The interscalene block is a valuable analgesic option in shoulder surgery. It targets the C5 and C6 roots of the brachial plexus that give rise to the suprascapular nerve, axillary nerve, and lateral pectoral nerves that innervate the shoulder joint. The most common technique of ultrasound-guided interscalene block involves imaging the C5 and C6 roots at approximately the level of the cricoid cartilage, just distal to where they emerge from behind their respective transverse processes and where they lie in the groove between the anterior and middle scalene muscles (Fig. 1). The block needle is usually advanced in plane in a lateral-to-medial direction through the middle scalene muscle to contact the nerve roots and inject local anesthetic around them.1

Fig. 1
figure 1

Dissection of the right brachial plexus above the clavicle. The conventional ultrasound-guided interscalene block targets C5 and C6 roots in the interscalene groove between the middle scalene muscle (MSM) and the anterior scalene muscle (ASM), just distal to where they emerge from under the sternocleidomastoid muscle (SCM). More distally, the C5 and C6 roots rise out of the groove and unite to form the superior trunk. The probe image and the dotted rectangle indicate the probe locations for the conventional interscalene block and superior trunk block, respectively. Observe the phrenic nerve (black arrows) descending in a medial direction over the ASM away from the brachial plexus as well as the artery crossing the C7 and C8 roots. (Image adapted with permission from D. Jankovic and reproduced with permission from www.usra.ca)

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Although this is a highly effective technique, it has some limitations, including a reported incidence of three cases per 1,000 of long-term (more than six months) postoperative neurologic symptoms (most commonly paresthesia or numbness; very rarely motor weakness)2 and an ever-present risk of phrenic nerve palsy despite injection of volumes as low as 5 mL.3,4 In this case report, we describe an alternative approach to C5/C6 blockade - the ultrasound-guided superior trunk block.

Case illustration

Written informed consent was obtained from the patient for inclusion in this report. A 55-yr-old male was scheduled for right arthroscopic shoulder surgery (Bankart repair with remplissage procedure) as an ambulatory surgery case. He was a heavy smoker with an 80-pack-year history and daily recreational use of marijuana, with resulting moderate chronic obstructive airways disease. Pulmonary function tests revealed post-bronchodilator values of FEV1/FVC 52% and FEV1% 62%. His medications included inhaled bronchodilator therapy (budesonide, formoterol, tiotropium bromide) and a statin for hypercholesterolemia. After discussing risks and benefits, the patient agreed to receive an ultrasound-guided superior trunk block for postoperative analgesia.

After applying standard monitoring,5 an ultrasound-guided superior trunk block was performed using a linear high-frequency (5-12 MHz) probe and a 22G 50-mm Stimuplex® block needle (B. Braun, Mississauga, ON, Canada). An injection of 0.25% bupivacaine 12 mL with epinephrine 0.0025 mg·mL−1 was administered, and an additional 5 mL of local anesthetic was injected superficial to the middle scalene muscle to block the supraclavicular nerves (lower branches of the superficial cervical plexus supplying the “cape” area of the shoulder).6 The patient exhibited motor and sensory blockade at 15 min without any subjective increase in work of breathing. Ultrasound of the ipsilateral diaphragm did not show paradoxical movement, further supporting an absence of clinically significant phrenic nerve paralysis.

The patient subsequently underwent a successful surgical repair under general anesthesia. His recovery was rapid and uneventful, and he was discharged four hours after emergence from anesthesia. At telephone follow-up the next day, the patient reported excellent postoperative analgesia with onset of pain evident approximately 23 hr after block completion. This was adequately controlled with oral analgesics. He had no respiratory or neurological symptoms.

Review of anatomy and sonoanatomy

The C5 and C6 roots are visible on ultrasound as dark hypoechoic circles emerging from their respective transverse process (Fig. 2A) and entering the groove between the anterior and middle scalene muscles (Fig. 2B and 2C). It is noteworthy that, in up to 30-35% of individuals, the C5 nerve root takes an anomalous course over or through the anterior scalene muscle rather than the interscalene groove (Fig. 3).7,8 Ultrasonographic identification of this anomaly is complicated by the fact that the C6 nerve root almost always splits into two hypoechoic bundles within the interscalene groove and can thus be mistaken for two separate roots (Fig. 2B and 2C; Fig. 3).8 It is vital to recognize this normal splitting of C6, as injection between these two structures would constitute an intraneural injection.

Fig. 2
figure 2

A sequential series of images of the brachial plexus above the clavicle from proximal to distal. (A) A transverse oblique view of the C5 (smaller hypoechoic circle) and C6 (larger circle) in the most proximal part of the interscalene groove, just under the lateral border of the sternocleidomastoid muscle (SCM). (B) The C6 root has split in characteristic fashion into two separate hypoechoic circles. The phrenic nerve (arrow) is visible in this individual on the surface of the anterior scalene muscle (ASM). (C) The C7 root has entered the interscalene groove, lying deep to C5 and C6. (D) The C5 and C6 roots have coalesced into the superior trunk, which has a well-defined hyperechoic boundary. The block needle is advanced in plane under the deep cervical fascia overlying the middle scalene muscle (MSM) until its tip is adjacent to the lateral aspect of the superior trunk. Injection here should produce spread around, but not within, the superior trunk. (Image courtesy of www.usra.ca)

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Fig. 3
figure 3

An example of an anomalous course of the C5 root, which, in this case, travels through and then superficial to the anterior scalene muscle (ASM), remaining outside the interscalene groove. As the brachial plexus is followed more distally, C5 will be seen to unite eventually with C6 to form the superior trunk. Observe that both the C6 and C7 roots in this individual have a “double bundle” appearance. (Image courtesy of www.usra.ca)

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The C5 and C6 roots become increasingly superficial within the interscalene groove and eventually unite to form the superior trunk (Fig. 1), which is visible as a single complex structure with a hyperechoic connective tissue boundary lying just below the prevertebral (deep cervical) fascia (Fig. 2D). More distally in the root of the neck, the C8 and T1 roots unite to form the inferior trunk, while the C7 root forms the middle trunk.9 The trunks bifurcate into anterior and posterior divisions posterior to the clavicle,10 and these comprise the complex “bunch of grapes” appearance of the brachial plexus as it crosses the first rib in the ultrasonographic view that is used in the supraclavicular block.

