The mediastinum is the central compartment of the chest cavity, located between the two lungs. This protective and connective region houses many of the body’s most active organs and pathways. It acts as a conduit for structures traveling between the neck, chest, and abdomen, while also supporting the function of the heart and major blood vessels. Understanding the mediastinum involves recognizing its anatomical location, the structures it contains, and its significance in clinical scenarios, from inflammation to tumor development.
Defining the Mediastinum and its Boundaries
The mediastinum is a partition situated in the middle of the chest, extending vertically from the neck’s upper opening down to the diaphragm. It is bounded anteriorly by the sternum (breastbone) and posteriorly by the vertebral column (thoracic vertebrae). On either side, the mediastinum is flanked by the two pleural cavities, which contain the lungs and their protective membranes.
The entire region is divided into two major sections: the superior and inferior mediastinum, separated by the transverse thoracic plane. This horizontal plane extends from the sternal angle, where the manubrium meets the body of the sternum, backward to the intervertebral disc between the fourth and fifth thoracic vertebrae (T4/T5).
The inferior mediastinum lies below this plane and above the diaphragm. It is further subdivided into three smaller compartments relative to the heart’s position: the anterior, middle, and posterior mediastinum. This allows for a precise anatomical classification of structures within the area.
Key Structures Contained Within the Space
The middle mediastinum holds the most significant structure: the heart, enclosed within the pericardium, a protective sac. It is joined by the roots of the great vessels, including the ascending aorta, pulmonary trunk, and the lower portion of the superior vena cava. The middle section also contains the main bronchi and the phrenic nerves, which are responsible for controlling the diaphragm.
The anterior mediastinum is the smallest subdivision, located immediately in front of the pericardium and behind the sternum. This space primarily contains loose connective tissue, fat, and the remnants of the thymus gland, which tends to atrophy and be replaced by fatty tissue in adults.
The superior mediastinum, positioned above the transverse thoracic plane, acts as a passageway for many structures moving between the neck and the chest. These structures include:
- The aortic arch and its major branches.
- The upper part of the superior vena cava.
- The trachea.
- The esophagus.
The posterior mediastinum is situated behind the heart and pericardium, extending down to the diaphragm. This area accommodates the descending thoracic aorta, the esophagus as it passes toward the stomach, the azygos system of veins, and the thoracic duct, the body’s largest lymphatic vessel. The posterior space also transmits important neural pathways, such as the vagus and sympathetic nerves.
Common Medical Conditions Affecting the Area
The confined space and variety of organs within the mediastinum make it susceptible to several serious conditions, most commonly involving the development of masses or inflammation. Mediastinal masses are abnormal growths that can be either benign or malignant, and their type often correlates with the specific compartment they originate from.
Masses in the anterior mediastinum are the most common and include thymomas, which arise from the thymus gland, as well as lymphomas and germ cell tumors. Posterior masses are frequently neurogenic tumors, originating from nerve tissue, while middle mediastinal masses are rarer and often present as cysts, such as bronchogenic or pericardial cysts, or enlarged lymph nodes.
The danger of any mass is not just malignancy but the compression of adjacent structures, which can impede the function of the trachea, esophagus, or major blood vessels. Symptoms often arise when this compression occurs, leading to generalized chest pain, a persistent cough, or difficulty swallowing (dysphagia).
Another serious, though less common, condition is mediastinitis, which is an inflammation or infection of the tissues within the compartment. This infection is often a complication of a surgical procedure, such as heart surgery, or can result from a perforation of the esophagus, allowing contents to leak into the sterile mediastinal space. Because the mediastinum is continuous with the neck’s connective tissue, severe infections can spread rapidly, representing a high-risk medical emergency.
Diagnostic Tools Used to Examine the Mediastinum
Due to the crowded nature of the mediastinum, specialized tools are required to accurately diagnose conditions in this region. The initial step in investigation is typically a standard chest X-ray, which can reveal a widened mediastinum or the presence of a mass, though it offers limited detail.
To obtain a clearer, more detailed picture, a Computed Tomography (CT) scan is commonly used, often with an intravenous contrast agent. This imaging technique provides excellent cross-sectional views, helping doctors determine the exact location, size, and relationship of a mass to surrounding organs. Magnetic Resonance Imaging (MRI) is another valuable non-invasive tool, particularly useful for distinguishing between soft tissues and for assessing the relationship of a mass to major blood vessels and nerves.
While imaging can identify a suspicious lesion, a definitive diagnosis, especially for cancer, often requires a tissue sample. Minimally invasive procedures like endobronchial ultrasound (EBUS) or endoscopic ultrasound (EUS) allow physicians to guide a needle through the airway or esophagus, respectively, to obtain a fine-needle aspiration (FNA) biopsy of a mass or enlarged lymph node.
More invasive surgical procedures, such as mediastinoscopy, involve making a small incision above the sternum to insert a scope and visually inspect the area. This allows for the direct collection of tissue samples. This procedure is routinely used for staging lung cancer by checking for metastasis in the mediastinal lymph nodes. These various diagnostic approaches, from basic X-rays to highly specialized biopsy techniques, are essential for characterizing disease and planning appropriate treatment.