Thoracic and mediastinal lymph nodes and lymphatics

Lymphatic drainage of the skin

The skin of the thorax achieves lymphatic drainage via a superficial and a deep system.

• The superficial lymphatic vessels converge in the subcutaneous space and take lymph back to the axillary lymph nodes.
• The deep lymphatic system, on the other hand, drain to the intercostal nodes, as well as to the parasternal and diaphragmatic nodes.

Lymphatic drainage of the breast

Another clinically relevant lymphatic pathway associated with the anterior chest wall is that of the breast lymphatics. The breasts, or mammary glands, are anatomically and functionally dichotomous structures associated with reproduction and human sexuality.

Of note, breast lymph channels are avalvular; therefore any occlusion (secondary to tumors) can result in reversal of blood flow. There are numerous periductal and perilobular vessels that parallel the venous tributaries of the breast.

In both males and females, the breast can be divided into quadrants.

• The regions superior to the nipple-areolar complex are the upper quadrants, while those inferior to the complex are the lower quadrants.
• Similarly, the quadrants lateral to the complex are outer quadrants and those medial to it are the inner quadrants.

From this notion, the inner quadrants, and the lower outer quadrant drain primarily to the parasternal and inframammary lymph nodes. The upper outer and part of the lower outer quadrants drain primarily to the axillary nodes. The medial part of each breast also drains to the internal thoracic lymph nodes, which subsequently drains to the superior and inferior epigastric lymph pathways to the groin. Furthermore, the left and right breasts, although symmetrical, have different terminal lymph drainage points. The left breast lymphatics drain via the thoracic duct to the left subclavian vein, while the right breast drains to the right jugulo-subclavian junction. Additionally, lymphatic channels originating from one mammary gland may cross the midline and terminate in nodes associated with the contralateral breast.

Parasternal lymph nodes

The parasternal lymph nodes are a group of four to five nodes residing at the anterior limit of the intercostal space, adjacent to the thoracic arteries.

In addition to draining the breast, they also receive lymph from deep structures of the epigastrium, deeper parts of anterior thoracic wall and the cranial surface of the liver. The efferent vessels of these nodes then anastomose with vessels associated with the tracheobronchial and brachiocephalic nodes to form the bronchomediastinal trunk. This vessel has a variety of terminal points, including in the jugular or subclavian veins, or at the junction of these vessels (jugulo-subclavian junction), the left thoracic duct or the right lymphatic duct.

Diaphragmatic lymph nodes

Additionally, there are lymph nodes located on the thoracic (superior) surface of the diaphragm known as diaphragmatic nodes (also known as superior phrenic nodes). These nodes are further subdivided into anterior, lateral and posterior groups.

• The anterior diaphragmatic group of nodes are comprised of about three nodes posterior to the xiphoid process. There are additional small nodes at the level of the seventh costochondral junction. Afferent lymphatic vessels to these nodes arise from the anterior lymph vessels of the diaphragm; while efferent lymphatic vessels from the same pass to the parasternal nodes.
• The lateral group of thoracic diaphragmatic lymph nodes are further subdivided into left and right groups of two or three nodes. They are adjacent to the point at which the phrenic nerves pierce the diaphragm. The right group of nodes, which are juxtaposed to the pericardium and the distal part of the intrathoracic inferior vena cava, drain the central diaphragm and the convex surface of the liver. They subsequently drain to the parasternal, posterior mediastinal and brachiocephalic lymph nodes.
• The posterior group of diaphragmatic nodes will be discussed below with the nodes of the posterolateral thoracic wall.

Intercostal lymph nodes

The intercostal lymph nodes have the responsibility of draining the posterolateral chest. They can be found at the costovertebral junction along the thoracic vertebrae. The efferent vessels of the nodes of the caudal four to seven intercostal spaces anastomose to form a single trunk that travels inferiorly and inserts into the proximal part of the thoracic duct. Efferent lymphatics of node of the cranial left and right intercostal spaces drain to the thoracic duct and right lymph trunks, respectively.

Posterior thoracic diaphragmatic group of nodes are found on the posterior surface of the diaphragmatic crura. They form a communication between the posterior mediastinal and lateral aortic nodes.

Thoracic duct

The thoracic duct and right lymphatic duct are two major lymph channels found close to the posterior thoracic wall.

The thoracic duct commences at the L1/L2 level, anterior and slightly to the right of the vertebral bodies and travels about 38 to 45 cm to the base of the neck in adults. The vessel is wider at its origin, where the lumbar lymph trunks unite. Occasionally, there is a lymphatic cistern (a swelling or waterproof receptacle) known as the cisterna chyli.

The thoracic duct becomes narrower as it moves cranially before dilating again prior to terminating in the jugulo-subclavian junction. The thoracic duct contains several valves at high pressure points where some tributaries join the vessel to prevent the reflux of lymph. The thoracic duct travels adjacent to the azygous and hemiazygous veins, and the aorta in the retrocrural part of the diaphragm.

It continues to the right of the midline of the thoracic cavity through the posterior mediastinum, with the thoracic aorta to its left and the azygous vein on the right. Posteriorly related structures include the vertebral column, terminal hemiazygous and accessory hemiazygous veins, and the right intercostal arteries; while anteriorly related structures include the diaphragm and oesophagus.

The fifth thoracic vertebra marks the point at which the thoracic duct courses towards the left as it enters the superior mediastinum, before progressing towards the thoracic inlet. It is noteworthy that at this point the thoracic aorta is located to the right of the duct.

At the seventh cervical vertebra, it curves laterally, after which it courses posterior to the left common carotid artery, internal jugular vein and vagus nerve (CN X) before descending towards and terminating in the jugulo-subclavian junction. It is not uncommon for the duct to terminate in either the subclavian or internal jugular veins, rather than the junction of the two. As previously mentioned, there are several tributaries to the thoracic duct. These include:

• Bilateral vessels from the intercostal lymph nodes of the caudal 6 – 7 intercostal spaces descending to join the duct. 
• Bilateral lumbar lymph vessels travelling cranially from the superior aortic nodes to anastomose and form the thoracic duct. 
• Efferents from the cranial 5 – 6 intercostal nodes on the left hand side. 
• Mediastinal nodes from a myriad of nodal groups. 
• The left jugular trunk and left bronchomediastinal trunk on occasion drain into the thoracic duct, but may also drain into their respective independent venous openings.

Right lymphatic duct

The right lymphatic duct has several anatomical presentations including unilateral or bilateral termination, duplicate ducts or left-sided termination. It is approximately 1.25 cm long and travels on the medial edge of scalenus anterior. The vessel then terminates in the right jugulo-subclavian junction. Like the thoracic duct, the right lymphatic duct also has semilunar valves at its opening to prevent the reflux of lymphatic fluid. Afferents to this vessel arise from the right jugular and subclavian trunks as well as from the right upper limb (lateral group of axillary nodes). The right side of the heart, right lung, and the right sides of the head, neck and thorax also drain into the right lymphatic trunk.