Anatomy
ANATOMY
The brachial plexus is formed by the ventral rami of C5 to T1 nerve roots. The C5, C6 and C7 nerve roots travel superficially (1–3cm depth) through the interscalene groove, which is found between the scalenus anterior and medius muscles. In the interscalene groove the C5 and C6 roots can lie on top of one another or may be separated by a muscular bridge between the anterior and median scalene muscles. The C7 root is found deep to the C6 root and is often in close proximity to the vertebral artery. The C5 and C6 roots combine to form the upper trunk and C7 continues to form the middle trunk of the brachial plexus. Both upper and middle trunks supply sensation to the shoulder and upper arm.
Positioning
POSITIONING
Patient supine, head turned slightly to the contralateral side. Transducer applied at the level of the cricoid cartilage in the axial, oblique plane to obtain the best possible transverse view of the brachial plexus roots.
Figure 1: Patient, transducer and needle positioning for a) in-plane interscalene block and b) out-of-plane interscalene block.
Preliminary Scan
PRELIMINARY SCAN
Identify carotid artery, internal jugular vein and sternocleidomastoid muscle. Sternocleidomastoid muscle lies superficially with the internal jugular vein lying underneath. Slowly move your transducer posteriorly to visualise the nerve roots as distinct round or oval hypoechoic structures (string of pearls) arranged linearly between the anterior scalene and middle scalene muscles. The scalene muscles are distinct round structures posterior to the vessels and deep to the sternomastoid muscle.
The interscalene groove between these muscles may be accentuated by asking the patient to sniff. Now adjust the transducer position to obtain the best picture and optimal entry point.
Figure 2: a) ultrasound image showing the brachial plexus within the interscalene groove, and surrounding structures b) annotated image (SCM=sternocleidomastoid muscle, JV=jugular vein, CA=carotid artery, AS=anterior scalene muscle, MS=middle scalene muscle, C7-TP=transverse process of C7, C5=C5 nerve root, C6=C6 nerve root, C7=C7 nerve root).
ALTERNATIVELY
Place the transducer in the supraclavicular fossa and obtain a view of the brachial plexus (see chapter 3.3). Now scan cephalad maintaining the plexus in the middle of the image. Observe the lung and subclavian artery fall away, the dark scalene muscles appear. The plexus divisions here form into the roots and can be seen in a line (C5, C6, C7 from supercial to deep).
Technique
TECHNIQUE
|
Positioning |
Supine, head turned to contralateral side |
|
Transducer |
High frequency linear probe |
|
Depth |
1–4 cm |
|
Needle |
25–50 mm |
|
Plane |
Transverse |
|
Needle approach |
In-plane or out-of-plane |
|
Injection |
5–10 ml for analgesia / 15–20 ml for anaesthesia |
Our preference is to initially position the needle tip deep to the C6 nerve root and then inject 2 ml of local anaesthetic whilst observing the spread. If local anaesthetic is seen to spread anterior and posterior to the nerve roots to surround both the C5 and C6 roots then this needle position may be suf cient. It may be necessary to reposition the needle tip superficial to the C5 root or between the C5 and C6 roots (if there is sufficient space between them) to achieve adequate spread to both the C5 and C6 roots. Spread anteriorly over the superficial part of the scalenus anterior will consistently block the phrenic nerve.
Low volume interscalene block will miss the superficial cervical plexus. This may need to be blocked separately to reduce intraoperative pain during arthroscopic shoulder surgery. You can block either the whole superficial cervical plexus or selectively block the medial and lateral supraclavicular nerves (See Figure 3).
Figure 3: a) ultrasound image of the brachial plexus in the interscalene groove and the medial and lateral supraclavicular nerves b) annotated image
(M SCN=medial supraclavicular nerve; L SCN=lateral supraclavicular nerve). Long thoracic and the dorsal scapular nerve often run in the middle scalene and can be seen here.
Tips
TIPS
1. The roots leave the vertebral foramina and pass inferiorly and laterally. In order to optimise your image, the transducer needs to be directed slightly caudad so that the ultrasound beam meets the roots perpendicularly (not perpendicular to skin).
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Anatomical variation is extremely common in the interscalene region, with variability in the size of the roots and the level at which the trunks and divisions are formed. Muscular bridges may occur within the scalene muscles, separating the roots. Separate injections will then be required to block the brachial plexus roots.
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With increasing age the scalene muscles can appear indistinct and the interscalene groove difficult to identify. Identification of individual nerve roots can be achieved by following them proximally towards the intervertebral foramen. C5 and C6 nerve roots and vertebral level can be identifed using the characteristic shape of each transverse process: C5 and C6 have a bifid shape created by the presence of both anterior and posterior tubercles. The C5 TP has a larger posterior tubercle (Figure 4a), whereas the C6 transverse process has a larger anterior tubercle (Figure 4b). The C7 transverse process only has a posterior tubercle and no anterior tubercle (Figure 4c).
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Injections close to C7 root should be undertaken with extreme caution due to the risk of puncturing the vertebral artery of other vessels. The vertebral artery can be seen anterior to the C7 transverse process using colour Doppler (Figure 5).
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In patients with short necks this technique can be performed with the patient in the lateral position (side to be blocked up).
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The incidence of ipsilateral phrenic nerve palsy can be reduced by a) reducing the injected volume of local and b) positioning the needle adjacent to C6 rather than C5 nerve root.
Figure 4: a) C5 transverse process with prominent posterior tubercle (PT) and smaller anterior tubercle (AT)
b) C6 transverse process with small posterior tubercle (PT) and prominent anterior tubercle also know as Chassaignacs’s tubercle (CT). Note the C5 and C6 nerve roots superficial to the transverse process.
c) C7 transverse process with a posterior tubercle (PT) and no anterior tubercle. Note the C5, C6 and C7 nerve roots super cial to the transverse process.
Figure 5: a) Colour Doppler applied anterior to the C7 transverse process
Other Resources
Plan B/C/D blocks for the shoulder include: Superior trunk block, combined axillary and suprascapular nerve blocks.
More information about the interscalene block can be found at the excellent 
article here
Text and images have been reproduced from the 2nd edition of the RA-UK Handbook, which can be purchased on Amazon, or is received on joining RA-UK. This excellent resource also contains practical descriptions of all of the advanced blocks referenced in the editorial.