The mediastinum is the central compartment of the thoracic cavity, the area inside the ribcage. It is surrounded by areolar connective tissue and contains all thoracic organs except for the lungs, including the heart and its great blood vessels, the esophagus and trachea, the phrenic and cardiac nerves, the thoracic duct, the thymus, and lymph nodes. The mediastinum lies between the right and left pleural cavities, which house the lungs. It extends from the breastbone to the spine.
This area may be descriptively divided into four sections. The superior mediastinum is the area above the upper level of the pericardium, the sac containing the heart. The other three sections are located below the upper level of the pericardium. From from to back, these sections are the anterior, middle, and posterior mediastina. The anterior section lies in front of the pericardium, and the posterior section lies behind the pericardium, while the middle section contains the pericardium and heart. The lower boundary of the mediastinum is the diaphragm, a muscle extending across the bottom of the ribcage.
Common medical problems affecting the mediastinal cavity include tumors, inflammation or mediastinitis, pneumomediastinum or the presence of air in the mediastinum, and widened mediastinum. All of these conditions are serious and require immediate medical attention. Mediastinitis is usually bacterial in nature and caused by the rupture of one of the organs in the mediastinum. If acute, mediastinitis progresses rapidly and is a medical emergency. Chronic mediastinitis accompanies such conditions as tuberculosis and histoplasmosis, and may be treated with steroid therapy or surgery.
There are many types of mediastinal tumors, including germ cell tumors, lymphoma, pheochromocytoma, and thymoma. Cancer can spread into the mediastinal lymph nodes from the lungs. Tumors in the anterior mediastinal cavity, in front of the heart, are more likely to be malignant, or cancerous, than those in other areas.
Pneumomediastinum can lead to pneumothorax, pneumoperitoneum, and pneumopericardium, or air in the pleural, abdonimal, and pericardial cavities respectively. Sometimes one of these other conditions is the cause of pneumomediastinum. These conditions are commonly associated with Boerhaave's syndrome, in which the esophagus ruptures due to vomiting.
A widened mediastinum is usually diagnosed through x-ray, and can be caused by a variety of medical problems. It often presents with rupture of the aorta or fracture of the vertebrae in the upper back. It is also a hallmark of anthrax poisoning.