mediastinum
Meaning of mediastinum: Nothing in this compartment has room to expand, so a mass here tends to declare itself by pressing on whatever lies beside it.
Definition of mediastinum
The mediastinum is the central compartment of the chest, lying between the two lungs. It runs from the breastbone in front to the spine behind, and from the base of the neck down to the diaphragm. It holds the heart and great vessels, the windpipe, the esophagus, the thymus and a great many lymph nodes. The lungs sit beside it, not within it.
How is it divided?
Anatomists split the space into compartments, and radiologists use a similar scheme, because location within it carries real diagnostic weight.
| Compartment | Main contents |
|---|---|
| Anterior | Thymus, fat, lymph nodes, occasionally thyroid tissue extending down from the neck |
| Middle | Heart within its sac, great vessels, windpipe and main bronchi, lymph nodes |
| Posterior | Esophagus, descending aorta, thoracic duct, sympathetic nerve chains |
Why does the compartment matter?
Because the tissues present in each one differ, so does the list of things that can grow there. A mass in the front compartment raises the possibility of a thymic tumor, a germ cell tumor, downward extension of the thyroid or a lymphoma. A mass in the middle is more often enlarged lymph nodes, a developmental cyst or an abnormality of a great vessel. A mass at the back most commonly arises from nerve tissue. Position on a scan therefore narrows the possibilities substantially before any tissue is sampled.
What happens when something enlarges here?
Everything in the compartment is packed together with no spare room, so pressure effects appear early. Compression of the large vein returning blood from the head and arms causes swelling of the face and arms with distended neck veins. Pressure on the windpipe produces cough or noisy breathing, and pressure on the esophagus makes swallowing difficult. Involvement of the nerve that loops around the great vessels to supply the voice box causes hoarseness. Infection of the space, which can follow a tear in the esophagus or heart surgery, is dangerous partly because the compartment connects upward into the neck. Air escaping into the space from a damaged airway or lung is called pneumomediastinum and shows as fine dark streaks on imaging, sometimes reaching up under the skin of the neck. The lower boundary is the diaphragm.
Used in a sentence
Imaging revealed a mass within the anterior mediastinum that required further evaluation.
Good to know
A mass in the mediastinum can arise from several different tissue types native to the space, so its precise compartment often narrows the likely diagnosis considerably before any biopsy is taken.
Word origin
Where “mediastinum” comes from
From Medieval Latin mediastinus, meaning situated in the middle, from Latin medius, meaning middle.