At this level, the anterior and middle scalene muscles lie just deep to the SCM, with the interscalene groove located between the two prominent muscles. The nerve roots of the brachial plexus will be visualized as oval or round hypoechoic structures located in the groove between the anterior and middle scalene muscles, where they are tightly clustered within their own fascial sheath. Here the C5-7 roots of the brachial plexus appear as three vertically aligned, round hypoechoic structures, sometimes referred to as the “traffic light” sign (Fig. 5).
Explore This IssueACEP News: Vol 31 – No 02 – February 2012
The C8 root may be located within this fascial plane or may be deeper as previously mentioned. Lateral to the brachial plexus is the middle scalene muscle, comprising the lateral border of the interscalene groove. The transverse cervical and suprascapular arteries typically run laterally and caudad to the brachial plexus at this level. After the brachial plexus is located, color Doppler is recommended to confirm that vasculature structures will not be present in the path of the needle and/or mistaken for hypoechoic nerve roots.
An alternative approach to finding the brachial plexus (if the provider is having difficulty locating the structures at the level of the larynx) involves identifying the brachial plexus distally at the level of the supraclavicular fossa and following the nerve trunks proximally into the interscalene groove. Here, the transducer is placed transversely in the supraclavicular fossa and aimed caudally until the subclavian artery is visualized. The brachial plexus lies just posterolateral to the artery at this level and will appear as a tight group, a hypoechoic “cluster of grapes.” Follow these hypoechoic structures cephalad until they form the traffic light sign within the interscalene groove at the level of the larynx (Fig. 6).
Needle Insertion and Injection
Two distinct injection techniques have been described:
- The classic periplexus approach targets the potential space between the middle scalene muscle and the brachial plexus sheath. This approach reduces the risk of intraneural injection but requires higher volumes of local anesthetic to achieve a complete block. Unfortunately, large volumes of anesthetic are thought to be more likely to track around the anterior scalene muscle to the phrenic nerve, causing paralysis of the ipsilateral hemidiaphragm.
- The intraplexus approach targets the space between the nerve roots within the brachial plexus sheath and requires smaller volumes of local anesthesia to achieve an adequate block. Lower volumes of injected anesthetic limits spread to the phrenic nerve but may also result in anesthetic tracking superiorly within the plexus sheath, affecting the recurrent laryngeal nerve and/or sympathetic chain, causing temporary hoarseness or Horner’s syndrome, respectively.
The authors recommend performing the interscalene brachial plexus with an in-plane technique so that the needle is visualized in its entirety. After cleaning the skin with a sterilizing solution such as chlorhexidine, and covering the cleaned ultrasound probe with a sterile adhesive dressing, place 2-3 mL of lidocaine (with or without epinephrine) about 0.5-1 cm lateral to the ultrasound probe. This will be the location of entry for the block needle, and adequate local anesthesia will allow for patient comfort during the block.