The brachial plexus arises from the cervical nerves C5 through C8 as well as T1.
It originates from the anterior primary rami of these nerves and after leaving their corresponding intervertebral foramina, these nerves run anterolaterally and inferiorly where they end up between the anterior and middle scalene muscles. The anterior scalene muscle arises from the anterior tubercles of the cervical vertebra and insert into the scalene tubercle of the first rib. The middle scalene muscle arises from the posterior tubercles of the cervical vertebra and inserts on the first rib posterior to the subclavian artery. C5 and C6 unite to form the superior trunk, while C7 makes up the middle trunk. Lastly, C8 and T1 come together to form the inferior trunk. All of this occurs while the nerves are coursing between the anterior and middle scalene muscles. These scalene muscles are invested with a prevertebral fascia which fuses laterally and encloses the brachial plexus. After this the nerves continue to subdivide and recombine eventually forming the peripheral nerves that innervate the upper extremities and lead to the above pictured cutaneous distributions.
The patient is generally positioned in the supine, semi-supine, or even a semi-lateral decubitus position with their head turned away from the side being blocked (and if semi-lateral decubitus with the side being block being the side of the patient semi-raised). The semi-supine position is often most comfortable as it allows the patient’s head to be slightly raised which in turn allows for better venous drainage and less engorgement of the neck veins.
Nerve Stimulation/Paresthesia Technique:
The easiest landmark to first identify is the posterior boarder of the sternocleidomastoid muscle. Have the patient lift their head while it is turned away from the side being blocked and this will make the SCM very prominent. Palpate the posterior border and then slide your fingers posteriorly and laterally to feel the bellies of the anterior and middle scalene muscles. There is a grove between these muscles called the interscalene grove. This is your goal. You should then trace a line laterally from the cricoid cartilage. Where this line intersects the interscalene grove is approximately the level of the transverse process of C6. This is your insertion point.
Using proper sterile techniques make a skin wheal of local anesthetic using a small gauge needle. Then take your chosen block needle and insert it at your previously identified spot perpendicular to the skin with a 45 degree caudad and slightly posterior angle (see above image from Miller’s Anesthesia, 7th ed.). Advance the needle until a paresthesia is elicited (usually C5 or C6 dermatome – see above cutaneous distribution image) or, if using nerve stimulation, there is twitch of the pectoralis, deltoid, triceps, biceps, forearm, or hand muscles. You may feel a pop as you pass through the prevertebral fascia. A twitch should be elicited with only 0.2-0.4 mA, > 1.0 mA results in exaggerated responses and is unnecessarily uncomfortable for the patient. Never inject local anesthetic when there is a twitch response at <0.2 mA, because the needle could be intraneural. Also, you will get a response at a depth of 1-2 cm in most patients, never advance beyond 2.5 cm to avoid the risk of complications (such as cervical cord injury, pneumothorax, and carotid artery puncture). If you contact bone within 1-2 cm of depth, you likely contacted a transverse process and the needle is too posterior. If you get twitches of the diaphragm, this is from stimulation of the phrenic nerve and the needle is positioned too far anterior. When you are in a proper position, first aspirate to ensure no blood return through the needle/tubing and then slowly inject 10-40 mLs of local anesthetic solution. Be sure to gently aspirate after every 2-5 mLs of injection to look for possible intravascular injection. The needle tip could move during the injection process and could end up in an intravascular space.
Similar to the nerve stimulation approach you must get the patient in the proper position, disinfect their skin, and locate your landmarks. Then you position the transducer in a transverse plane at the level of the cricoid cartilage just medial to the sternocleidomastoid muscle. One of the easiest structures to first identify is the carotid artery. Once you have seen this, move the transducer laterally to identify the anterior and middle scalene muscle bellies. In between these two structures you should see the branches of the brachial plexus. Depending on your relative position the plexus may appear as the three trunks or several smaller nerves meaning you are looking at the nerve roots themselves before they form the trunks. If you are having trouble viewing identifying the plexus nerves this way, bring the transducer down to the supraclavicular position where the brachial plexus can be seen sitting lateral/superficial to the subclavian artery. Once identified, trace the nerves back up the neck and stop once you find them between the scalene muscle bellies.
After a suitable image has been obtained, inject a small amount of local anesthetic in the skin at the lateral end of the ultrasound probe. Then insert the block needle in-plane moving towards the plexus in the lateral to medial direction. It is possible to do this in the medial to lateral direction if more convenient, however the other direction is what is generally used. While slowly inserting the needle under direct visualization, you may feel a slight pop/give sensation as you pass through the prevertebral fascia. Once the tip of the needle is seen close to the plexus, aspirate to ensure the needle is not in a blood vessel and then injection a small amount of local anesthetic to further assess the position of the needle. If the LA spreads around the brachial plexus then the needle is in a proper position. In general, 15-25 mLs of LA is adequate for an adult patient. The needle can be repositioned as needed to help redirect some of the LA to fully surround the plexus.
Generally the interscalene block is most appropriate for surgeries involving the shoulder. Using this technique the superior and middle trunks are often well anesthetized, however there can be partial or complete sparing of the inferior trunk. This means that surgery involving the ulnar nerve distribution may require a supplemental ulnar nerve block.