The mediastinum is an area of the body in which a wide range of tissue variability exists. Therefore, tumors and cysts that occur in this area can represent many different clinical entities and pathologic processes. An understanding of the embryology of this area and of the anatomic relationships of the normal structures within the mediastinum are essential in the proper determination of the exact nature of a mass or tumor located in this area.
History of the Procedure
Although the entire field of surgery is an ancient one, successful surgical procedures of the thorax are a relatively recent advancement. Until the era when the airway and ventilation could be controlled artificially, the mediastinum, like other parts of the thorax, was deemed a dangerous area to approach.
A few surgeons in the late 1800s and early 1900s attempted and described surgical approaches to the mediastinum. In 1888, Nassiloff first showed that the esophagus was accessible using a posterior approach. In this time frame, with no ability to manage the airway or to ventilate safely, such a surgical approach had to remain completely extrapleural because perforation of the pleura would result in a fatal pneumothorax.
In 1893, Bastinelli described the removal of an anterior mediastinal dermoid cyst. The procedure required resection of the manubrium, but the patient recovered.
In 1897, Milton wrote extensively on mediastinal surgery using the median sternotomy approach. He tried this approach first on human cadavers, finding that median sternotomy gave him excellent access to the mediastinum. He then used the same approach to explore the mediastinum of a live goat. Although he did enter the pleural cavity of the animal, Milton was able to perform a tracheostomy and give artificial respiration through it. This support enabled him to explore the mediastinum successfully and allowed the animal to have an uneventful recovery. Milton then described a human case in which he resected most of a tuberculous sternum plus 2 large tuberculous lymph nodes from the mediastinum, successfully avoiding the pleural spaces. This patient did well.
In 1940, Heuer published a monograph on mediastinal tumors. Most of the cases referenced in the monograph were from the 1920s and 1930s, and, in spite of Milton's previously described work, no reference was made to the use of median sternotomy as an acceptable surgical approach to the mediastinum.
Heuer noted that at that time, dermoid cysts and teratomas were the most commonly found tumors of the mediastinum. He also described successful removal of neurogenic tumors from the posterior mediastinum and described several types of thymic tumors. [1, 2]
Any discussion of neoplasms or other masses found within the mediastinum requires delineation of the boundaries of that area. When defining the location of specific mediastinal masses, the portion of the thorax defined as the mediastinum extends from the posterior aspect of the sternum to the anterior surface of the vertebral bodies and includes the paravertebral sulci. The mediastinum is limited bilaterally by the mediastinal parietal pleura and extends from the diaphragm inferiorly to the level of the thoracic inlet superiorly.
Because a number of mediastinal tumors and other masses are most commonly found in particular mediastinal locations, many authors have artificially subdivided the area for better descriptive localization of specific lesions. Most commonly, the mediastinum is subdivided into 3 spaces or compartments (ie, anterior, middle, posterior) when discussing the location or origin of specific masses or neoplasms. The anterior compartment extends from the posterior surface of the sternum to the anterior surface of the pericardium and great vessels. The middle compartment, or middle mediastinum, is located between the posterior limit of the anterior compartment and the anterior longitudinal spinal ligament. The posterior mediastinum is the area posterior to the heart and trachea and includes the paravertebral sulci. 
The most common tumors and masses in the anterior compartment are of thymic, lymphatic, or germ cell origin. [4, 5] More rarely, the masses found are associated with the aberrant parathyroid or thyroid tissue. Neoplasms and other masses originating from vascular or mesenchymal tissues also may be found. Rarely, bronchogenic cysts may be found in the anterior mediastinum, and some have been reported within the thymus. 
While neoplasms of the middle mediastinum are most commonly of lymphatic origin, neurogenic tumors also may occasionally occur in this area.  Another significant group of masses identified in this compartment is cystic structures associated with a developmental abnormality of the primitive foregut or the precursors of the pericardium or pleura.  They include bronchogenic, esophageal, gastric, and pleuropericardial cysts. In addition, more complex cysts related to embryologic abnormalities, such as neurenteric or gastroenteric cysts, can be found. Isolated cystic abnormalities of lymphatic origin, such as hygromas or lymphangiomas,  can develop within the middle mediastinal compartment, but, more commonly, they are extensions of these abnormalities from the cervical lymphatics.
