The axilla is a fat-filled space, the shape of a truncated cone, lying between the lateral thoracic wall and the arm. At its apex it is bounded by the upper border of the scapula, the outer border of the first rib and the middle third of the clavicle and it is through this that structures pass from the posterior triangle of the neck to the upper limb. The base of the axilla is formed by skin and subcutaneous tissue. It is bounded anteriorly by the pectoralis major and, deep to that, pectoralis minor. The posterior wall extends lower than the anterior wall and is formed, from above downwards by subscapularis, latissimus dorsi and teres major muscles. The axilla is limited medially by the upper ribs and intercostal spaces, which are here covered by slips of the serratus anterior muscle. The lateral wall is thin, being represented by the narrow intertubercular groove of the humerus into the lips of which the muscles of the anterior and posterior walls are inserted. It contains the axillary artery and vein, cords and branches of the brachial plexus, coracobrachialis and biceps muscles, axillary lymph nodes and vessels and fat.
The axillary artery begins as the continuation of the subclavian artery at the outer border of the first rib below the middle of the clavicle. It passes through the axilla to become the brachial artery as it leaves at the lower border of teres major. Throughout its course it lies on the muscles of the posterior wall of the axilla and is surrounded by the cords and branches of the brachial plexus. The axillary vein lies medial to this neurovascular bundle; coracobrachialis and the short head of biceps are lateral. Anteriorly, the vessels and nerves are crossed by the pectoralis minor. The axillary artery supplies branches to the upper thoracic wall, the muscles of the axilla and the shoulder joint. Its largest branch, the subscapular artery, gives an important contribution to the scapular anastomosis, which provides a collateral circulation to the upper limb in the event of obstruction of the axillary artery, by virtue of its communication with branches of the thyrocervical trunk that arise from the subclavian artery.
The axillary vein is a continuation of the brachial vein and ascends through the axilla, medial to its artery, before leaving it to become the subclavian vein at the outer border of the first rib. Its branches correspond to those of the artery, apart from the cephalic vein, which drains the superficial tissues of the upper limb. It ascends in the groove between pectoralis major and deltoid muscles before piercing the deep fascia just below the clavicle. Obstruction of the axillary vein by a thrombus, though not as common as that of the leg veins, occasionally occurs after prolonged infusion of hyperosmotic fluids and/or intravenous nutrition. It results in acute swelling of the upper limb and possibly pulmonary embolus.
The lymph vessels within the axilla are superficial or deep. The superficial group drains the skin. Medially the vessels accompany the basilic vein and terminate in the axillary nodes. Laterally the vessels accompany the cephalic vein and drain via infraclavicular nodes into the axillary nodes.
The axillary lymph nodes drain the upper limb, the upper abdominal wall and the pectoral region and receive most of the lymphatic drainage of the breast. They are arranged in five groups (Figure 1).
The anterior group lie deep to pectoralis major and drain the lateral and anterior chest wall, the breast and the upper abdominal wall.
The lateral group lie on the lateral wall of the axilla receiving the efferent vessels from the upper limb.
The central group are arranged around the axillary vessels in the axillary fat.
The posterior group lie to the lateral edge of the subscapularis muscle on the posterior wall of the axilla.
The apical group lie at the apex of the axilla immediately behind the clavicle, they are continuous with the inferior deep cervical nodes and receive drainage from all the preceding groups.
A subclavian lymph trunk conveys the lymph from the apical group to the right jugular trunk or the thoracic duct.
The brachial plexus lies in the posterior triangle of the neck along a line from the middle of the posterior border of the sternocleidomastoid muscle to the middle of the clavicle (Figure 2; for brachial plexus block see page 114). It is formed from the anterior, (ventral) primary rami of the lower four cervical and the first thoracic nerves. These five roots of the plexus emerge between the middle and anterior scalene muscles and unite to form three trunks. The upper two roots (C5, 6) form the upper trunk, the middle root (C7) the middle trunk, and the lower two roots (C8, T1) form the lower trunk. Behind the middle of the clavicle, at the apex of the axilla, each trunk divides into anterior and posterior divisions. The three posterior divisions join to form the posterior cord, the anterior divisions of the upper and middle trunks form the lateral cord, and the anterior division of the lower trunk continues as the medial cord. The cords lie around the axillary artery, related to it as their names imply (Figure 3), and are enclosed with the artery in a neurovascular sheath, the axillary sheath (Figure 2). The posterior cord and its branches supply structures on the dorsal surface of the limb. The posterior cord ends by dividing into axillary and radial nerves.
