Mediastinal lesions include mediastinal tumors (benign and malignant), cysts, acute and chronic mediastinitis, mediastinal hernia, and mediastinal emphysema. The human thoracic cavity is divided into two pleural cavities, left and right, and the middle part of the pleural cavity on both sides is called the mediastinum. The mediastinum contains the heart, large intra-thoracic blood vessels, trachea, esophagus, nerves and lymphatic tissues. The mediastinum can be divided into several areas, from the sternal angle (i.e., where the sternal stalk meets the sternal body, which can be felt on the body surface as a distinct transverse ridge) to the lower edge of the fourth thoracic vertebral body by drawing a horizontal line backward, above this line is called the upper mediastinum, below the line is called the lower mediastinum. The upper mediastinum is bounded by the trachea, the anterior part is the anterior superior mediastinum, and the posterior part is the posterior superior mediastinum. The lower mediastinum is divided into three parts: the anterior mediastinum before the pericardium, the middle mediastinum where the pericardium is located, and the posterior mediastinum between the pericardium and the spine. The anterior superior mediastinum mainly contains the thymus and the intrathoracic thyroid gland, while the posterior superior mediastinum contains the trachea, esophagus, aortic arch and its three cephalic-arm vascular branches, thoracic duct, vagus, and nerves. The lower anterior mediastinum has the lower thymus, lymph nodes, fat and connective tissues, etc. The lower posterior mediastinum has the esophagus, thoracic duct, descending aorta and its branches, odd vein, hemi-odd vein, vagus and sympathetic nerves. Etiology of mediastinal lesions: Intrathoracic goiter (or tumor): Intrathoracic goiter is mostly due to adenoma or nodule in the lower pole of the thyroid gland and isthmus in the neck, which gradually descends into the mediastinum along the anterior fascia of the vertebral body before and after the anterior fascia of the trachea due to gravity, flexion and extension of the neck, swallowing activities, and negative pressure in the thoracic cavity. Because the aorta is on the left side of the upper mediastinum, the descending thyroid gland is mostly on the right side, located in front of the pre-tracheal carotid sheath, the innominate vein and superior vena cava, and a few in front of and behind the esophagus; sometimes it can also be located in the left upper mediastinum, pushing the trachea to the right side; the other is a relatively rare embryonic developmental abnormality, namely the vagal ectopic thyroid gland. In the embryonic stage, the thyroid and parathyroid glands come from the 3rd and 4th gill arches (on the medial side of the gill arches and gill slits) adjacent to the large pericardial vessels. If there is a fibrous band in the upper mediastinum connected to the cervical thyroid gland. Sometimes it can be located posterior or inferior to the sternum, behind the trachea and esophagus, etc. Thymus tumor: Thymus is a primary lymph-like organ of the immune system, which produces regulatory immune lymphocytes and participates in the immune response of the body together with bone marrow-regulated lymphocytes. It is also associated with autoimmunity. For example, the generalized myasthenia gravis manifested by thymoma is associated with abnormal immune response. Pregnancy, lactation, exposure to radiation, and application of adrenocorticotropic hormones can affect thymus function. Teratocystic tumors: The cause of this disease is now mostly agreed with the view proposed by G.R. Meinert, that it has the same origin as the thymus, thyroid and parathyroid glands. It may explain the composition of the tumor as a polypoidal tissue. In the past, these tumors were mostly classified as epithelial cysts, dermatomatous cysts, and teratomas. Epithelial cysts originate from ectodermal tissue; dermatomatous cysts contain ectodermal and mesodermal tissue; and teratomas contain ectodermal, mesodermal, and endodermal tissue. These three types of tumors cannot be clearly distinguished histologically, so they are called teratomas. They occur mostly in the anterosuperior mediastinum, protruding to one side, ranging in size from the size of a pigeon’s egg to the full size of one side of the chest, but rarely in the neck, upper sternal border, posterior mediastinum, and rarely in the bronchi. Mediastinal neurogenic tumor: This tumor is derived from Schwann’s cell, ectoderm, and is mostly called Schwann’s tumor. The most common mediastinal neurogenic tumors are neurofibroma, ganglioneuroblastoma and Schwannoma. Other tumors include malignant Schwann’s tumor, sympathetic neurofibroma, sympathetic ganglioneuroblastoma, neurofibrosarcoma, neuroblastoma, parasympathetic ganglioneuroblastoma, chemoreceptor tumor, and pheochromocytoma. Malignant mediastinal neurogenic tumors are rare, and the ratio of benign to malignant is 10:1. The tumor sites are mostly in the posterior mediastinum, and the upper mediastinum is more common than the lower mediastinum. Posterior mediastinal neurofibroma and Schwann’s tumor originate from the spinal nerve and intercostal nerve and are located in the paraspinal groove. Mediastinal lymphatic tumors and other sarcomas Lymphomas are located in the middle mediastinum. They are divided into two categories: Hodgkin’s disease and non-Hodgkin’s lymphoma, the latter including lymphosarcoma and reticulocytoma, among others. Other mediastinal sarcomas are fibrous, lipodystrophy and smooth muscle tumors. Hodgkin’s disease is a separate type of lymphoma. It is characterized by the presence of Reed-Sternberg diastematous cells (R-S cells for short) found in the tumor tissue, which are multichromosomal giant mesenchymal cells with highly lobulated nuclei and multiple large nucleoli, a cellular histomorphology characteristic for Hodgkin’s disease. Pathologically, there are two types of lymphosarcoma, lymphoblastic and lymphocytic lymphosarcoma. Reticulocytic