The mediastinum corresponds to a tissue space of the thoracic cavity that houses all thoracic organs except the lungs. The organs are embedded within the mediastinum in connective tissue, which maintains their shape and performs supportive as well as protective functions. The mediastinum often becomes clinically relevant due to mediastinal tumors, which can displace the mediastinal cavity, including the organs, into one half of the thorax.
What is the mediastinum?
Various systems with vital functions are located in the human thoracic cavity. In addition to the heart, for example, the immunologically relevant thymus and the trachea are embedded in the thoracic cavity. The mediastinum is understood by physicians to be a tissue space of the thoracic cavity that houses all thoracic organs with the exception of the lungs. Literally translated, mediastinum means middle pleural space. The structure lies median within the thoracic cavity and is bounded on the ventral side by the sternum. The dorsal boundary is formed by the spine. Laterally, the mediastinum is bordered by the pleurae and cranially lies the superior thoracic aperture. Caudal to the mediastinum is the diaphragm, which, similar to the lungs, is not part of the mediastinal cavity. Anatomically, the thoracic cavity is divided into two distinct sections: a lower tissue space and an upper tissue space. Within the mediastinum, in addition to veins, arteries and nerves, lie primarily lymphatic channels. Organs such as the heart are also a part of the structure.
Anatomy and structure
Loose connective tissue lines the mediastinum and embeds all chest organs except the lungs within it. The upper mediastinum houses the thymus as well as vessels near the heart such as the aortic arch or its branches, the truncus pulmonalis or the superior vena cava. The trachea, esophagus, mediastinal and tracheobronchial lymph nodes are also located in the upper mediastinum. The nerve structures of the upper region correspond to the phrenic nerve, the vagus nerve, and the recurrent laryngeal nerve. As a so-called thoracic duct, the thoracic duct belongs to the lymphatic system. Between the aortic arch and the left pulmonary artery lies the so-called ‘aortopulmonary window’: a space that plays a role in radiology. Adjacent to the upper mediastinum is the lower mediastinal space. This lower space is further subdivided into an anterior portion between the sternum and pericardium, a middle portion encompassing the heart, and a posterior portion located between the heart and the thoracic spine. The posterior portion is home to the esophagus, aorta, and inferior vena cava, as well as the azygos vein, hemiazygos vein, and vagus nerve structures. The origins of various cervical muscles are located in the upper layer of the mediastinum. The organs are located in the deeper layers.
Function and tasks
The mediastinum houses most of the organs of the chest cavity. The main function of the structure is protective. The entire mediastinum is equipped with connective tissue, which forms an ideal protective space for the vital chest cavity structures. Connective tissue performs mainly supportive functions in the human body and consists of comparatively few cells. Because of the large intercellular mass of connective tissue structures, connective tissue can maintain the shape of organs. In the mediastinum, the connective tissue lining thus primarily protects the organs from damage. Different types of fibers are located in the intercellular substance for this purpose. Fibers made of fibrillar proteins such as collagen form a tight meshwork that supports the organs. Between these fibers lie strongly swelling proteoglycans, which resist tensile forces and at the same time have a compression-damping effect. In interaction, the fibrous meshwork and the proteoglycan buffer of the mediastinal connective tissue perform supporting and protective functions for the thoracic organs. In addition, the mediastinal connective tissue serves as a water reservoir and also performs important functions in the defense against disease. While the structures of the thoracic cavity are already relatively shock-protected by the ribs, it is the connective tissue protection of the mediastinum that maintains the organ shape and localization. Thus, the organs of the thoracic cavity are essentially multiply protected and secured. The ribs correspond to skeletal protection, while the mediastinum provides fine tissue protection. The multiple protection of the thoracic organs refers to the vitality that organs such as the heart have in the human body.
Diseases
In the clinic, the physician is confronted with different diseases of the mediastinum day by day. The most common of these are mediastinal tumors such as thymomas and lymphomas. Somewhat less common are space-occupying lesions in the thoracic region that displace the mediastinum. Mediastinal displacement is not necessarily associated with a mass, but may also be indicative of pneumothorax. For imaging of the mediastinum, the physician performs a mediastinoscopy. Mediastinal tumors cause typical symptoms by locally displacing adjacent structures. Common signs of a tumor in the mediastinum include compression of the superior vena cava accompanied by hoarseness. In this scenario, the tumor displaces the superior vena cava as well as the recurrent laryngeal nerve. Due to the displacement, the two structures henceforth lie jammed, resulting in circulatory disturbances and nerve conduction disorders. Tumors of the mediastinum can also cause the symptom complex Horner’s syndrome, resulting in paralysis of the smooth eye muscles. Tumor-induced tissue displacement may also cause diaphragmatic elevation associated with increasing shortness of breath. This scenario results primarily from tumor-induced compression of the trachea. Nonspecific accompanying symptoms of a tissue-displacing mass within the mediastinum include cough and the general symptoms of night sweats and fever. In the context of tumor diseases of the mediastinal space, a mediastinal shift often occurs, which corresponds to a radiographically depictable displacement of the mediastinum. This displacement shifts the mediastinal space along with its organs to one of the thoracic halves.
