The axillary region is the area between the superior portion of the upper limb and the thorax. Bordered by the serratus anterior and the thoracic wall medially, a part of the humerus laterally, the pectoralis muscles anteriorly, together with the latissimus dorsi, but not only, posteriorly, the axilla is an important region, both in terms of neurovasculature and clinical relevance.
The axilla allows the passage of several muscles, blood vessels such as the axillary artery and vein, and crucial nerves like the brachial plexus. In this article, we will explore all of the contents more deeply, and we will examine the anatomy and relevant clinical conditions of the axillary region.
Medially - serratus anterior, thoracic wall
Laterally - intertubercular sulcus of the humerus
Anteriorly - pectoralis major and minor
Posteriorly - subscapularis, latissimus dorsi, teres major muscles
Muscles - coracobrachialis, pectoralis minor, and the biceps brachii
Blood vessels - axillary artery and vein
Nerves - brachial plexus
Lymphatics - axillary lymph nodes (pectoral, subscapular, lateral, central, infraclavicular groups)
The axilla is a three dimensional pyramidal space which changes shape due to its location and arm movement.
Its apex is bound superiorly by the root of the neck. The apex is also referred to as the axillary inlet. The borders of the apex are made by the lateral surface of the first rib, the posterior surface of the clavicle, and the superior margin of the scapula.
The medial border is created by the serratus anterior, as well as the thoracic wall which includes intercostal muscles and costals (ribs) of that region.
The lateral border is made by the intertubercular sulcus (groove) of the humerus.
The floor, or base, of the axilla is the axillary fascia and its skin on the surface of the axilla, aka the ‘armpit’. The axilla is filled with adipose (fat) and allows passage for: vessels, nerve plexus, lymphatics, and muscles. The muscles include the coracobrachialis, pectoralis minor, and the biceps brachii.
The coracobrachialis arises from the coracoid process of the scapula and inserts onto the medial aspect of the humerus. The pectoralis minor arises from costals (ribs) 3-5 and inserts onto the coracoid process of the scapula. The long and short heads of the biceps brachii arise from the supraglenoid tubercle (long head) and coracoid process (short head). They insert onto the radial tuberosity and bicipital aponeurosis.
The posterior border of pectoralis major and minor defines the anterior border (fold).
The axilla contains both the axillary artery and axillary vein. The vein runs medial to the artery along its length. The cords of the brachial plexus surround the axillary artery and are named after their position relative to the artery.
The subclavian artery is renamed the axillary artery once it passes the lateral border of the first rib. The axillary artery supplies the upper limb and is divided into three parts: the region proximal to pectoralis minor, the region beneath it, and the region distal to it.
The first section gives rise to the superior thoracic artery, which supplies the first and second intercostal spaces.
The second part gives rise to the lateral thoracic artery and the thoracoacromial artery ,which has a pectoral branch, acromial branch, clavicular branch, and deltoid branch. The pectoral branch supplies the pectoral muscles and anastomoses with the internal and lateral thoracic arteries. The acromial branch supplies the deltoid and also anastomoses with arteries around the shoulder (thoracoacromial, subscapular, and posterior humeral circumflex arteries). The clavicular branch supplies subclavius, and the sternoclavicular joint. The deltoid branch runs in the deltopectoral groove and supplies the deltoid and pectoralis major muscles.
The third section gives rise to the subscapular artery, the anterior circumflex humeral artery, and the posterior circumflex humeral artery. The subscapular artery supplies the supraspinatus and the infraspinatus muscles. The anterior and posterior circumflex humeral arteries supply the muscles near the surgical neck of the humerus.
The axillary vein drains the upper limb and is formed by the unification of the basilic and brachial veins. The cephalic vein joins at the proximal region of the vein. It is renamed the subclavian vein once it passes the lateral border of the first rib.
The brachial plexus arises from the ventral rami of C5 to T1. The medial cord gives rise to the ulnar nerve (C8-T1). This nerve is the main nerve that innervates the intrinsic hand muscles, apart from the thenar eminence and radial two lumbricals. Flexor carpi ulnaris and the ulnar head of flexor digitorum profundus are also innervated by the ulnar nerve. The ulnar nerve also supplies sensation to the palmar and dorsal surfaces of the ulnar one and a half fingers.
