The Lymphatic System of the Thoracic Cavity and Mediastinum
Lymph nodes and associated vessels are widely distributed throughout the body. They are strategically located at areas at which the body would be susceptible to pathogenic invasion. Furthermore, they screen interstitial fluid that is being returned to the general circulation for potentially harmful agents. The main focus of this article will be on the lymphatic system associated with the thoracic cavity and mediastinum. The thorax is the region of the body extending from the base of the neck and thoracic inlet (the latter being at the supraclavicular fossae) to the diaphragm (marked anteriorly by the xiphisternal joint).
Within the thoracic cavity is the mediastinum. The mediastinum is the region of the thorax between the lungs. It extends from the level of the first rib, superiorly, to the diaphragm, inferiorly. Additionally, it is bounded anteriorly by the posterior surface of the manubriosternum, and posteriorly by the anterior surface of the thoracic vertebra. The mediastinum is further subdivided into four compartments. A horizontal line traveling anteroposteriorly at the level of the manubriosternal angle of Luis divides it into the superior and inferior mediastina. The inferior mediastinum is further subdivided such that the anterior mediastinum is found anterior to the heart and posterior to the sternum; while the posterior mediastinum is anterior to the thoracic vertebrae (lower border of T4 to T12) and posterior to the heart. The middle mediastinum is occupied by the heart, its pericardium along with segments of the great vessels of the heart, the tracheal bifurcation and the main bronchi.
Special attention will be paid to drainage pathways of the skin and other superficial structures, as well as the viscera of the thorax and mediastinum.
Lymphatics of the Anterior Thoracic Wall
The anterior thoracic extends craniocaudally from the level of the clavicle and jugular notch to the level of the xiphisternal joint. It also extends bilaterally from one anterior axillary line to the other. 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.
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. In both males and females, the breast can be divided into quadrants. 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. The regions cranial to the nipple-areolar complex are the upper quadrants, while those caudal 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.
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.
Additionally, there are lymph nodes located on the thoracic (superior) surface of the diaphragm known as diaphragmatic nodes. These nodes are further subdivided into anterior, lateral and posterior groups, the latter of which will be discussed below. 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.
Lymphatics of the Posterolateral Thoracic Wall
The posterolateral thoracic wall extends superoinferiorly from the level of the T1 vertebra to the T12 vertebra, and bilaterally from one anterior axillary line to the next across the posterior surface of the thorax. 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.
The thoracic and right lymphatic ducts are two major lymph channels found on the posterior thoracic wall. The thoracic duct commences at the 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 box 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 and bronchomediastinal trunks on occasion drain into the thoracic duct, but may also drain into their respective independent venous openings.
The right lymphatic trunk 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.
Lymphatics of the Intrathoracic Respiratory Tract
The intrathoracic part of the respiratory system is comprised of the distal half of the trachea (commencing at the inferior border of T1) and its subsequent branches as well as the lungs and their associated pleura. These structures are fundamental to the processes of gaseous exchange and respiration.
There are several groups of nodes associated with lymphatic drainage of the lungs and conducting airway. These nodes, which are found bilaterally, are:
- the pretracheal and paratracheal nodes are located anterior to, and along the sides of the trachea (respectively);
- the superior tracheobronchial nodes, are seen at the superior border of the bifurcation of the trachea;
- the bronchopulmonary or hilar nodes, are located in the hilum of the lungs where the main-stem bronchi enter the lungs,
- the pulmonary or intrapulmonary nodes, are just deep to the hilum and surround the bronchi;
- and the inferior tracheobronchial or subcarinal nodes are inferior to the carina of the trachea.
The trachea drains its lymph to the pretracheal and paratracheal lymph nodes. Drainage of the lungs can be considered from superficial to deep. The most superficial layer, which is the parietal pleura, drains its lymphatic content ventrally to the parasternal nodes and dorsally to the intercostal nodes. The visceral pleural, on the other hand, drain via the deep pulmonary plexus to the hilar nodes.
There are superficial and deep sub-pleural plexuses that have few communications with each other (one being at the hilar region). There are additional small channels that are capable of dilating to facilitate divergence of deep lymph to the superficial system in the instance that there is an obstruction in the former system. The efferent vessels of the superficial plexus follow the borders of the lungs and margins of the fissures until they terminate in the bronchopulmonary nodes. The upper lobes typically drain to the superior tracheobronchial nodes, while the inferior lobes drain to the subcarinal nodes.
Lymphatics of the Heart
The heart is the muscular, four-chambered, autonomic pump responsible for circulating blood throughout the body. Like any other organ, it also has lymphatic fluid that needs to be drained from its interstitial space. The lymphatics of the heart are divided into three plexuses found in the subepicardial, myocardial and subendocardial spaces. The subepicardial plexus is the recipient of efferent lymphatics arising from the subendocardial and myocardial plexuses. After receiving those tributaries, the subepicardial plexuses give rise to the left and right cardiac collecting trunks. The trunks arising on the left side travel superiorly in the anterior interventricular groove while receiving afferents from the right and left ventricles. In the atrioventricular groove, the trunks merge with the tributaries from the diaphragmatic surface of the left ventricle. This newly formed vessel then courses between the pulmonary artery and left atrium before terminating in the tracheobronchial node.
The right trunks receive tributaries from the right border of the heart, diaphragmatic surface of the right ventricle and the right atrium. The newly formed vessel then travels adjacent to the right coronary artery in the atrioventricular groove. It subsequently courses anterior to the ascending aorta on its way to the brachiocephalic node on the left.