The medial and lateral cords and their branches supply structures on the flexor surface of the limb; the lateral cord ends by dividing into the musculocutaneous nerve and the lateral head of the median nerve, the medial cord ends as the ulnar nerve and the medial head of the median nerve (Figure 4).
Branches of the plexus
From the roots there are small branches to the back muscles. The long thoracic nerve passes posteriorly to the medial wall of the axilla to supply serratus anterior.
From the upper trunk arises the suprascapular nerve, which crosses the root of the neck to supply supraspinatus and infraspinatus.
From the lateral cord branches the lateral head of the median nerve, the lateral pectoral nerve that supplies pectoralis major, the musculocutaneous nerve that supplies coracobrachialis, biceps and brachialis before becoming the lateral cutaneous nerve of the arm.
From the medial cord: the medial pectoral nerve supplies pectoralis major and minor, the ulnar nerve supplies flexor muscles in the forearm, small muscles in the hand and the skin of the medial side of the hand. Also branching from the medial cord are the medial head of median nerve, the medial cutaneous nerves of the arm, which supplies skin on the medial side of the arm, and the medial cutaneous nerve of the forearm, which supplies the skin of the medial side of the forearm.
From the posterior cord the subscapular nerves descend the posterior axillary wall to supply subscapularis and teres major. The thoracodorsal nerve supplies latissimus dorsi by entering its upper border. The axillary nerve descends on subscapularis and turns posteriorly around the surgical neck of the humerus, supplying deltoid, teres minor and the shoulder joint before ending as the upper lateral cutaneous nerve of arm, which supplies a small patch of skin over the insertion of deltoid muscle. The radial nerve is a terminal branch of the posterior cord and descends through the posterior compartment of the arm supplying triceps, brachioradialis and extensor carpi radialis longus and brevis and sensory branches to the skin on the posterior and medial sides of the limb and the posterior surface of the lateral three and a half fingers.
The median nerve (C6–T1), descends deep to the axillary artery to reach its medial side midway down the arm. It crosses the elbow deep to the common flexor origin anterior to the medial epicondyle of the humerus at which point it is accessible for regional anaesthesia. Through the forearm it descends deep to flexor digitorum superficialis but, at the wrist, it becomes superficial, and accessible for regional anaesthesia, in the midline on the ulnar side of flexor carpi radialis before entering the hand deep to the flexor retinaculum. It gives cutaneous supply to the radial side of the palm, and the palmar and a variable portion of the dorsal surface of the radial three and a half digits (Figure 5a).
The ulnar nerve descends medial to the axillary artery until, midway down the arm, it pierces the medial intermuscular septum and crosses the elbow behind the medial epicondyle (where it is palpable and accessible for regional anaesthetic procedures). It descends the forearm deep to flexor carpi ulnaris and on the ulnar side of the ulnar artery. 5 cm above the wrist a dorsal cutaneous branch emerges to supply the dorsal aspects of the ulnar one and a half digits. The main nerve crosses superficial to the flexor retinaculum to supply intrinsic muscles of the hand and the palmar one and a half digits (Figure 5b). Blocking the dorsal and palmar skin of these digits requires separate injections.
The radial nerve descends posterior to the axillary artery and passes posteriorly in the spiral groove of the humerus between the long and medial heads of triceps. In the lower third of the arm it re-enters the anterior compartment, lateral to the humerus, and crosses the elbow anterior to the lateral epicondyle deep to brachioradialis. Its muscular branches to the forearm extensors are all conveyed in the posterior interosseous nerve. Its cutaneous branches, supplying the radial side of the dorsal surface of the hand (Figure 5c), are conveyed in the superficial radial nerve, which emerges posterior to brachioradialis tendon above the wrist. The nerve can be blocked at the elbow lateral to the tendon of biceps and, at the wrist, 2–3 cm proximal to the radial styloid.
Digital nerve block
Two palmar and two dorsal nerves supply each digit and these can be blocked by dorsal injections into the webspaces, one on each side of the finger.
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