Neurogenic tumors, by far, are the most common neoplasm of the posterior mediastinum. Tumors originating from lymphatic, vascular, or mesenchymal tissues can also be found in this compartment. Bronchogenic cysts can also be found in this area and have been found in the paravertebral sulcus.
A review of collected series reveals that many mediastinal neoplasms and masses vary in incidence and presentation depending on patient age. Also, numerous mediastinal tumors characteristically occur in specific areas within the mediastinum.
In adults, historically, the most common type of mediastinal tumor or cyst found has been the neurogenic tumor. This was followed in frequency by thymic tumors, lymphomas, and germ cell tumors. Foregut and pericardial cysts follow these 4 lesions in frequency of occurrence. However, more recent data suggest that thymic tumors have become the most common type of mediastinal tumor. 
In children and infants, neurogenic tumors are the most commonly occurring tumors or cysts, followed by foregut cysts, germ cell tumors, lymphomas, lymphangiomas and angiomas, tumors of the thymus, and pericardial cysts. 
Bronchogenic and other types of foregut cysts comprise 10-18% of all mediastinal masses identified in infants and children and 20-32% of all mediastinal masses when all age groups are included. 
Approximately two thirds of mediastinal tumors and cysts are symptomatic in the pediatric population, while only approximately one third produce symptoms in adults. The higher incidence of symptoms in the pediatric population is most likely related to the fact that a mediastinal mass, even a small one, is more likely to have a compressive effect on the small, flexible airway structures of a child.
Roughly 50-66% of cystic lesions found in persons of all age groups are bronchogenic cysts.
Males have a slightly greater incidence of bronchogenic cysts compared to females.
Foregut and other mediastinal cysts do not display a greater incidence in any specific racial, ethnic, or geographic population.
Neurenteric and gastroenteric cysts are considered rare lesions. Enteric cysts, also known as esophageal duplications, are relatively rare.
The tumors and cysts found in the mediastinum have various causes.
Various cysts can originate in the mediastinum. While not actually neoplasms, they represent space-occupying lesions that usually result from abnormal embryologic development. These include foregut cysts, gastroenteric cysts, neurenteric cysts, mesothelial cysts, and thoracic duct cysts. Cysts can also be associated with teratomas within the mediastinum. Thymic cysts are discussed in Thymic Tumors.
Foregut cysts are believed to develop from abnormal primitive foregut development. 
Bronchogenic cysts likely arise from an abnormality of the normal budding of the ventral foregut, the precursor of the trachea and major bronchial structures. The walls of these cysts are lined by ciliated pseudostratified columnar epithelium and may contain bronchial glands, smooth muscle bundles, and other tissues found in the tracheobronchial tree.
Enterogenous cysts arise from abnormal development of that portion of the dorsal foregut that becomes the gastrointestinal tract. Most commonly, these cysts are lined with some form of gastrointestinal epithelium. Esophageal duplication cysts are believed to arise in early development, when vacuolization of the solid early esophagus occurs to form the esophageal lumen. If an isolated vacuole fails to merge with the central esophageal lumen, a duplication cyst may occur.
Neurenteric cysts develop at a location in which the dorsal foregut and the primitive notochord are in close relationship. Many theories have been offered about the development of these cystic abnormalities; however, the common feature noted in many of them is that an adhesive process of some type appears to cause a vacuole of the foregut to become incorporated into the notochord tissue. The classic cyst is lined with enteric and neural tissue. They are often associated with other defects and anomalies of the vertebral column, and many cases described in the literature are those in which the cyst communicates with, or extends into, the spinal canal.
Mesothelial cysts are generally made up of a capsule of fibrous tissue with an inner single-cell layer of mesothelial cells. The most common type of mesothelial cyst found in the mediastinum is the pleuropericardial cyst, which is generally located at the anterior cardiophrenic angle. Other mesothelial cysts occurring in the mediastinum are simple mesothelial cysts and lymphogenous cysts.