The posterior cord gives rise to the radial nerve (C5-T1), which innervates the triceps brachii and the extensor muscles of the forearm. It also supplies sensation over the posterior surface of the arm, forearm, and hand. The axillary nerve (C5-6) is also a branch of the posterior cord, and innervates the deltoid and teres minor muscles. It also supplies sensation over the regimental patch area of the shoulder. The thoracodorsal nerve (C6-8) is also a branch of the posterior cord and innervates the latissimus dorsi.
The lateral cord gives rise to the musculocutaneous nerve (C5-6/7), which innervates the flexor compartment of the arm: biceps brachii, brachialis, and coracobrachialis. It also is renamed the lateral cutaneous nerve of the forearm (antebrachial) after it leaves the flexor compartment of the arm.
The median nerve (C6-T1) is formed by the unification of branches from the medial and lateral cords. It innervates all the muscles of the flexor compartment of the forearm, aside from flexor carpi ulnaris and the ulnar head of flexor digitorum profundus. In the hand, the medial nerve supplies the thenar eminence and radial two lumbricals. It also supplies sensation over the palmar aspect of the radial 3 ½ fingers, as well as the nailbeds of these fingers dorsally.
Compression of these cords, or nerves in the axillary region, can result in muscle wasting, fasciculations, and weakness. Loss of sensation may also result. The axillary region is therefore an important clinical region, simple due to the multitude of structures that pass through the region.
The long thoracic nerve arises from the C5-7 nerve roots and innervates the serratus anterior. This nerve descends through the axillary region. The intercostobrachial nerves are the sensory (cutaneous) branches of the intercostal nerves. They descend in the axilla and supplies the posterior and medial surface of the arm.
Axillary Lymph Nodes
These are divided into five groups.
The anterior group (pectoral group) drains the lymph from the anterolateral aspect of the abdominal wall, superior to the level of the umbilicus and the lateral quadrants of the breast.
The posterior group (subscapular group) drain the superficial lymphatic vessels from the back, as far back as the superior border of the iliac crests.
The lateral group drains the majority of lymphatic fluid from its upper limb.
The central nodes receive lymph flow from the anterior, posterior, and lateral group of lymph nodes. These all drain into the apical nodes (subclavicular group), which are located at the lateral border of the first rib at the apex of the axilla. The lymph then flows to the subclavian lymph trunk. The drainage is different on the left and right sides. The left side axillary drainage flows into the thoracic duct. Whereas on the right side the drainage is into the right lymphatic trunk.
The infraclavicular (deltoid group) of lymph nodes do not strictly reside within the axilla, however, they lie in the deltopectoral groove and drain the superficial lymphatic vessels of its associated upper limb (arm, forearm, and hand).
Radial Nerve Palsy
Axillary region compression or stretching leading to damage can result in palsy (paralysis with tremors) of the radial nerve. This can also result due to midshaft humeral fracture. The radial nerve supplies the triceps brachii, as well as all the extensor muscles of the forearm. It also supplies cutaneous sensation to some of the dorsal portions of the hand. The radial nerve is particularly vulnerable as it resides deep in the axilla. Compression from prolonged pressure on the axilla can therefore result in wrist drop. Grip strength is dependent on the wrist being in slight extension, which is not possible in cases of wrist drop.
The long thoracic nerve (arises from the C5-7 nerve roots) supplies the serratus anterior. This nerve descends in the axillary region and compression or damage of the nerve may give rise to winged scapula deformity. The serratus anterior is no longer able to protract or stabilize the scapula against the thoracic wall. Compression of the nerve may be a result from a tumour, an enlarged lymph node, or an aneurysm.
Breast cancer is the most common known cancer to affect women worldwide. The vast majority of lymphatic drainage of the breast flows through the axillary lymph nodes. Fine needle aspiration of lymph nodes can aid in diagnosis of breast cancer, as well as determining if the cancer has spread.
Chest Tube (Drain) Insertion
If a patient has a pneumothorax, pleural effusion, or haemothorax (air, fluid, or blood in the intrapleural space, respectively), a chest tube is likely to be inserted. Local anaesthetic and skillful technique are required for the procedure to be safe. The safe region to insert this is in the triangle of safety. This is marked by the lateral border of pectoralis major anteriorly, the apex is just below the axilla, the horizontal line at the level of the nipple marks the base, and posteriorly by the anterior border of latissimus dorsi.
The tube should also be placed above the 6th rib, in the 5th intercostal space. This avoids the intercostal neurovascular bundle under the 5th rib. The drain should be kept below the level of the heart to ensure drainage. Risks of the procedure include damage to the lung, inducement of a pneumothorax, or infection (pleuritis) of the intrapleural space.