Interscalene blocks using the nerve stimulator are probably the most technically difficult to perform. The anatomy in the region of the brachial plexus in the neck is complex and redundant tissue can make landmark identification difficult. Complications are many, and studies report success rates of only 75-80%. But its utility for arm, particularly shoulder surgery in regards to reduction in pain, opioid requirements during and after surgery, and economic impact related to reductions in overall hospital stay is tremendous. Most regionalists will agree that becoming proficient at stimulator-guided interscalene blocks hinges on accurate twitch interpretation. The basic injection techniques discussed earlier are essential to follow with this approach. Many different approaches are described to accomplish interscalene brachial plexus blocks. Some favor a low approach citing better coverage for the ulnar distribution. Studies also suggest that many procedures including many types of shoulder surgery, and manipulations under anesthesia can be done under regional. Generally, post-op therapies are also better tolerated by patients having blocks. 

The foundation of peripheral blocks of the upper extremity revolve around a basic understanding of the brachial plexus. For the purposes of this block only the essential anatomy will be presented. The brachial plexus derives innervation from cervical roots of C-4 through T-1. Each root at these levels joins with others to form a framework or plexus that innervates both sensory and motor function to the arm. After leaving their respective ganglions, the plexus forms a bundle, somewhat resembling a net, that surrounds the subclavian vessels in the supraclavicular region. Distally, the plexus divides into five distinct nerves that provide sensory and motor innervation to the arm. They are the ulnar, median, radial, musculocutaneous, and the axillary nerves. The proximal brachial plexus is surprisingly superficial. It becomes most accessible for this technique in the interscalene groove. This groove is comprised by the anterior and middle scalene muscles. Visit interactive 24.1 to review the interscalene groove. Many novices mistake the posterior border of the sternocleidomastoid muscle (SCM) as the proper landmark, and consequently find only the phrenic nerve able to be stimulated. Phrenic stimulation, indicated by rhythmic hiccuping, should prompt the provider to reposition the needle posteriorly as the phrenic nerve is located anterior to the brachial plexus. The spinal accessory nerve (cranial nerve XI) lies just posterior to the brachial plexus. Stimulation of this nerve, displayed as twitching of the trapezius muscle, should prompt the provider to reposition the needle anteriorly. 

Because of the blind nature of the nerve stimulator technique, anatomic variations, and complexity of the brachial plexus and surrounding nerves of the neck it is difficult  to predict which nerve will be initially stimulated. However, once a twitch is elicited, it is usually easy to reposition the needle to the desired location. Fortunately, the axillary nerve is located superficially, and is stimulated first many times. Many articles advocate that any twitch of the brachial plexus can be used, however there are increased success rates when the target nerve is sought. For example, if shoulder surgery is planned, the axillary nerve should be sought to ensure proper distribution blockade.

Begin this block by slightly turning the patients head away from the side of the block. Position yourself on the block side and have all equipment easily accessible. It is desirable to be able to see the patients face, the entire arm, and the monitor. Novices should have an assistant standing by to reduce the stimulator output, and perform the injection. Place your hand on the forehead and have the patient lift their turned head against your hand. This will distend the interscalene and SCM muscles making the interscalene groove easier to identify. Once the groove is identified, place a mark and begin the prep. Recent articles advocate the use of chlorhexidine solution noting fewer untoward reactions and a decrease in infection rates. See gallery 24.1 to see the process of the nerve stimulator interscalene block, and see the movie 24.1 to see an example of an acceptable twitch prior to injection for the interscalene block.

As with all regional approaches monitors, sedation and analgesia should be instituted. After identification and marking the site of needle entry, raise a skin wheel with a 25-27G needle and lidocaine solution. It should be remembered that liberal use of local for skin wheals in this area can result in contact with the superficial brachial plexus. 

This will make twitch elicitation difficult and can create a dangerous situation. If a twitch can't be elicited and many needle passes are required, it stands to reason that nerve injury is more likely to ensue. Introduce the insulated needle perpendicular to the skin plane, keeping in mind that the superficial cord of the brachial plexus is usually 1/2 to 1 cm in depth. It is important to keep the patient still, and steady the needle hand by placing it securely on the patient. As the needle approaches  a nerve a twitch will be elicited. Position the needle tip as close to the nerve as possible by eliciting the twitch at outputs from 0.2- 0.5 mA. Carefully begin the injection paying close attention to the first few mLs. Adults should receive a customized volume and concentration local. Risks are an inevitable part of any anesthetic. But the interscalene block has more than its share. Horner's syndrome is the most common. This term comprises five symptoms that are mostly nuisance problems. They are ptosis, (drooping of the eyelid) which can be mistaken for stroke; myosis (unilateral pupillary constriction), also appearing stroke-like; anhydrosis (unilateral inability to produce facial sweating); phrenic nerve palsy which is potentially problematic if the patient has pre-existing respiratory compromise; and lastly recurrent laryngeal nerve palsy, causing hoarseness. All of these symptoms will resolve as the local is metabolized. Appreciating these concerns will help the provider make better decisions regarding risk vs. benefit for patients receiving interscalene blocks. The image below shows placement of an interscalene block.

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.