Mediastinum: Definition, Structure, Mediastinal Diseases, Mediastinoscopy and Treatment

It is an important region of the body located in the thorax, in the upper part of the chest between the breastbone and the spine.

Structures found in this region include the swallowing tube ( esophagus ), trachea , heart, and large blood vessels.

Including the ascending aorta (the large artery that carries blood from the heart’s left ventricle on its way to the rest of the body) and the right and left pulmonary arteries, it also houses the lymph nodes.

It essentially contains all the organs of the thorax and other important structures, except the lungs. The lungs are located behind the mediastinum.

There are many conditions that can affect the mediastinum or the lymph nodes of the mediastinum, such as cancer, benign tumors, infections, and more.

Structure

The word mediastinum is translated as “midway” in Latin, referring to the middle part of the chest.

You can visualize this area by viewing the diaphragm as the bottom, the sternum as the front, the blood vessels entering and leaving the heart (the entrance to the tube) as the top, and the spine as the back.

On the lateral sides (edges), the mediastinum is attached by the membranes that line the lungs.

The mediastinum is generally considered to include two areas, the upper and lower.

The lower half is divided into 3 main regions:

  1. Anterior mediastinum (or anterosuperior mediastinum): The anterior mediastinum is present only on the left side and contains some small arteries in addition to the lymph nodes.
  2. Middle (or visceral) mediastinum : The middle mediastinum is the largest portion and contains the heart, the blood vessels, including those that travel from the lungs to the heart, and the lymph nodes.
  3. Posterior (or paravertebral) mediastinum : The posterior mediastinum contains the esophagus, many blood vessels, and mediastinal nerves and lymph nodes.

The mediastinal masses

There are many medical conditions that can affect the structures in the mediastinum, and at first, most of these conditions have no symptoms.

Mediastinal masses are often first noticed when a scan is done to evaluate chest-related symptoms, such as cough, shortness of breath, or other symptoms.

Most of the masses in the mediastinum are small and do not have any symptoms.

When they are large, they can cause respiratory failure (difficulty breathing or getting enough oxygen for tissues), as well as heart problems , such as a drop in blood pressure or a decrease in blood flow.

The age of an individual and the location of a mediastinal mass are important when considering a diagnosis.

In children, mediastinal masses occur most often in the posterior mediastinum and are usually benign (not cancer).

In contrast, adult mediastinal masses are more common in the anterior mediastinum and often malignant (cancerous).

Masses in the middle mediastinum most often represent lymph nodes enlarged by a malignant, infectious, or inflammatory process.

Masses in the posterior mediastinum are usually benign tumors or cysts that originate in the nerves that are present in this area (neurogenic tumors) or in the esophagus (duplication cysts of the foregut).

Cancers like lymphomas (both Hodgkin lymphoma and non-Hodgkins lymphomas ), some germ cell tumors occur in the mediastinum.

Mediastinal diseases

Timomas

The anterior mediastinum contains the thymus gland behind the breastbone, and tumors in this region can include thymomas (tumors of the thymus, an organ in the chest that is quite large in childhood but essentially disappears in adults).

Malignant lymph nodes

“Swollen” lymph nodes in the mediastinum can occur with some infections, especially viral infections.

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.

The term malignant lymph nodes refers to lymph nodes that are cancerous.

This can occur due to cancers that primarily affect the lymphatic system, such as lymphomas, as well as cancers that spread to the lymph nodes.

In addition to cancers such as lymphoma, which are not uncommonly found in these nodes, cancers such as lung cancer can spread to the lymph nodes in the mediastinum.

Tumors

Germ cell tumors (such as teratomas) and retrosternal (behind the sternum) thyroid masses and dermoid cysts occur in the mediastinum.

Common anterior mediastinal tumors are germ cell tumors (tumors that originate in cells similar to testicular or ovarian cells but are abnormally located in the chest).

Malignant mediastinal germ cell tumors are relatively rare.

They are often large enough to cause symptoms by invading or compressing other areas such as the lungs, pericardium, and chest wall.

Bronchogenic cysts

Bronchogenic cysts are benign fluid-filled cysts that can become infected. But generally, they must be removed.

Hematopoyesis extramedular

One mass in this region is extramedullary hematopoiesis.

This is a condition in which blood cells form in areas outside of the bone marrow, and while it is often normal in young babies, it is usually abnormal in adults.

Neumomediastino

Pneumomediastinum refers to the presence of air in the mediastinum, most commonly related to a collapsed lung ( pneumothorax ).

Mediastinitis

Mediastinitis is a term that refers to an infection that involves the mediastinum.

Mediastinitis is a life-threatening condition that most often develops after heart surgery.

Aneurysms

Thoracic aortic aneurysms are a serious condition that affects the mediastinum.

Nerve damage

Nerves that pass through the mediastinum can be damaged, for example, by pressure from a tumor or cancerous lymph nodes in this area.

Myasthenia gravis

Myasthenia gravis is an autoimmune disorder that causes muscle fatigue from the effects of abnormal antibodies on nerve endings.

Myasthenia gravis can be associated with various abnormalities of the thymus gland.

This possible association has led doctors to recommend that the gland be removed, through a procedure called thymectomy.

Other possibilities include paraspinal abscesses and ectopic thyroid tissue (thyroid tissue that grows in an area outside the thyroid gland).

It is important to note that there are many more causes of mediastinal abnormalities, and more tests are often needed to define the exact cause.

Mediastinoscopy

With cancers such as lung cancer and lymphomas, doctors often evaluate the mediastinum to see if the cancer has spread to this region.

A procedure that allows surgeons to visualize this area is called a mediastinoscopy.

If a cancer has spread to the mediastinal nodes, it is often treated differently than a cancer that has not spread to this region, the presence of mediastinal lymph nodes is important in the staging of lung cancer.

In a mediastinoscopy, a surgeon uses a thin scope called a mediastinoscope to examine the mediastinum.

The procedure is often used to check for upper chest problems or to check the lymph nodes in the mediastinum before considering lung removal surgery to treat lung cancer or to diagnose certain infections.

The surgery involves making an incision at the base of your neck, just above your breastbone or on the left side of your chest between your ribs.

The scope is inserted to allow the surgeon to see inside the area. When finished, you will receive some stitches and a small bandage.

Mediastinoscopy was a common procedure for people diagnosed with lung cancer, but the same information can sometimes be available now through a Positron Emission Tomography scan.

Treatment

Tumors, masses, and cysts can form in this space. The appropriate treatment for mediastinal tumors depends on the type and location. Sometimes minimally invasive surgery is selected.

Other tumors are best treated with an open approach or with additional chemotherapy or radiation.

Removal of the thymus can sometimes be done with a procedure called minimally invasive transcervical thymectomy. With it, a horizontal incision is made in the lower part of your neck.

Then an endoscope is inserted to allow the surgeon to see the gland. A small drain is left in the incision for a couple of days.

This technique is associated with the least amount of pain and has the shortest recovery period. It is important to note that it is not recommended for all patients.

Some patients require thymectomy by the traditional route through the sternum.