Chapter 56: Mediastinum Anatomy

The mediastinum constitutes the central visceral area situated between the two lungs, bordered laterally by the parietal pleura, anteriorly by the sternum, and posteriorly by the thoracic vertebral column, extending vertically from the thoracic aperture to the respiratory diaphragm. Due to the oblique orientation of the apertures, the posterior boundary is longer than the anterior, and the entire structure appears asymmetric relative to the median plane. This critical region is broadly segmented by the sternal plane, which passes horizontally from the manubriosternal joint to the vertebral column (T4/superior T5 body), dividing the area into the superior and inferior mediastina. The superior division houses major vasculature, including the aortic arch and its principal branches (brachiocephalic trunk, left common carotid, left subclavian arteries), the superior vena cava, the trachea, the esophagus, the vagus and phrenic nerves, and the thymus. The inferior mediastinum is further subdivided into the anterior, middle, and posterior parts. The middle mediastinum is the broadest section, containing the fibrous pericardium, the heart, the ascending aorta, and the pulmonary vessels. Conversely, the posterior mediastinum is the longest and narrowest, accommodating the descending thoracic aorta, the esophagus, the thoracic duct, the sympathetic trunks, and the azygos and hemiazygos venous systems. The azygos system is notable for its variability and its role in providing collateral drainage pathways, essential if the inferior vena cava is interrupted or thrombosed. Mediastinal lymph node groups are classified into regional stations and zones, which are crucial for the clinical staging of conditions like lung cancer. The esophagus, a muscular conduit 20 to 25 cm long in adults, descends through the mediastinum largely anterior to the vertebral column and is constricted at multiple points, including its origin, where it is crossed by the aortic arch and left main bronchus, and at the diaphragm. Its wall features an inner circular and outer longitudinal layer, transitioning from striated muscle superiorly to smooth muscle inferiorly, with function regulated by the lower esophageal sphincter and the right crus of the diaphragm. The thymus, a bilobed primary lymphoid organ, is essential for T-lymphocyte production and development, reaches its largest size relative to body mass at birth, and subsequently involutes through fibrofatty infiltration, although it retains active areas throughout life. The chapter concludes by discussing how structures are visualized using imaging techniques, such as identifying normal contours like the aortic knuckle or the pediatric "thymic sail sign" on chest radiographs, and delineating spaces and lymph node stations via CT and MRI.