Axillary Block

INDICATION

Surgery of the forearm and elbow (and hand)

The aim is to block the terminal branches of the brachial plexus which include the median, ulnar, radial and musculocutaneous nerves.

Contraindications:

See general contraindications for regional anaesthesia

Complications:

See general complications

Blocking multiple separate nerves – consider needle trauma/intraneural injections

NOTE: This is the safest of the approaches to the brachial plexus as there is no risk of pneumothorax or blocking the phrenic nerve at this level and the vessels at this location are compressible should unintended vascular injury occur.

NOTE: for hand and finger surgery, give preference to more distal peripheral nerve blocks where possible

ANATOMY

The brachial plexus consists of the anterior rami of the spinal nerves C5-T1. It provides motor and sensory innervation to the entire upper limb (except for the skin of the upper medial half, where sensory innervation is supplied by the intercostal brachial nerve).

The roots leave the spinal cord via the respective intervertebral foramina, pass between the anterior and medial scalene muscles to enter the base of the neck. They converge and divide into trunks, divisions and cords before arriving in the axilla where they give rise to five major branches.

Axillary nerve (C5-6)

This nerve is NOT covered by axillary block

Musculocutaneous nerve (C5-6-7)

Runs in the fascial plane between the coracobrachialis muscle and the short tendon of the biceps muscle

Motor: elbow flexors (brachialis, biceps brachii and coracobrachialis muscles) + supination

Sensory: lateral cutaneous branch of the forearm: skin of the anterolateral forearm

NOTE: A secondary needle insertion point is often needed to anaesthetise this nerve when performing an axillary block

Radial nerve (C5-T1)

Runs posteriorly to the axillary artery before travelling along the humeral groove

Motor: elbow and wrist extensors (triceps brachii, brachioradialis, extensor carpi radialis longus)

Sensory: lower lateral cutaneous nerve of the arm (lateral aspect of the arm, posterior cutaneous nerve of the arm and forearm (posterior surface of arm, forearm) and superficial branch (dorsal skin of hand and digits I-IV)

Median nerve (C6-T1)

At level of the axilla, this nerve is located laterally to the axillary artery

Motor: flexors of wrist and digits I-IV and pronation (pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, pronator quadratus, and the lateral half of the flexor digitorum profundus), thenar muscles (flexor pollicis longus) and intrinsic hand muscles (lateral lumbricales muscles)

Sensory: palmar cutaneous branches (lateral and palmar surface of palm and digits I-IV and dorsal fingertip of digits I-III

Ulnar nerve (C8-T1)

At level of the axilla, this nerve is located medially to the axillary artery and often in proximity to the axillary vein(s)

Motor: finger flexors medial side (flexor carpi ulnaris, medial half of flexor digitorum profundus), hypothenar muscles (flexor digiti minimi brevis, abductor digiti minimi, opponens digiti minimi muscles), thenar (adductor pollicis muscle) and intrinsic hand muscles (medial lumbricales muscles, palmaris brevis and interosseus muscles of the hand)

Sensory: palmar cutaneous branch (medial half of the palm), dorsal cutaneous branch (dorsal surface of medial palm and digit IV and V) and superficial branch (palmar surface of digit IV and V)

PATIENT POSITION

SONOANATOMY

Ultrasound probe selection: linear high frequency probe

Place the ultrasound transverse in the axillary crease across the junction of the biceps and pectoralis muscles.

Locate the axillary artery lying on the conjoint tendon, and the axillary veins around it (confirm with colour doppler).

NOTE: Balloting with the probe will cause the nerves to move in relation to the surrounding vascular structures and will aid in identification.

The nerves are hyperechoic round structures around the artery. The best approach to identify the nerves and to accommodate for anatomical variations is to scan up and down the upper arm to trace the individual nerves and confirm their location in the axilla in relation to the artery.

Musculocutaneous nerve

Axilla: fascial plane between the coracobrachialis muscle and the short tendon of the biceps muscle.

Upper arm: in the flexor compartment between brachialis and biceps brachii muscles to finally pierce the latter and emerge on the lateral surface of the elbow to continue into the forearm as the lateral cutaneous nerve.

NOTE: due to its more laterally and slightly more distal location in the axilla (not inside the axillary neurovascular sheath), a secondary needle insertion point is often needed when performing an axillary block

Radial nerve

Axilla: deep to the artery (posterior), 4-6 o’clock

Upper arm: dips toward the humerus to travel in a shallow depression in the bone (radial groove) which spirals around the posterior surface towards the lateral epicondyle. Here the radial nerve splits into a deep and superficial branch.

Median nerve

Axilla: lateral and superficial to the artery, 9-12 o’clock

Upper arm: travels alongside the axillary and then brachial artery, initially lateral but crosses over to the medial side halfway down the upper arm to enter the anterior compartment of the forearm via the cubital fossa.

Ulnar nerve

Axilla: medial to the artery, 1-3 o’clock

Upper arm: travels away from the artery to continue down the medial aspect of the upper arm. Halfway down it enters the posterior compartment to reach the ulnar tunnel (small space between medial epicondyle and olecranon) in which it passes the elbow posteriorly.

NOTE: Once you've identified the individual nerves and their positions relative to the artery, ensure to return the probe back to the starting position in the axillary crease where the axillary artery rests on the conjoint tendon (diagonal line across the screen). This is the desired level for conducting the block.

Note: anatomical variations exist. The median nerve may appear as two separate structures or is sometimes bundled with the musculocutaneous nerve.