The mediastinum is a central anatomical compartment within the chest cavity, serving as a partition between the two lungs. This area is not an organ itself but rather a complex space that houses and protects many vital structures. Understanding the mediastinum is fundamental to comprehending overall thoracic health, as it contains the heart, major blood vessels, and airways. Compromise in this region can immediately affect multiple life-sustaining systems.
Location and Boundaries
The mediastinum is located precisely in the midline of the thorax, situated directly between the left and right pleural sacs containing the lungs. It is an upright, box-like space extending vertically through the center of the chest. The superior boundary is the thoracic inlet, the upper opening of the rib cage connecting the chest to the neck.
The inferior limit is the diaphragm, the large muscle separating the chest from the abdominal cavity. The anterior boundary is the sternum (breastbone), and the posterior boundary is formed by the twelve thoracic vertebrae of the spinal column. The lateral confines are defined by the pleural membranes of the lungs, creating a central corridor through the torso. The space is filled with connective tissue and fat, allowing flexibility for the movement of the heart, esophagus, and great vessels.
How the Mediastinum is Divided
For anatomical and clinical purposes, the mediastinum is systematically divided by imaginary planes. The primary division separates the region into the superior mediastinum and the inferior mediastinum. This horizontal dividing line, known as the transverse thoracic plane, runs from the sternal angle to the intervertebral disc between the fourth and fifth thoracic vertebrae (T4-T5).
The inferior mediastinum, which lies below this plane, is further subdivided based on its relationship to the pericardium surrounding the heart. The middle mediastinum is defined by the pericardium and its contents, primarily the heart itself.
The space located in front of the pericardium and behind the sternum is called the anterior mediastinum. The posterior mediastinum is the space situated behind the pericardial sac, extending back to the vertebral column. This compartmentalized view is useful when locating the origin of a mass or infection.
Primary Structures Found Inside
Superior Mediastinum
The superior mediastinum houses major structures passing through to the neck or abdomen, or originating from the heart. This includes the aortic arch, the upper portion of the superior vena cava, the trachea, and the descending esophagus.
Anterior Mediastinum
The anterior mediastinum is the smallest subdivision and primarily contains loose connective tissue, fat, and lymph nodes. In youth, the thymus gland is prominent here, though it typically atrophies into fatty tissue in adults. This area acts as a protective cushion for the heart and great vessels.
Middle Mediastinum
The middle mediastinum is dominated by the heart and its protective pericardium. The roots of the great vessels, such as the ascending aorta and the pulmonary trunk, emerge directly from the heart within this space. Additionally, the phrenic nerves, which control the movement of the diaphragm, pass through this region.
Posterior Mediastinum
The posterior mediastinum serves as a major conduit for structures moving between the chest and the abdomen. The descending thoracic aorta travels down this space. The esophagus continues its path here, along with the thoracic duct and the azygos system of veins.
Understanding Mediastinal Conditions
The complexity of the mediastinum means various medical conditions can arise from the specific structures within its compartments. The presence of abnormal growths, or masses, is a common issue, and the location often suggests the origin.
The anterior mediastinum is the most frequent site for tumors, including lymphomas, germ cell tumors, and thymomas originating from the thymus gland. Growths in the posterior mediastinum frequently arise from nervous tissue, known as neurogenic tumors, due to the presence of sympathetic nerves. Masses in the middle mediastinum are less common and often present as benign fluid-filled structures like bronchogenic or pericardial cysts.
Beyond masses, the mediastinum is susceptible to infectious and inflammatory conditions. Mediastinitis is a serious inflammation of the tissues, often resulting from a tear or perforation of the esophagus that allows contents to leak into the area. It can also occur following certain surgical procedures on the heart or chest. Due to the concentration of vital structures, conditions affecting the mediastinum can rapidly become severe, causing symptoms like chest pain, cough, or difficulty breathing as nearby organs are compressed.