Leonard V. Bunting, M.D.
Interscalene Plexus Block
The brachial plexus is a neural bundle that provides sensory and motor innervation to the upper extremity. Nerve roots of C5-T1 undergo complex congregation before forming the terminal nerves of the upper extremity (Illustration 1). The plexus can be blocked at several locations, with more proximal blocks anesthetizing more proximal portions of the arm
Illustration 1 – Course of the Brachial Plexus
Illustration 2 – Anatomy of the Brachial Plexus
The interscalene space (or interscalene groove) is a potential space between the anterior and middle scalene muscles. Here, the cervical roots C5-T1 of the brachial plexus join at the level of the cricoid cartilage, at the hight of the transverse process of C6. They form a vertical bundle consisting of superior, middle and inferior trunk before meeting the subclavian artery infero-laterally (Illustration 2). In this location, the plexus is found superior and posterior to the subclavian artery, the dome of the lung is found anteromedial to the inferior trunk. A successful block at this level anesthetizes the shoulder and upper arm, but does not reliably block nerve roots innervating the forearm, as the inferior trunk is often not included into the blockand the ulnar nerve (Illustration 3). Therefore it should not be used for isolated injuries below the elbow.
Illustration 3: Common distribution of anesthesia, interscalene block.
The patient is positioned supine with their head rotated 45 degrees to the contralateral site (Figure 1). Assure proper room and equipment setup and prepare scanning field and ultrasound transducer accordingly.
Figure 1: Probe positioning for interscalene plexus block.
Use a high-frequency (9-18 MHz) linear probe and begin scanning over the anterior neck at the level of the cricoid in the transverse plane. You can start by identifying the hyperechoic arc of the trachea and then move the probe posterior-laterally to identify the sternocleidomastoid (SCM, Figure 2). The SCM is a triangular shaped muscle located anteriorly to the carotid artery and internal jugular vein. Continue to move the probe postero-laterally around the neck. After identifying the great vessels of the neck your next step is to localize the anterior scalene muscle (ASM) deep to the lateral border of the SCM and lateral to the internal jugular vein. The middle scalene is found further postero-lateral. Between anterior and middle scalene muscle, visualize the roots or trunks of the brachial plexus in the interscalene groove (Figure 3). These can appear as triangular, round or oval bundles with hypoechoic centers. The individual elements of the plexus may be tightly packed or loosely associated. If the plexus or interscalene space is difficult to identify, the probe is moved slightly cephalad and caudal along the lateral border of the SCM to identify the ASM. Remember that the anterior scalene muscle can be much smaller in the more cephalad region and will increase in size when scanned more caudal. This can help identify the plexus in the interscalene area. Also, if the head is not turned adequately, the SCM will overlay the plexus.
Figure 2: Ultrasound image of the anterior-lateral neck.
Figure 3: Ultrasound image of the interscalene region with roll-over.
In this approach, the brachial plexus is first identified in the supraclavicular fossa and then tracked cephalad into the interscalene space. Scanning begins over the sternocleidomastoid, 1-2 cm superior to the head of the clavicle. Trachea and thyroid lobe are identified medially and carotid artery and internal jugular vein are located deep to the SCM. The subclavian artery is identified by its thick wall and brisk pulsations. Immediately superior and posterior to the artery, the brachial plexus is seen as a grouping of small hyperechoic circles with hypoechoic centers, similar to a cluster of grapes. The plexus is then traced cephalad to the preferred block region at the level of C6 ( Figure 3).
Keep in mind that the probe indicator should always point to the right of the patient. This means that the marker is pointing anterior for left-sided blocks and posterior for blocks of a right brachial plexus. The depth needed to detect the plexus is often around 2-3 cm, but can reach up to 6 cm depending on patient anatomy and location of the target area.
An in-plane approach from the posterior-lateral side of the probe is preferred (Figure 4). After the appropriate equipment is setup, the skin is anesthetized and the block needle is inserted at an angle of about 45 degrees to the skin surface. The needle tip is located and slowly advanced towards the plexus, avoiding any sensitive structures. When passing through the prevertebral fascia, a "click" may be felt. Once movement of the needle causes movement on the plexus, injection may begin. A common target area for injection is between the top and middle trunk. Appropriate needle placement is confirmed by movement of the plexus with the flow of anesthetic and spread of anesthetic around the entire plexus. This will appear as a hypoechoic fluid collection (Video 1). Readjustment of the needle position may be necessary to achieve adequate distribution of anesthesia.