Other primary cysts of the mediastinum include thymic cysts and thoracic duct cysts, the latter being very rare. Thymic cysts usually have an inner lining that is a single layer of cuboidal cells and islands of normal thymic tissue in the wall. Thoracic duct cysts may or may not communicate with the duct itself. They are composed of the same tissue as the normal lymphatic channels.
Although these abnormalities are considered benign lesions, a few cases have been reported in which malignant tissue has been found within the wall of a resected bronchogenic cyst. Malignant cell types found include squamous cell carcinoma and adenocarcinoma.
Tumors and cysts of the mediastinum can produce abnormal effects at both systemic and local levels.
Because of the malleable nature and small size of the pediatric airway and other normal mediastinal structures, benign tumors and cysts can produce abnormal local effects. These effects are more evident in children than in adults. Compression or obstruction of portions of the airway, the esophagus, or the right heart and great veins by a large or enlarging tumor or cyst can occur, resulting in a number of symptoms. Rarely, bronchogenic cysts have been described as causing superior vena cava syndrome or pulmonary artery compression. Infection can occur primarily within some of these mediastinal lesions (particularly those of a cystic nature) or can result secondarily in nearby structures (eg, lungs) as a result of local compression and subsequent obstruction.
Malignant mediastinal tumors can cause all of the same local effects as those associated with benign lesions but can also produce abnormalities by invasion of local structures. Local structures most commonly subject to invasion by malignant tumors include the tracheobronchial tree and lungs, esophagus, superior vena cava, pleura and chest wall, and any adjacent intrathoracic nerves. Pathophysiologic changes that can be produced by invasion of specific structures are obstructive pneumonia and hemoptysis; dysphagia; superior vena cava syndrome; pleural effusion, and various neurologic abnormalities such as vocal cord paralysis, Horner syndrome, paraplegia, diaphragmatic paralysis, and pain in the distribution of specific sensory nerves.
A number of mediastinal tumors can produce systemic abnormalities. Many of these manifestations are related to bioactive substances produced by specific neoplasms.
Foregut and other cysts of the mediastinum generate no bioactive substances that produce systemic effects. They can be responsible for secondary systemic symptoms related to infection, or even sepsis, if they are secondarily infected or if their location imposes on airway structures, causing obstructive pneumonia. Additional complications that can arise from the cysts themselves are rupture or perforation into the airway. Complications, such as superior vena cava syndrome and unilateral pulmonary artery hypoplasia or stenosis, have been ascribed to the effects of long-standing compression by a cyst. Finally, some authors have attributed the onset of atrial, and even ventricular, arrhythmias to the presence of a bronchogenic cyst.
A large percentage of mediastinal tumors and cysts produce no symptoms and are found incidentally during a chest radiograph or other imaging study of the thorax performed for some other reason as depicted in the image below.  Symptoms are present in approximately one third of adult patients with a mediastinal tumor or cyst but are more commonly observed in the pediatric population, in which nearly two thirds of individuals present with some symptoms. In adults, asymptomatic masses are more likely to be benign.  See the image below.
Symptoms associated with the respiratory tract predominate in pediatric patients because airway compression is more likely. This occurs because of the significant malleability of the airway structures and the small size of the chest cavity in infants and children. Symptoms most commonly observed include persistent cough, dyspnea, and stridor. If the location and size of the mass produces partial or complete obstruction, obstructive pneumonia can also occur. Infectious symptomatology, and even signs of sepsis, can occur if a mediastinal cyst becomes infected.
Constitutional symptoms, such as weight loss, fever, malaise, and vague chest pain, commonly occur in association with malignant tumors in pediatric patients but also may be signs associated with secondary infection of a congenital cyst, such as a bronchogenic cyst.
Symptoms associated with compression of some portion of the respiratory tract can be produced by benign lesions in adults. This is much less likely to occur in adults compared to children. Infectious symptoms or sepsis from infection of a mediastinal cyst can also occur in adults, although, again, this is very unlikely in persons in this age group.
