The mediastinum is a central compartment located within the chest cavity, positioned between the lungs. It serves as a protective conduit for numerous vital organs and structures. It plays an integral role in the body’s overall function by housing components necessary for circulation, respiration, and other essential systems.
Location and Divisions of the Mediastinum
The mediastinum occupies the central region of the thoracic cavity, extending vertically from the thoracic inlet at the neck’s base down to the diaphragm. It is bounded anteriorly by the sternum and costal cartilages, posteriorly by the thoracic spine, and laterally by the mediastinal pleura. This compartment is filled with loose connective tissue, allowing for the movement and volume changes of the structures it contains, such as during breathing or swallowing.
For descriptive purposes, the mediastinum is divided into two main parts: the superior mediastinum and the inferior mediastinum. A transverse plane, known as the thoracic plane, separates these two divisions. This plane extends from the sternal angle to the intervertebral disc between the fourth and fifth thoracic vertebrae (T4-T5).
The inferior mediastinum is further subdivided into three distinct compartments: the anterior, middle, and posterior mediastinum. The anterior mediastinum lies behind the sternum and in front of the heart. The middle mediastinum contains the heart itself. The posterior mediastinum is situated behind the heart and extends to the vertebral column.
Key Structures Within the Mediastinum
The superior mediastinum contains several structures that pass between the neck and the chest, including:
The thymus gland, which is more prominent in children and plays a role in the immune system.
The trachea, or windpipe, and the esophagus, which transports food.
Major blood vessels: the aortic arch and its major branches (brachiocephalic artery, left common carotid artery, and left subclavian artery), along with the superior vena cava and brachiocephalic veins.
Nerves: vagus and phrenic nerves, which control various bodily functions.
Anterior Mediastinum
The anterior compartment primarily contains the lower portion of the thymus gland, especially in younger individuals. It also includes loose connective tissue, fat, and lymph nodes.
Middle Mediastinum
The middle mediastinum, the broadest part, is dominated by the heart, enclosed within its protective sac, the pericardium. This central area also includes the roots of the great vessels connected to the heart, such as the ascending aorta, pulmonary trunk, and the lower half of the superior vena cava. The bifurcation of the trachea into the main bronchi also occurs within this compartment, alongside the phrenic nerves and various lymph nodes.
Posterior Mediastinum
The posterior mediastinum is a pathway for structures descending through the chest. It contains:
The esophagus, which continues its path to the stomach.
The descending thoracic aorta, responsible for supplying blood to the lower body.
The azygos and hemiazygos veins, which drain blood from the thoracic wall and are also located here.
The thoracic duct, the body’s largest lymphatic vessel.
Components of the sympathetic nervous system.
Common Conditions Affecting the Mediastinum
The mediastinum can be affected by various medical conditions, often due to the presence of numerous organs and tissues within this confined space. One common issue is mediastinal masses, which can be either benign (non-cancerous) or malignant (cancerous). These masses may arise from structures within the mediastinum, such as the thymus gland, lymph nodes, or germ cells. Examples include thymomas, lymphomas, and germ cell tumors, with thymomas being common in the anterior mediastinum.
Inflammatory or infectious conditions can also impact the mediastinum. Mediastinitis refers to inflammation or infection of the mediastinal tissues. This can occur after cardiac surgery or due to the spread of infection from nearby structures. Enlarged lymph nodes, known as lymphadenopathy, can also be found in the mediastinum due to infections, inflammatory conditions like sarcoidosis, or the spread of cancer.
Other conditions include hemorrhage, which involves bleeding into the mediastinal space, often related to trauma or issues with major blood vessels like aortic aneurysms. Cysts, which are fluid-filled sacs, such as bronchogenic cysts or pericardial cysts, can also form within different mediastinal compartments. While many mediastinal issues may be asymptomatic, their growth can put pressure on structures, leading to symptoms like chest pain, difficulty breathing, or swallowing problems.
Diagnosing Mediastinal Issues
Diagnosing conditions within the mediastinum involves a combination of imaging techniques and procedural interventions. Initial evaluation begins with imaging studies like a chest X-ray, which can reveal a mass or widening of the mediastinal space. More detailed anatomical information is obtained through computed tomography (CT) scans, which provide cross-sectional images, and magnetic resonance imaging (MRI), offering superior soft tissue contrast. These imaging modalities help pinpoint the exact location, size, and characteristics of any abnormalities.
To determine the nature of a mass or enlarged lymph node, a tissue sample, or biopsy, is necessary. Biopsies can be performed using image guidance, where a needle is inserted through the skin and guided by CT or ultrasound to the target area. Another procedure is mediastinoscopy, where a surgeon inserts a thin, lighted tube with a camera through a small incision to visualize the mediastinum and collect tissue samples from lymph nodes or masses. This procedure is valuable for diagnosing and staging cancers, such as lung cancer or lymphoma. The collected tissue is then analyzed by a pathologist to establish a definitive diagnosis.