The axilla and breast lymphatics: walls, contents, node levels and the nerves at risk

A pyramid between the arm and the chest wall

The axilla is the pyramidal space below the shoulder joint through which the vessels and nerves of the upper limb pass between the root of the neck and the arm. Its apex, the cervico-axillary canal, is bounded by the clavicle in front, the outer border of the first rib medially and the upper border of the scapula behind: the subclavian artery becomes the axillary artery as it crosses the first rib here, and the cords of the brachial plexus enter with it. Its base is the skin and axillary fascia of the armpit, stretched between the anterior axillary fold, made by the lower border of pectoralis major, and the posterior axillary fold, made by latissimus dorsi wrapping round teres major. The anterior and posterior walls converge laterally on the narrow intertubercular groove of the humerus, so the space is widest medially against the chest wall, where the surgeon works.

Inside the pyramid, embedded in fat, lie the axillary artery and its branches, the axillary vein and its tributaries, the cords and branches of the brachial plexus wrapped with the artery in the axillary sheath (a sleeve of prevertebral fascia), the axillary lymph nodes, and the intercostobrachial nerve crossing to the arm. The long thoracic nerve lies on the medial wall and the thoracodorsal nerve on the posterior wall rather than in the fat, which is what allows a clearance to remove the fat and nodes and leave both nerves intact.

The axillary artery and vein

The axillary artery runs from the outer border of the first rib to the lower border of teres major, where it becomes the brachial artery. Pectoralis minor crosses in front of it and divides it into three parts, which conveniently have one, two and three branches, and the cords of the brachial plexus are named lateral, posterior and medial by their relation to the second part, behind the muscle. The subscapular artery from the third part joins the scapular anastomosis with the suprascapular and dorsal scapular arteries from the subclavian system, so a ligature placed above the origin of the subscapular artery is usually tolerated, whereas one beyond it carries a much greater risk of ischaemia of the limb. The axillary vein begins at the lower border of teres major as the continuation of the basilic vein, joined by the venae comitantes of the brachial artery. It lies medial to the artery, receives the cephalic vein after that vein has pierced the clavipectoral fascia, and becomes the subclavian vein at the outer border of the first rib. It marks the upper limit of a standard level I and II clearance.

Nerves at risk in axillary surgery

Long thoracic nerve (C5–C7)

Arises from the roots of the plexus, passes down behind it and then runs on the outer surface of serratus anterior along the medial wall, roughly in the mid-axillary line. Divided, it paralyses serratus anterior: the medial border of the scapula lifts off the chest wall (winging) when the patient pushes against a wall, and the arm cannot be raised fully above the head. It is kept safe by leaving the fascia of the chest wall undisturbed.

Thoracodorsal nerve (C6–C8)

Arises from the posterior cord and runs down the posterior wall with the thoracodorsal vessels to latissimus dorsi. Injury weakens extension, adduction and medial rotation of the arm. The thoracodorsal vessels are also the pedicle of a latissimus dorsi flap for breast reconstruction.

Intercostobrachial nerve (T2)

The lateral cutaneous branch of the second intercostal nerve crosses the axilla through the fat to the upper medial arm. It is often divided in a clearance, leaving numbness of the axillary floor and the inner upper arm, and patients are warned of this beforehand.

Medial and lateral pectoral nerves

Named after their cords, not their positions. The medial pectoral nerve (C8, T1) pierces pectoralis minor to supply it and the lower sternocostal part of pectoralis major; the lateral pectoral nerve (C5–C7) pierces the clavipectoral fascia above pectoralis minor to supply the clavicular and upper sternocostal parts. Dividing pectoralis minor to reach level III nodes risks the medial nerve and later wasting of the lower pectoralis major.

Axillary nodes: anatomical groups and surgical levels

Anatomists describe five groups of axillary nodes by their position and the vessels they follow; surgeons and pathologists describe three levels by their relation to pectoralis minor, and that is the language of an operation note or a pathology report. Lymph generally passes from level I to level II to level III, then through the subclavian lymph trunk to the great veins at the root of the neck, with some channels to the supraclavicular nodes. Spread that misses out a level, a skip metastasis, is uncommon.

The breast and its lymphatic drainage

The adult breast lies in the superficial fascia over pectoralis major and part of serratus anterior, from about the second to the sixth rib and from the edge of the sternum towards the mid-axillary line, with an axillary tail that passes through the deep fascia into the axilla. A loose retromammary space separates it from the pectoral fascia and lets it move on the chest wall; a carcinoma that has invaded the fascia becomes fixed, which is tested by examining with pectoralis major contracted. Fibrous suspensory ligaments (of Cooper) run from the skin to the deep fascia, so a tumour that shortens them dimples the skin, and blocked dermal lymphatics give the oedematous pitting of peau d'orange. Arterial blood comes from perforating branches of the internal thoracic artery, from the lateral thoracic and thoraco-acromial branches of the axillary artery and from the posterior intercostal arteries. The veins follow the arteries and, through the intercostal veins, connect with the vertebral venous plexus, a route for spread to the spine.

About three-quarters of the breast's lymph drains to the axillary nodes, mostly to the anterior (pectoral) group, some to the posterior group and some from the upper breast directly to the apical nodes. Most of the remainder passes through the intercostal spaces to the internal thoracic (parasternal) nodes along the internal thoracic vessels; every quadrant drains partly by this route, but the medial quadrants contribute the larger share. The lymphatics of all quadrants communicate through the subareolar plexus, which is why an injection beneath the areola maps the same axillary sentinel node as an injection beside the tumour. When the usual channels are blocked, lymph can reach the opposite breast, the supraclavicular nodes or, from the lower inner quadrant, the rectus sheath and beyond it the peritoneal cavity and the liver.

Staging and treating the axilla

Everyone investigated for early invasive breast cancer has an ultrasound of the axilla, with needle sampling of any abnormal node. If the axilla is normal on ultrasound, or the needle biopsy is negative, it is staged by sentinel lymph node biopsy rather than clearance, using the dual technique of radioisotope and blue dye (NICE NG101). The blue dye has effects of its own worth knowing: blue-green urine, temporary staining of the skin, rare anaphylaxis and a spuriously low pulse oximetry reading, because the circulating dye absorbs light at the wavelengths the oximeter uses. After sentinel node biopsy, macrometastases in one or more nodes lead to further axillary treatment, either clearance or radiotherapy, although after breast-conserving surgery the option of no further axillary treatment for one or two macrometastases can be discussed. Micrometastases alone need no further axillary treatment, and isolated tumour cells are classified as node-negative. Nodal disease proved by needle biopsy before surgery generally means axillary clearance. A clearance leaves a risk of seroma, shoulder stiffness and lymphoedema, which is greater when radiotherapy to the axilla is added; long-standing lymphoedema is the setting for lymphangiosarcoma (Stewart–Treves syndrome).