sarcomas are divided into mature and immature types. Mediastinal sarcomas composed of other tissues include fibrous, fatty, and smooth muscle sarcomas. Most mediastinal lymphatic system tumors are highly malignant, fast-growing, and easily metastasized. Fibrous sarcomas are slow growing, and those with poor cell differentiation may become malignant and metastasize to distant areas. Smooth muscle sarcoma and liposarcoma are less malignant. Mediastinal hemangiomas and lymphangioleiomas: Both are rare. Most of the hemangiomas are located in the anterior and posterior superior mediastinum, and most of the lymphangioleiomas are located in the anterior and posterior superior mediastinum, and the growth locations of the left and right chests are roughly similar. Congenital mediastinal cysts: These include pericardial cysts, tracheal cysts and esophageal cysts. Pericardial cysts are formed by the tissue left over from the buds that make up the pericardial cavity during the embryonic period, and are mostly attached to the outer wall of the pericardium, with thin, transparent walls and mesothelial cells on the inner wall, containing clarified fluid. Bronchial cysts come from the embryonic foregut area and are formed with the development of bronchi and lungs into the thoracic cavity. The wall of the capsule is covered with pseudo-complex ciliated epithelium with scattered smooth muscle and cartilage. The bursa contains yellow blood-colored mucus. It is often located near the tracheal bulge and may protrude from the anterior-posterior or superior mediastinum, with many variations in location and very few malignant changes. Esophageal cysts occur when the vacuoles of the upper gastrointestinal tract can fuse with each other at the end of the embryonic period. The mucosa of the cyst is mostly typical of the gastric mucosa, and part of it has the function of acid secretion. The outer wall is similar to the esophageal wall and consists of two circular longitudinal layers of smooth muscle. The muscle layer of the cyst is mostly fused with the esophageal muscle layer without obvious boundaries, and there is no plasma membrane outside the muscle layer, and there is usually no fistula connection between the cyst and the esophagus. Acute mediastinitis: Acute mediastinal connective tissue purulent inflammation caused by various causes of infection. For example, penetrating trauma to the chest, rupture or perforation of the esophagus or trachea. Perforated esophagus, tracheoscopy, and perforated esophageal cancer ulcer. Post-surgical infection, post-operative anastomotic fistula of the esophagus, retroperitoneal infection extending upward to the mediastinum, and infection of the oral cervical region spreading downward can all cause mediastinitis. The etiology of chronic mediastinitis is unknown. According to the literature, tuberculosis, upper respiratory tract infection, influenza, pneumonia, septic infection, histoplasmosis, actinomycosis, radiation therapy, and syphilis can cause this disease, mostly due to nonspecific inflammation. Chronic mediastinitis is one of the important causes of superior vena cava obstruction and is also an advanced manifestation of the disease. Mediastinal hernia: The cause is that the intrathoracic pressure on one side is greater than the opposite side, and the side with the greater pressure compresses the mediastinal hernia into the weaker side. For example, one side is pushed by large pulmonary alveoli, high pressure pneumothorax, large pleural effusion, huge lung cysts and lung tumors. Or the side where the diaphragmatic hernia occurs may pull on the healthy side and produce a mediastinal hernia due to severe fibrous constriction caused by thoracic pathology, pulmonary atelectasis, or after total lung resection on that side. A mediastinal hernia is different from a mediastinal displacement. However, the two are often present together. Mediastinal displacement is due to high pressure on the diseased side or extensive fibrous constriction pushing or pulling on the diseased side, causing the entire mediastinal organ to be displaced to the other side. Mediastinal emphysema: The filling of the mediastinal pleural connective tissue space with gas is called mediastinal emphysema and is a sign rather than a separate disease. Mostly due to alveolar rupture, gas gradually invades the mediastinum from the perivascular space of interstitial emphysema; it can also be caused by rupture of the pleura of the visceral layer of the lung and the mediastinal pleura, gas enters the mediastinum through the extra-tubular space caused by gas entering the pleura, trachea, bronchus or esophagus perforation. The gas in the mediastinum may continue to travel up to the neck along the anterior fascial space of the spine, trachea, and the space around the great vessels, forming a subcutaneous emphysema of the neck; or pneumothorax caused by gas distending through the mediastinal pleura and entering the pleural cavity. In addition, gastrointestinal rupture, gas can travel up to the mediastinum through the mesenteric and retroperitoneal spaces; it can also travel down to the mediastinum from subcutaneous emphysema occurring in the neck; mediastinal emphysema can be formed by using artificial respirator with high pressure and peritoneal gas injection after artificial pneumoperitoneum surgery. In trauma, such as tracheal, bronchial and esophageal ruptures caused by esophageal and tracheal stab wounds and closed chest injuries, gas can enter the mediastinum; medical trauma, such as endoscopy and tracheotomy, can also cause gas to spill into the subcutaneous mediastinum and spread to the mediastinum, resulting in mediastinal emphysema. The severity of the symptoms of mediastinal emphysema may vary depending on the amount of gas and the presence of secondary infection. Pure mediastinal emphysema may include shortness of breath, chest tightness and dyspnea. Trauma with high pressure pneumothorax and internal bleeding may cause respiratory distress and even life-threatening.