Always perform aspiration and incremental injection to avoid systemic distribution of the anesthetic. The usual volume of local anesthetic administration is between 15 to 45 cc (1), although sufficient anesthesia has been reported with smaller volumes (2). Digital pressure superior to the block and up to 45-degree head elevation may facilitate deep distribution of the anesthetic and blockade of the lower trunk (3). One-time injection can provide 8-10 hours of anesthesia and up to 18 hours of analgesia when long-acting medication is used (4). Patients and other medical providers will need to be made aware.
Video 1: Interscalene plexus block.
Figure 4: Probe and patient positioning for In-plane approach for the interscalene block.
Pearls and Pitfalls
The ipsilateral phrenic nerve lies just anterior to the interscalene space towards the SCM (Figure 5). It is frequently blocked utilizing this approach (6), suspected through anterior spread of the anesthetic. Although this complication is associated with a significant reduction in lung function tests (6,7), it is well tolerated in most healthy patients (6,7). Care should be taken in patients with respiratory compromise.
A temporary Horner’s syndrome or a hoarse voice may also result from block of sympathetic afferents or the recurrent laryngeal nerve, respectively. These are self-limited and will resolve as the block resolves. However this block is not recommended in patients with known contralateral laryngeal nerve palsy.
It is important to avoid local anesthetic injection immediately adjacent to the transverse process and the nerve root emerging from the neural foramen because of the risk of unintentional epidural or spinal anesthesia.
Pneumothorax can be caused by injections close to the inferior trunk and is best avoided by needle insertion and correct angulation at the C6 level. This will keep the dome of the lung fairly distant from the needle tip.
The effect of head turning: Incomplete head rotation can place the SCM over the plexus and block the needle path. (Figure 6). Additional rotation will move the SCM muscle out of the path (Figure 7).
Remember that the anterior scalene muscle can be much smaller in the more cephalad region and will increase in size when scanned more caudal. (Figure 8 and 9). Careful aspiration and incremental injection is paramount to avoid systemic toxicity.
Figure 5: Phrenic Nerve
Figure 6: Incomplete rotation of the head will place the SCM over the interscalene plexu.
Figure 7: Appropriate rotation will move the SCM out
Figure 8: ASM more cephalad.
Figure 9: ASM caudal shown with increased diameter.
- Borgeat A, Blumenthal S.
Interscalene Brachial Plexus Block. In Hadzic A (ed). Textbook of Regional Anesthesia. McGraw-Hill, 2007, p 413.
- Riazi S, Carmichael N, Awad I, Holtby RM, McCartney CJ.
Effect of local anaesthetic volume (20 vs 5 ml) on the efficacy and respiratory consequences of ultrasound-guided interscalene brachial plexus block. Br J Anaesth, 2008, 101(4):549-56.
- Borgeat A, Blumenthal S.
Interscalene Brachial Plexus Block. In Hadzic A (ed): Textbook of Regional Anesthesia. McGraw-Hill, 2007, p 413.
- Borgeat A, Blumenthal S.
Interscalene Brachial Plexus Block. In Hadzic A (ed):Textbook of Regional Anesthesia. McGraw-Hill, 2007, pp 414-5.
- Urmey WF, Talts KH, Sharrock NE.
One hundred percent incidence of hemidiaphragmatic paresis associated with interscalene brachial plexus anesthesia as diagnosed by ultrasonography. Anesth Analg, 1991,72:498–503.
- Hortense A, Perez MV, Amaral JL, Oshiro AC, Rossetti HB.
Interscalene brachial plexus block. Effects on pulmonary function. Rev Bras Anesthesiology, 2010;60:130-7.
- Casati A, Fanelli G, Aldegheri G, Berti M, Colnaghi E, Cedrati V, Torri G.
Pulmonary function changes after interscalene brachial plexus anesthesia with 0.5% and 0.75% ropivacaine: a double-blinded comparison with 2% mepivacaine. Br J Anaesth.1999;83(6):872-5.