Bronchogenic cysts are asymptomatic in as many as 20-30% of children. When symptomatic, children most commonly present with vague respiratory problems or dysphagia. They may have a history of recurrent respiratory problems or pneumonia. Symptoms in infants can range from none to significant upper airway distress (see the image below) manifested as severe stridor and wheezing. Likewise, physical examination findings can range from normal to significantly decreased breath sounds on the affected side. See the image below.
A large percentage of adults with bronchogenic cysts are asymptomatic, although several authors report that more than 50% of those followed nonsurgically become symptomatic and usually manifest symptoms of airway or esophageal compression or infection. In those adults who present with symptoms, respiratory complaints or dysphagia predominate.
Neurenteric cysts can manifest in persons of any age group. When identified in symptomatic infants or children, clinical findings are usually those of airway compression. A triad consisting of the presence of a mediastinal mass, symptoms referable to the airway, and the presence of a vertebral anomaly has been observed in approximately 70% of pediatric patients with neurenteric cysts. Communication of a neurenteric cyst with the spinal canal has been reported. Patients with this added abnormality may develop neurologic symptoms, including gait abnormalities, back pain, and motor or sensory deficits.
Treatment selection for a given mediastinal tumor or cyst depends on the diagnosis of the lesion being investigated. Surgical resection is indicated in a large percentage of cases.
Surgical resection is indicated for most mediastinal cysts. All large and symptomatic bronchogenic cysts are included in the group, as are all enteric gastroenteric and neurenteric cysts. Some authors do not recommend resection of small asymptomatic bronchogenic cysts, while others advise aspiration of such cysts and resection only for those with symptoms or recurrence. Excision of pleuropericardial cysts and simple mesothelial cysts is indicated for diagnostic purposes only. Thymic cysts require excision or enucleation. Thoracic duct cysts require excision and accompanying ligation of the thoracic duct.
When defining the location of specific mediastinal masses, the portion of the thorax defined as the mediastinum extends from the posterior aspect of the sternum to the anterior surface of the vertebral bodies and includes the paravertebral sulci. The mediastinum is limited bilaterally by the mediastinal parietal pleura and extends from the diaphragm inferiorly to the level of the thoracic inlet superiorly.
Traditionally, the mediastinum is artificially subdivided into 3 compartments for better descriptive localization of specific lesions. Most commonly, when specific masses or neoplasms are discussed, the location or origin is defined as being in the anterior, middle, or posterior compartments or spaces.
The anterior compartment extends from the posterior surface of the sternum to the anterior surface of the pericardium and great vessels. The anterior compartment normally contains the thymus gland, adipose tissue, and lymph nodes.
The middle compartment, or middle mediastinum, is located between the posterior limit of the anterior compartment and the anterior longitudinal spinal ligament. This area contains the heart, pericardium, ascending and transverse portions of the aorta, brachiocephalic vessels, main pulmonary arteries and veins, superior and inferior vena cavae, trachea and mainstem bronchi, and numerous lymph nodes.
The posterior mediastinum comprises the area posterior to the heart and trachea and includes the paravertebral sulci. The posterior mediastinum contains the descending thoracic aorta and ligamentum arteriosum, esophagus, thoracic duct, the azygos vein, and numerous neural structures (including the autonomic ganglion and nerves, lymph nodes, and adipose tissue).
Foregut cysts of the mediastinum are found most commonly in the middle and posterior compartments but have been reported in the anterior compartment. [1, 8] These cysts are usually found in close association with the tracheobronchial tree or the esophagus. Foregut cysts have also been found in the lung and pericardium and (rarely) have been reported in areas outside the chest such as the subcutaneous tissues of the anterior and posterior chest wall and the abdominal cavity.
Cysts of other types may be found in any of the 3 compartments.
Surgical removal is not indicated as primary treatment for (1) pleuropericardial cysts, lymphogenous cysts, and simple mediastinal cysts, provided an accurate nonsurgical diagnosis has been performed and (2) some cases of bronchogenic cysts found in adult patients in which the cyst is small and the patient has no symptoms. (This opinion varies among authors.)
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