Anatomy
ANATOMY
The cords of the brachial plexus divide into the terminal branches within the axilla. The lateral cord divides into 3 nerves: lateral root of the median nerve, musculocutaneous and lateral pectoral nerve. The posterior cord divides into
5 nerves: radial, axillary, thoracodorsal, lower subscapular and upper subscapular. The medial cord also divides into
5 nerves: medial root of the median nerve, ulnar nerve, medial pectoral, medial cutaneous nerve of arm and medial cutaneous nerve of forearm. The nerves supplying the arm pass through the base of the axilla into the upper arm where they lie in close proximity to the brachial artery.
The musculocutaneous nerve often leaves the plexus high in the axilla, to lie within the coracobrachialis muscle
at the level of the axillary block.
Positioning
POSITIONING
Patient supine, with arm abducted and elbow flexed. Apply the transducer transversely across the axilla at the junction of the biceps and pectoralis muscles.
Figure 1: Patient, transducer and needle positioning for a) in-plane axillary block b) out-of-plane axillary block.
Preliminary Scan
PRELIMINARY SCAN
Identify the axillary artery (pulsatile, anechoic) and veins (compressible, anechoic, number and position variable). Scan anteriorly to identify the two heads of biceps and the coracobrachialis beneath (posteriorly). The musculocutaneous nerve is a bright, echogenic structure between these muscles and ‘swims’ anteriorly as the transducer is moved distally. Scan posteriorly to identify either the conjoined tendon of teres major and latissimus dorsi or long head of triceps.
Figure 2: a) ultrasound image of the brachial plexus and surrounding structures in the axilla b) annotated image (MCN=musculocutaneous nerve; MN=median nerve; UN=ulnar nerve; RN=radial nerve; AA=axillary artery; AV=axillary vein; Tendon=conjoined tendon of teres major and latissimus dorsi).
Now look for structures around the artery that do not change with gentle compression and release. These are likely to be nerves. The nerves can be traced along their course in the upper arm to confirm their identity:
Median nerve: lies on the biceps side of the artery, usually more superficial to it. It is a hypoechoic structure, which may appear as two components – the lateral and medial heads. The nerve traverses the artery in the lower arm to lie medial to the artery at the antecubital fossa.
Ulnar nerve: lies on the triceps side of the artery, often beneath the main axillary vein. It is a hyperechoic structure and often the easiest nerve to identify. Scanning distally the ulnar nerve remains superficial, moving away from the axillary artery to stay on the triceps side, as it heads towards the medial epicondyle of the humerus.
Figure 3: a) ultrasound image of the median and ulnar nerves in the mid humerus b) annotated image (MN=median nerve; AA=axillary artery; AV=axillary vein; UN=ulnar nerve).
Radial nerve: The radial nerve can be difficult to visualise clearly as its path is not perpendicular to the transducer. Scanning distally it dives deep heading towards the triceps side of the humerus, accompanied by the profunda brachii artery. It travels between the heads of triceps to pass around the spiral groove of the humerus. To improve visibility, rotate the transducer slightly and tilt cephalad. High in the axilla the nerve lies consistently on the conjoint tendon of teres major and latissimus dorsi often deep to the artery.
Figure 4: a) ultrasound image of the radial nerve and surrounding structures in the mid humerus. Note how the median nerve is not visible next to the artery due to the cephalad tilt on the transducer required to visualise the radial nerve. b) annotated image (AA=axillary artery; RN=radial nerve; PB=profunda brachii artery; MCN=musculocutaneous nerve; MHT=medial head triceps; LHT=long head triceps).
Musculocutaneous nerve: A hyperechoic structure, may lie close to the median nerve adjacent to the artery, but more commonly is identifed intramuscularly between biceps and coracobrachialis muscles. Scanning distally, it travels away from the artery running towards the biceps muscles in the intersection between the two heads of biceps and coracobrachialis.
Figure 5: a) ultrasound image of the musculocutaneous nerve as it lies between biceps and coracobrachialis muscles in the arm b) annotated image (MCN=musculocutaneous nerve; AA=axillary artery).
The smaller terminal nerves (e.g. medial cutaneous nerve of arm and forearm) that accompany the major nerves in the axilla are usually blocked with the technique described below.
The intercosto-brachial nerve lies outside of the axillary sheath and needs to be blocked independently if required.
Technique
Ultrasound guidance in the axillary approach allows the use of reduced volume of local anaesthetic and selective blockade of all or some of the nerves as indicated. The block can
be performed with either an in-plane or out-of-plane technique. Using the in-plane technique
start with either the median or ulnar nerve. Then pass deep to the artery to infiltrate around
the radial nerve. Using the out-of-plane technique, keep the needle close to the artery.
The musculocutaneous nerve can often be approached using the same needle insertion point used for the other nerves, however if it is a signi cant distance from the artery a separate needle entry site may be required, often slightly more distal and overlying the biceps muscle. Inject up to 5 ml of local anaesthetic around each nerve in 1–2 ml aliquots whilst observing spread.
Other Resources
Plan B/C/D blocks for the upper limb below shoulder include: Infraclavicular block, supraclavicular block
More information about the axillary 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.