Technical description of the superior trunk block

The C5 and C6 nerve roots are identified within the interscalene groove, but instead of targeting them at this level, they are traced distally to where they coalesce into the superior trunk. The block needle is advanced in plane in a lateral-to-medial direction under the deep cervical fascia and superficial to the middle scalene muscle until the tip is just adjacent to the lateral boundary of the superior trunk (Fig. 2D). An out-of-plane approach may also be used if preferred. Injection of local anesthetic at this point will hydrodissect the tissue plane around the superior trunk, and 10-15 mL will spread around it with minimal need for needle repositioning (see Video; available as Electronic Supplemental Material).

It is crucial to target the superior trunk proximal to the takeoff of the suprascapular nerve. This nerve is identified as a small hypoechoic circle that separates from the lateral aspect of the superior trunk (Figure 4) and courses laterally under the omohyoid muscle away from the brachial plexus.11 Therefore, if low volumes of local anesthetic are used, a more distal approach, such as the conventional ultrasound-guided supraclavicular plexus block, may miss the suprascapular nerve.

Fig. 4
figure 4

In this view, the suprascapular nerve (solid arrow), which is the first major branch of the brachial plexus, has begun to separate from the superior trunk. It will be seen to continue to move laterally as the plexus is scanned more distally. The superior trunk lies just under the deep cervical fascia (dotted line). The middle and inferior trunk are less visible in this image but lie deep to the superior trunk and immediately adjacent (lateral) to the subclavian artery. (Image courtesy of www.usra.ca)

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Discussion

Ip and Tsui12 recently described a low interscalene approach that is similar to the superior trunk block in that the trunks, and not the roots, of the brachial plexus are targeted. Where their approach differs is that the low interscalene block is intended as an alternative to the supraclavicular plexus block for anesthesia of the upper limb distal to the shoulder. As a result, their site of needle insertion is slightly more distal. The needle is inserted through the middle scalene muscle to place the tip deeper, between the superior and middle trunks, and a larger volume of local anesthetic is injected.

In contrast, the superior trunk block is intended as an alternative to the conventional ultrasound-guided interscalene block for anesthesia of the shoulder and may offer several theoretical advantages.

First, the superior trunk has a clearly visible and well-defined connective tissue sheath which enhances resilience to needle-nerve contact. In contrast, the roots are surrounded by relatively little connective tissue and the epineurium is closely applied to the investing fascia of the interscalene groove, increasing the risk of sub-epineurial injection if visible needle-nerve contact is routinely sought.13 This may partly explain why registry data indicate that ultrasound-guided interscalene block continues to be associated with a higher risk of neurologic deficit compared with other blocks.2

Second, injury to the dorsal scapular and long thoracic nerves has recently been described in association with ultrasound-guided interscalene block.14,15 These nerves course through the middle scalene muscle at the level of the interscalene groove but are not always easy to visualize.16 The needle path in the superior trunk block does not traverse the middle scalene muscle and thus reduces the possibility of inadvertent needle trauma to these nerves.

Phrenic nerve palsy is a well-recognized risk of interscalene block but has received increased attention following recent reports of persistent phrenic nerve palsy, attributed in part to inflammatory scarring, perhaps from the myotoxicity of local anesthetics.17-19 The use of lower volumes of local anesthetic in ultrasound-guided interscalene block has been shown to reduce, but not eliminate, the incidence of phrenic nerve palsy.3,4 This is not surprising given that the phrenic nerve lies within 2 mm of the brachial plexus at the level of the cricoid cartilage (where it can often be seen superficial to the anterior scalene muscle).20 Nevertheless, it diverges from the brachial plexus by an additional 3mm for every 1cm that it descends into the root of the neck.20 It therefore seems prudent to inject local anesthetic as distal as possible in an attempt at further minimizing the risk of involvement of the phrenic nerve.

Finally, in the 30-35% of individuals whose C5 nerve root takes an anomalous course through the anterior scalene muscle,7,8 the scanning process associated with the superior trunk block facilitates recognition of this anatomic variation and improves the chance of block success, as the C5 root inevitably joins the C6 nerve root to form the superior trunk.

The main caveat to the superior trunk block is that the transverse cervical artery may lie across and superficial to the brachial plexus at this level.21 Its presence should always be excluded before commencing needle insertion by transiently releasing pressure on the probe (in case the vessel is compressed and therefore invisible) and use of colour Doppler.

One of the greatest advantages of ultrasound-guided regional anesthesia has been the enhanced knowledge of anatomy that it both demands as well as provides. It has also removed our dependence on surface anatomical landmarks, and these factors have allowed our specialty to devise new block techniques and refine existing ones. Further research is planned to confirm the efficacy and safety of the technique, in particular the incidence of phrenic nerve palsy, compared with the conventional ultrasound-guided interscalene block.