The mediastinum is a central compartment within the chest cavity, located between the lungs. It houses several important organs and structures.
Anatomical Definition and Boundaries
The mediastinum lies centrally between the two pleural cavities, which contain the lungs. This compartment extends from the sternum, or breastbone, at the front of the chest to the vertebral column, or spine, at the back. Vertically, it stretches from the thoracic inlet, the upper opening of the chest, down to the diaphragm, the muscular sheet separating the chest from the abdomen. Its lateral borders are formed by the mediastinal pleura, the membranes that enclose the lungs.
Divisions and Key Contents
The mediastinum is separated into distinct regions. An imaginary line, known as the transverse thoracic plane, extends from the sternal angle (where the manubrium meets the body of the sternum) to the intervertebral disc between the fourth and fifth thoracic vertebrae. This plane divides the mediastinum into a superior and an inferior part.
Superior Mediastinum
The superior mediastinum, located above this imaginary line, contains structures passing between the neck and the chest. It includes the thymus gland, the trachea, and the esophagus. Major blood vessels like the arch of the aorta and its main branches (brachiocephalic trunk, left common carotid, and left subclavian arteries), along with large veins such as the superior vena cava and brachiocephalic veins, are also found here. Nerves like the vagus and phrenic nerves, and the thoracic duct, also traverse this upper region.
The inferior mediastinum, below the transverse thoracic plane, is further subdivided into three compartments based on their relationship to the pericardium.
Anterior Mediastinum
The anterior mediastinum is the smallest subdivision, located in front of the pericardium and behind the sternum. It primarily contains loose connective tissue, lymph nodes, and the lower part of the thymus.
Middle Mediastinum
The middle mediastinum contains the heart within the pericardium. It also contains the roots of the great vessels, 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 and the phrenic nerves also reside in the middle mediastinum.
Posterior Mediastinum
Behind the middle mediastinum and in front of the vertebral column is the posterior mediastinum. This region houses the descending thoracic aorta, the esophagus, and the thoracic duct. It also contains the azygos and hemiazygos veins, the vagus nerves, sympathetic trunks, and various lymph nodes.
Essential Functions
The mediastinum performs several functions. It acts as a protective space, housing and cushioning organs such as the heart and the major blood vessels that transport blood to and from it. This central location provides a degree of support and allows these structures to move during physiological processes like breathing and heartbeat.
The mediastinum also serves as a passageway. For instance, the esophagus passes through it, connecting the throat to the stomach, and the trachea extends through it to reach the lungs. Major nerves and vessels also traverse this region, ensuring communication and circulation.
Clinical Significance
Understanding the mediastinum is important due to various conditions that can affect this central chest area. Inflammation, known as mediastinitis, often results from infection, sometimes due to a tear in the esophagus or complications following chest surgery. Symptoms can include fever, chills, and chest pain.
The mediastinum can also be the site of masses or tumors, which may be benign or malignant. These masses can press on surrounding organs and structures, leading to symptoms such as chest pain, shortness of breath, coughing, difficulty swallowing, or hoarseness. Common types of mediastinal tumors include thymomas, lymphomas, and neurogenic tumors.
Changes in the size or shape of the mediastinum, known as mediastinal widening, can be observed on chest X-rays. It can signal serious underlying conditions such as aortic dissection, aneurysms, or enlarged lymph nodes. Recognizing abnormalities in the mediastinum is important for diagnosing and managing chest-related health issues.