved postoperative comfort and enhanced tolerance of physical therapy.

Essential Anatomy for Interscalene Block

A clear understanding of the brachial plexus is foundational for performing upper extremity blocks. The plexus is formed by the ventral rami of C4 through T1, which join to create a complex network of nerves providing both motor and sensory innervation to the arm. After exiting their respective intervertebral foramina, the nerve roots coalesce into a plexus that encircles the subclavian vessels in the supraclavicular region. Distally, the plexus gives rise to five terminal nerves: the musculocutaneous, median, ulnar, radial, and axillary nerves.

At the interscalene level, the plexus lies relatively superficial and is most accessible between the anterior and middle scalene muscles—known as the interscalene groove. Novice practitioners often misidentify the posterior border of the sternocleidomastoid (SCM) muscle as the interscalene groove, which can lead to unintended stimulation of the **phrenic nerve**, located anterior to the plexus. Phrenic nerve stimulation presents as rhythmic hiccuping and necessitates posterior redirection of the needle. Conversely, stimulation of the **spinal accessory nerve** (cranial nerve XI), which lies posterior to the plexus and results in trapezius contraction, requires anterior needle repositioning.

Technical Considerations

Due to the blind nature of nerve stimulator guidance, anatomic variation, and the density of neural structures in the neck, predicting which nerve will be stimulated first can be challenging. However, once a twitch is elicited, needle repositioning can typically guide the tip toward the desired target. The **axillary nerve**—often the first to be stimulated due to its superficial location—is particularly relevant for shoulder procedures. While any twitch of the brachial plexus may be acceptable, targeting the appropriate nerve has been associated with higher success rates and more effective surgical anesthesia.

Procedural Technique

Begin by turning the patient’s head slightly away from the side to be blocked. Position yourself on the side of the block, ensuring unobstructed access to the patient's face, operative arm, and monitor. For learners or less experienced providers, having an assistant nearby to manage the nerve stimulator and assist with the injection is recommended.

To facilitate identification of the interscalene groove, place your hand on the patient’s forehead and ask them to gently push their head against it. This maneuver engages the SCM and interscalene muscles, making the groove more prominent. Once identified, mark the site and proceed with sterile skin preparation. Current evidence supports the use of **chlorhexidine** as the antiseptic of choice due to its lower incidence of skin reactions and reduced infection rates.

Refer to **Gallery 24.1** for a visual guide to this technique and **Movie 24.1** for an example of an appropriate twitch response prior to injection.

Injection Preparation

As with all regional techniques, appropriate monitoring, sedation, and analgesia should be initiated. After identifying the insertion site, raise a skin wheal using a 25–27 gauge needle with lidocaine. It is important to avoid excessive local infiltration in this region, as inadvertent spread to the superficial brachial plexus may impair twitch detection and complicate needle placement.

Excessive local anesthetic infiltration during skin wheal formation can impair motor response (twitch) elicitation, complicating nerve localization and increasing the risk of adverse outcomes. When twitches are not elicited and multiple needle passes are required, the likelihood of nerve injury increases.

To perform the block, introduce an insulated needle perpendicular to the skin, bearing in mind that the brachial plexus at this level typically lies 0.5 to 1 cm beneath the surface. Ensure the patient remains still, and stabilize your needle hand by resting it securely on the patient for optimal control. As the needle advances toward the nerve, an appropriate motor response should be elicited. Optimal positioning is achieved when the twitch persists at a current of 0.2–0.5 mA. At this point, the injection may begin, with close attention to the initial few milliliters.

The volume and concentration of local anesthetic should be tailored to the individual patient's needs and the surgical context. As with any regional anesthetic, complications are possible—though the interscalene block is associated with a higher incidence of certain effects.

**Horner’s syndrome** is the most commonly encountered side effect and consists of five typically benign symptoms:

* **Ptosis** (drooping eyelid)

* **Miosis** (pupillary constriction)

* **Anhidrosis** (loss of facial sweating)

* **Phrenic nerve palsy** (which may impair respiration, particularly in patients with pre-existing pulmonary compromise)

* **Recurrent laryngeal nerve palsy** (which may cause transient hoarseness)

While these effects generally resolve as the local anesthetic is metabolized, awareness of these potential outcomes allows the provider to better assess the risk–benefit ratio for each patient receiving an interscalene block.

Refer to the image below for demonstration of proper needle placement during interscalene block administration.

Ultrasound guidance for the Interscalene Block

Imaging for this procedure requires a high frequency linear probe. An initial depth should be 1-2 cm.  Identifying a large, easily recognizable structure, in this case, imaging the carotid artery can provide an easily identifiable landmark for the posterior location of the brachial plexus. The next structures that can be identified as the probe is positioned posteriorly, are the large sternocleidomastoid heads followed by the smaller anterior and middle scalene muscles. These muscles are recognized by their oval and marbled appearance. Deep to the scalene muscles, the trunks of the plexus can be viewed as three bright white or three dark circles with the characteristic “starry night” appearances within them. This describes the circular nerve with dull hyperechoic dots within it. These dots represent the nerve fascicles. An excellent image may even allow the provider to view slight pulsations of the vessel rich nerves. After proper cleansing of the area, obtain the desired view of the three dark “starry night” dots. Either an in-plane or out-of-plane approach can be used; but it is desirable to see the tip of the needle in relation to the structure to be surrounded with local anesthetic. Aim the needle toward the side of, not directly at the nerve. This technique will minimize accidental contact with the structure, and allow easier maneuvering deep to it. After negative aspiration begin to administer local anesthetic. Allow it to diffuse circumferentially around the entire structure if possible. Some clinicians prefer to check the image with a nerve stimulator to identify which nerve is being imaged. Infiltrate the deepest structures first, as micro-bubbles in the injectate will obscure the view after administration. Observe the spread circumferentially and be sure to document this occurrence. It is necessary to capture an image of proper local anesthetic spread, without needle contact of the nerve, for medical/legal, as well as billing purposes. Visit gallery 242 for illustrated anatomy to re-familiarize yourself with the basics. Then visit  the other interactives to see the various aspects of ultrasound guided interscalene blocks and peri-neural catheter placement. See also movie 17.2 to see the ultrasound of the brachial plexus.