What is the Mediastinum and Three Main Regions?

Contents and Abnormalities of the Mediastinum

X-ray of normal healthy chest front view, 36 year old male
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What is the mediastinum and what structures and tissues are present in this area of the chest? What medical conditions may occur in the mediastinum? What types of mediastinal masses or tumors are found in the three parts of the mediastinum?

Definition: Mediastinum

The mediastinum is the area in the chest between the lungs that contains the heart, part of the windpipe (the trachea), the esophagus, and the great vessels including the ascending aorta (the large artery which carries blood from the left ventricle of the heart on its way to the rest of the body) and right and left pulmonary arteries—essentially all of the organs in the chest except the lungs.

It is also home to many lymph nodes and nerves. The word mediastinum is translated as "midway" in Latin, in reference to the middle part of the chest.

You can visualize this area by seeing the diaphragm as the bottom, the breastbone (the sternum) as the front, the blood vessels that enter and exit the heart as the top, and the backbone (the vertebral column) as the back side.

Structure - The mediastinum is divided into 3 main regions:

  • Anterior - The anterior mediastinum is present only on the left side and contains some small arteries as well as lymph nodes.
  • Middle - The middle mediastinum is the largest portion, and contains the heart, blood vessels including those that travel from the lungs to the heart, and lymph nodes.
  • Posterior - The posterior mediastinum contains the esophagus, many blood vessels and nerves, and mediastinal lymph nodes.

Conditions Affecting the Mediastinum

There are many medical conditions which can affect structures in the mediastinum.

 Some of these include:

  • Cancer - Cancers such as lymphomas (both Hodgkin's lymphoma and non-Hodgkins lymphomas), some germ cell tumors, and thymomas (cancers of the thymus gland) occur in the mediastinum.  
  • Benign tumors - Benign germ cell tumors such as teratomas and dermoid cysts may occur in this region.
  • Malignant lymph nodes (lymph nodes affected by cancer) - Cancers such as lung cancer may spread to lymph nodes in this region (mediastinal and tracheobronchial lymph nodes.)
  • Lymphadenopathy - "Swollen" lymph nodes in the mediastinum may occur with some infections, especially viral infections.
  • Aneurysms - Thoracic aortic aneurysms are a serious condition involving the mediastinum.
  • Nerve damage - Nerves that pass through the mediastinum may be damaged, for example by pressure from a tumor or cancerous lymph nodes in this area.

Mediastinal Masses

Mediastinal masses are often first noted when a scan is done to evaluate symptoms related to the chest, such as a cough, shortness of breath, or other symptoms. Most masses in the mediastinum are small and do not have any symptoms. When they are large they can cause respiratory insufficiency (difficulty breathing or getting enough oxygen to the tissues) as well as heart problems, such as a drop in blood pressure or decreased blood flow.

The age of an individual and the location of a mediastinal mass are important in considering a diagnosis. In children, mediastinal masses occur most often in the posterior mediastinum and are often benign (non-cancerous.) In contrast, mediastinal masses in adults are more common in the anterior mediastinum, and often malignant (cancerous.) 

Some causes of mediastinal masses based on location include:

  • Anterior mediastinum - Tumors in this regions may include thymomas (tumors of the thymus, an organ in the chest which is fairly large in childhood but essentially disappears in adults), lymphomas, and thyroid masses.
  • Middle mediastinum - Enlarged lymph nodes are a common cause of a mass in the middle mediastinum. These lymph nodes could, in turn, be related to an underlying cancer. Bronchogenic cysts may also be seen in this area, as well as cardiac abnormalities such as an enlarged aorta.
  • Posterior mediastinum - Esophageal cancers and other esophageal abnormalities may appear in the posterior mediastinum, and enlarged lymph nodes may also be noted here. Other possibilities include sarcomas and ectopic thyroid tissue (thyroid tissue growing in an area outside of the thyroid gland.

    Lung cancer may cause a mass in any part of the mediastinum. It's important to note that there are many more causes of abnormalities in the mediastinum, and most often further testing is needed to define the exact cause.

    Terms Involving the Mediastinum

    • Pneumomediastinum refers to the presence of air in the mediastinum, most commonly related to the collapse of a lung (pneumothorax.)
    • Mediastinitis is a term referring to an infection involving the mediastinum.

    Procedures - Mediastinoscopy

    With cancers such as lung cancer and lymphomas, doctors often evaluate the mediastinum to see if any cancer has spread to this region. One procedure that allows surgeons to visualize this area is called a mediastinoscopy. If a cancer has spread to nodes in the mediastinum it is often treated differently than a cancer which has not spread to this region; the presence of mediastinal lymph nodes is important in the staging of lung cancer.

    Mediastinoscopy was a common procedure for those diagnosed with lung cancer, but the same information may sometimes be available now via a PET scan.

    Examples: Joy had a procedure done to see if her lung cancer had spread to lymph nodes in her mediastinum.


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