Mediastinal Cysts

Updated: Dec 11, 2017
  • Author: Mary C Mancini, MD, PhD, MMM; Chief Editor: John Geibel, MD, DSc, MSc, AGAF  more...
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Overview

Background

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 an awareness of the anatomic relations 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.

Any discussion of neoplasms or other masses found within the mediastinum requires delineation of the boundaries of that area. In 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 three spaces or compartments in discussing the location or origin of specific masses or neoplasms, as follows [1] :

  • The anterior compartment extends from the posterior surface of the sternum to the anterior surface of the pericardium and great vessels
  • The middle compartment is located between the posterior limit of the anterior compartment and the anterior longitudinal spinal ligament
  • The posterior compartment 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. [2, 3]  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. [4]

Whereas neoplasms of the middle mediastinum are most commonly of lymphatic origin, neurogenic tumors also may occasionally occur in this area. [5]  Another significant group of masses identified in this compartment consists of cystic structures associated with a developmental abnormality of the primitive foregut or the precursors of the pericardium or pleura. [6]  These include bronchogenic, esophageal, gastric, and pleuropericardial cysts. In addition, more complex cysts related to embryologic abnormalities (eg, neurenteric or gastroenteric cysts) can be found.

Isolated cystic abnormalities of lymphatic origin, such as hygromas or lymphangiomas, [7]  can develop within the middle mediastinal compartment, but more commonly, they are extensions of these abnormalities from the cervical lymphatics.

Neurogenic tumors are by far the most common neoplasms 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.

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Anatomy

With respect to 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 three compartments for better descriptive localization of specific lesions. In most cases, 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, or anterior mediastinum, extends from the posterior surface of the sternum to the anterior surface of the pericardium and great vessels. It 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, the pericardium, the ascending and transverse portions of the aorta, the brachiocephalic vessels, the main pulmonary arteries and veins, the superior vena cava (SVC) and the inferior vena cava (IVC), the trachea and mainstem bronchi, and numerous lymph nodes.

The posterior compartment, or posterior mediastinum, comprises the area posterior to the heart and trachea and includes the paravertebral sulci. It contains the descending thoracic aorta and ligamentum arteriosum, the esophagus, the 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. [8, 6] 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 (eg, 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 three compartments.

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Pathophysiology

Tumors and cysts of the mediastinum can produce abnormal effects at both systemic and local levels.

Local

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 SVC syndrome (SVCS) 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 following:

  • Tracheobronchial tree and lungs
  • Esophagus
  • SVC
  • Pleura and chest wall
  • Any adjacent intrathoracic nerves

Pathophysiologic changes that can be produced by invasion of specific structures are as follows:

  • Obstructive pneumonia and hemoptysis
  • Dysphagia
  • SVCS
  • Pleural effusion
  • Various neurologic abnormalities, such as vocal cord paralysis, Horner syndrome, paraplegia, diaphragmatic paralysis, and pain in the distribution of specific sensory nerves

Systemic

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 (eg, SVCS 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.

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Etiology

The tumors and cysts found in the mediastinum have various causes.

Various cysts can originate in the mediastinum. Although they are not actually neoplasms, they represent space-occupying lesions that usually result from abnormal embryologic development. These include the following:

  • Foregut cysts
  • Gastroenteric (enterogenic) cysts
  • Neurenteric cysts
  • Mesothelial cysts
  • 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. [8]

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 where the dorsal foregut and the primitive notochord are in close relation. Many theories about their development have been proposed; however, the common feature noted in many of these theories is that some adhesive process appears to cause a vacuole of the foregut to become incorporated into the notochord tissue. Classic cysts of this type are 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.

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Epidemiology

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, followed in frequency by thymic tumors, lymphomas, and germ cell tumors. After these four types, foregut and pericardial cysts have been the next most common types. However, subsequent data suggest that thymic tumors have become the most common type of mediastinal tumor. [1]

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. [6]

Bronchogenic and other types of foregut cysts account for 10-18% of all mediastinal masses identified in infants and children and 20-32% of all mediastinal masses when all age groups are included. [5]

Approximately two thirds of mediastinal tumors and cysts are symptomatic in the pediatric population, whereas 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.

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Prognosis

The prognosis after resection of a mediastinal tumor varies widely, depending on the type of lesion resected. After resection of mediastinal cysts and benign tumors, the prognosis is generally excellent. [9]

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