Mediastinal Lymph Nodes

Mediastinal lymph nodes are a group of deep lymph nodes located within the mediastinum, the central compartment of the thoracic cavity. These nodes form a vital part of the thoracic lymphatic network, filtering lymph from the lungs, trachea, esophagus, heart, thymus, and surrounding structures. They are arranged in specific anatomical groups based on their location relative to key thoracic landmarks.

Classification

Mediastinal lymph nodes are categorized into several groups according to their location in relation to the trachea, bronchi, heart, and great vessels. The major categories include:

• Superior mediastinal nodes
• Anterior mediastinal nodes
• Middle mediastinal (tracheobronchial) nodes
• Posterior mediastinal nodes

1. Superior Mediastinal Lymph Nodes

These nodes are located in the superior mediastinum, around the trachea and great vessels.

• Paratracheal nodes: Located lateral to the trachea, along both its right and left sides
• Pretracheal nodes: Found anterior to the upper portion of the trachea
• Retrosternal nodes: Situated behind the upper sternum, sometimes overlapping with anterior mediastinal nodes

These nodes receive lymph from the upper lung lobes, trachea, esophagus, and thymus and drain into the bronchomediastinal trunks or thoracic duct (left side).

2. Anterior Mediastinal Lymph Nodes

Found in the anterior mediastinum between the sternum and pericardium, these nodes are small and often embedded within fatty connective tissue.

• Parasternal nodes: Located alongside the internal thoracic (mammary) vessels
• Thymic nodes: Associated with the remnants of the thymus gland

These nodes primarily receive lymph from the anterior thoracic wall, superior surface of the diaphragm, and thymus, and drain into the bronchomediastinal trunk or directly into thoracic veins.

3. Middle Mediastinal Lymph Nodes (Tracheobronchial Group)

Located around the carina and main bronchi, these nodes are often collectively referred to as tracheobronchial lymph nodes and are the most prominent mediastinal group.

• Superior tracheobronchial nodes: Located above the carina, at the bifurcation of the trachea
• Inferior tracheobronchial nodes (carinal nodes): Located below the carina between the right and left main bronchi
• Bronchopulmonary (hilar) nodes: Found at the hilum of each lung

These nodes receive lymph from the lungs, bronchi, and heart. They are key relay points in the lymphatic drainage of the pulmonary system. Lymph from these nodes passes to the paratracheal nodes or directly to the bronchomediastinal trunks.

4. Posterior Mediastinal Lymph Nodes

These nodes are located posterior to the pericardium and esophagus, adjacent to the thoracic vertebrae and descending thoracic aorta.

• Paraesophageal nodes: Along the length of the esophagus in the posterior mediastinum
• Intercostal nodes: Near the heads of the ribs, close to the vertebral column

They receive lymph from the esophagus, posterior aspect of the pericardium, and posterior thoracic wall. Efferent vessels from these nodes often drain into the thoracic duct on the left side and into bronchomediastinal trunks or venous angles.

Drainage Pathways

The mediastinal lymph nodes form a central part of thoracic lymphatic drainage and serve as key relay points between peripheral structures and major lymphatic ducts. Their drainage can be summarized as follows:

• Lymph from the lungs passes to bronchopulmonary nodes → tracheobronchial nodes → paratracheal nodes → bronchomediastinal trunks
• Lymph from the esophagus drains into paraesophageal and posterior mediastinal nodes → thoracic duct
• Lymph from the anterior chest wall and pericardium drains into anterior mediastinal and parasternal nodes → bronchomediastinal trunk

Ultimately, the bronchomediastinal trunks empty into:

• Right side: Right lymphatic duct or directly into the junction of the right internal jugular and subclavian veins
• Left side: Thoracic duct or the junction of the left internal jugular and subclavian veins

Structural Characteristics

Mediastinal lymph nodes are embedded in loose connective tissue and are variable in size, number, and location. They are more prominent in the peribronchial and subcarinal regions due to their role in filtering lymph from the lungs and conducting system. These nodes are not typically palpable and are primarily visualized through imaging techniques such as CT or MRI, or encountered during thoracic surgeries and dissections.

Embryological Basis

The lymphatic system begins developing around the fifth week of gestation. Mediastinal lymph nodes arise from mesenchymal condensations along the path of developing thoracic lymph vessels, particularly in relation to the developing lungs, trachea, and major vessels. They follow a segmental pattern and align closely with bronchial tree bifurcations and vascular structures.