Mediastinal Lymph Node - Anatomy and Significance

Introduction

The mediastinal compartment encompasses numerous vital organs and vessels, functioning as the central connection for lymphatic drainage. The mediastinum is traditionally categorized into three distinct functional divisions, namely the anterior (pre-vascular), middle (visceral), and posterior (paravertebral) mediastinum. Lymph nodes (LNs) are distributed across the three functional compartments of the mediastinum, with a higher concentration of lymphatic tissue observed in the anterior and middle compartments. The division of the mediastinum aids in narrowing down the extensive range of potential diagnoses that may manifest in the thorax.

What Is the Structure and Organization of Mediastinal Lymph Nodes?

The mediastinal lymph nodes are located within the mediastinum, the region in the thoracic cavity between the lungs. The mediastinal lymph nodes can be classified into two primary categories: anterior and posterior. The anterior mediastinal lymph nodes are located in the frontal region of the mediastinum, positioned anteriorly to the heart and posteriorly to the chest. They envelop the primary blood vessels of the heart, the pericardium, and the thymus. The posterior mediastinal lymph nodes are anterior to the spinal column and posterior to the heart. The veins travel parallel to the thoracic aperture linearly, near the stomach, trachea, and aorta.

What Are the Functions of Mediastinal Lymph Nodes?

The lymphatic system comprises a complex network of vessels and lymph nodes that cover the entire human body.

• Its primary functions include facilitating the optimal functioning of the immune system, aiding in the absorption of dietary fats, and maintaining fluid balance within the body. In contrast to arteries and veins, lymphatic vessels do not have open ends.

• They function to transport interstitial fluid, immune cells, and macromolecules, which are collectively referred to as lymph. This lymph is carried through lymph nodes and ultimately returned to the bloodstream via the subclavian veins in the chest.

• The lymphatic vessels facilitate the transportation of fluid and cells back to the chest. During its journey, lymph nodes, such as those in the mediastinum, perform sampling lymphatic fluid, effectively removing potential hazards and undesired cellular waste before reintegrating into the bloodstream.

• The lymph nodes contain macrophages that phagocytose undesirable substances, subsequently decomposing them for bodily recycling. T-lymphocytes are responsible for sampling antigens derived from pathogens and subsequently interact with B cells to initiate clonal expansion, thereby facilitating the production of antibodies.

• During immune cell activation, lymph nodes undergo cellular division. Rapid growth can induce discomfort, whereas slow mitotic activity, commonly observed in cancer cases, leads to the development of painless lymphadenopathy.

How Can the Landmarks of Mediastinal Lymph Nodes Be Determined?

The arrangement of mediastinal lymph nodes is determined by their proximity to key anatomical landmarks, namely the great vessels, trachea/bronchi, and pleura.

• Computed tomography (CT) is the preferred radiological modality for visualizing lymph nodes. In CT imaging, normal mediastinal nodes appear as reniform soft tissue structures with a fatty hilum.

• Lymph nodes that exhibit an increase in size can be attributed to reactive hyperplasia resulting from acute infection or chronic lung conditions like emphysema or pulmonary fibrosis. Enlarged lymph nodes raise concerns regarding pathological conditions such as lymphoma, malignant metastases, or sarcoidosis.

What Are the Various Categories of Mediastinal Lymph Nodes?

The mediastinal lymph nodes can be categorized and split based on several classifications or conventions. However, in general, the primary lymph node groups in each mediastinal region are as follows:

• Superior Mediastinum - The superior mediastinum contains the brachiocephalic/prevascular and paratracheal lymph nodes.

• Anterior Mediastinum - The anterior mediastinum contains the pre-pericardial lymph nodes.

• Middle Mediastinum - The middle mediastinum contains the superior and inferior tracheobronchial lymph nodes and bronchopulmonary lymph nodes, which drain the intrapulmonary lymph nodes.

• Posterior Mediastinum - The posterior mediastinum contains several types of lymph nodes and superior diaphragmatic lymph nodes.

What Are the Complications During Enlargement of Mediastinal Lymph Nodes?

The mediastinal lymph nodes collect lymphatic fluid from all organs within the thoracic cavity, such as the heart, lungs, and esophagus. Mediastinal lymph node enlargement, or lymphadenopathy, can occur due to many disorders and pathologies, such as primary lung or esophageal malignancy, metastatic malignancies from other locations, pulmonary infections, including pneumonia or tuberculosis, and occupational lung diseases.

What Is the Lymph Node Size?

The short-axis or least diameter, which refers to the smallest diameter of a lymph node observed in a cross-section, is typically employed to determine the size of the lymph node. Measuring the short-axis diameter is preferable to measuring the long-axis or largest diameter. This is because it provides a more accurate representation of the actual node diameter when nodes are at an angle away from the scan plane and exhibits less variation across healthy individuals.

CT scans typically provide clear visualization of healthy lymph nodes. The sizes of objects vary depending on their location. There are several overarching guidelines:

• Subcarinal nodes can be enlarged in individuals who are in good health.

• Nodes located in the pretracheal duct are typically smaller than those found in the subcarinal duct.

• Typically, the paratracheal nodes on the right side are larger than those on the left.

• Nodes in the upper mediastinum typically exhibit lesser dimensions than nodes closer to the carina.

• The nodes located within the breasts, around the heart, and along the spine are all of minute size, measuring only a few millimeters.

• The upper limits of normal short-axis node width vary for each set of mediastinal nodes. A short-axis node diameter of 1 cm or smaller is typical for therapeutic purposes, except in the subcarinal regions. The subcarinal region is typically considered to have a normal upper limit of 1.5 cm.

Conclusion

The thoracic region exhibits asymmetry in the distribution of lateral mediastinal lymph nodes and the drainage area associated with the lymph nodes in the anterior mediastinum. Nodes located on the left side exhibit a greater extent of drainage than their counterparts situated on the right side. Disrupting the primary lymphatic drainage pathway from the liver to the mediastinal nodes hinders the transmission of immunological signals to the nearby liver-associated regional nodes in the mediastinum.