Lymphatics of the Oesophagus
The oesophagus is a musculo-tubular structure that commences that the caudal end of the inferior pharyngeal constrictors and terminates at the cardio-oesophageal junction. The point of origin is located at the lower border of the cricoid cartilage, while the terminal point is at the T11 vertebra (intraabdominal). The 25 cm tube spans the cervical and thoracic regions to allow communication between the pharynx and the stomach, which facilitates eating and digestion.
Lymph nodes of the oesophagus – also known as the juxta-oesophageal nodes – are continuous craniocaudally and are found bilateral to the structure. There are submucosal lymphatic channels that facilitate drainage of the oesophagus. Since the oesophagus can be divided into cervical, thoracic and abdominal parts, their lymphatic channels also drain to different anatomical regions. The abdominal part drains to the left gastric nodes, the thoracic part drains to the posterior mediastinal nodes and the cervical part drains to the paratracheal and deep cervical nodes.
The thymus is a derivative of the third pharyngeal pouches. This encapsulated, bilobular structure – along with bone marrow – are the only two primary lymphoid organs in the body. Majority of the thymus is found in the superior mediastinum, with some extension into the anterior part of the inferior mediastinum. The thymus may also extend superiorly toward the thyroid gland and may also communicate with the same via the thyrothymic ligament. The thymus is largest earlier in life and undergoes progressive fibrofatty degeneration with time. It is primarily concerned with the production of T-lymphocytes (thymus-processed).
There are no afferent lymphatic vessels leading to the thymus. However, it gives off efferent lymphatic channels that originate from its corticomedullary junction, as well as directly from the medulla. The channels travel in the extravascular spaces with the arteriovenous supply of the gland. Thymic efferent lymphatics subsequently terminate in parasternal, brachiocephalic and tracheobronchial nodes.
One of the main clinical reasons for appreciating lymph node location is to accurately stage metastasis. Cancer cells can spread via the lymphatic system and subsequently result in secondary malignancies. It isn’t uncommon, for example, for gastroenteric carcinomas to spread to the lungs because their common lymphatic channels. The mediastinal nodes, in particular, have been arranged into stations in order to better appreciate the staging of lung cancer. While there are no clear-cut demarcations signifying transition points between stations, the approximation assists in assessing cancer staging:
- Station 1 nodes are superior to the horizontal line that marks the point at which the left brachiocephalic vein crosses the trachea;
- Station 2 nodes are inferior to the line of the uppermost mediastinal nodes, but inferior to the horizontal landmark at the upper border of the aortic arch. These are known as the paratracheal nodes;
- Station 3 nodes are prevascular and retrotracheal nodes that are anterior to the great vessels but posterior to the trachea;
- Station 4 nodes are the inferior paratracheal group of nodes found caudal to the superior border of the aortic arch and cranial to the border of the upper lobe bronchi bilaterally. The nodes are typically seen anterolateral to the trachea with the ligamentum arteriosum located laterally.
- The subaortic nodes are found in station 5. They are found lateral to the ligamentum arteriosum or left pulmonary artery and near the proximal end of the first part of the left pulmonary artery.
- Station 6 nodes reside lateral to the aortic arch and ascending aorta and caudal to the superior margin of the aortic arch. They are collectively known as the para-aortic nodes.
- Station 7 nodes are situated below the bifurcation of the trachea; a point known as the carina. Conveniently, the nodes are known as the subcarinal nodes and they have no association with the lower lobe bronchi.
- Para-oesophageal nodes distal to the carina and found bilateral to the thoracic oesophagus are found at station 8.
- Finally, nodes of the pulmonary ligament are called station 9.
Throughout nature there are several anatomical variations among members of the same species. The thoracic duct is no exception to this phenomenon, as there are differences in the course it travels in the thoracic cavity. The most common variant of the thoracic duct (60 - 65%), as described above, shows the duct commencing at L2 and travelling cranially. It then decussates at the level of T4 and continues cranially to the left of the vertebral column until it terminates at the jugulo-subclavian junction. In some cases, the thoracic duct does not decussate at all. It travels on the side of origin (4% incidence on the left and 4% incidence on the right) and drains into the ipsilateral jugulo-subclavian venous system. About 15 - 20 % of individuals may have duplicated ducts distally that coalesce on the left hand side before draining into the left jugulo-subclavian system. Another 12 - 15% of individuals have paired thoracic ducts that travel adjacent to the thoracic aorta in a cranial manner; each vessel subsequently drains to the ipsilateral jugulo-subclavian venous systems.
The walls of the lymphatic vessels are less robust than those of arteries and veins. As a result, they can be easily damaged iatrogenically or otherwise. Injury to the lymph vessels may also result in the formation of chyle fistulas. A fistula is an abnormal communication between two epithelialized surfaces (e.g. tracheoesophageal fistula, perianal fistula, etc.) With chyle fistulas, the lymphatic fluid leaves the vessels and accumulates in the surrounding cavities. In the abdomen, this condition is referred to as a chyloperitoneum; and in the thoracic cavity, it is called a chylothorax. Although damage to the thoracic duct is a rare occurrence, it is associated with a relatively high mortality rate. Depending on the aetiology of the chylothorax, patients may present with shortness of breath. On examination, it is likely to find decreased tactile vocal fremitus, dull percussion note and reduced chest expansion on the affected side. Depending on the severity of the chylothorax, the physician may also not muffled heart sounds and tracheal deviation as well as displacement of the apex beat away from the affected side (mediastinal shift).