The discovery of a finding on a chest scan can often lead to immediate concern, especially when that finding involves lymph nodes. The mediastinum is the space situated directly between the lungs, containing structures such as the heart, the trachea, the esophagus, and major blood vessels. Lymph nodes are small, bean-shaped organs distributed throughout the body that function as filtering stations for the immune system. The mediastinal lymph nodes are those specific immune organs located within this central thoracic compartment.
The Function of Mediastinal Lymph Nodes
These nodes serve a specialized immunological role within the chest cavity. They are part of the body’s lymphatic system, which collects fluid and material that drains from surrounding tissues. The mediastinal nodes primarily filter lymph that drains from the lungs, the heart, the esophagus, and the upper airways.
Their primary function involves trapping and neutralizing foreign particles, cellular debris, and infectious organisms like bacteria or viruses. This filtration process prevents harmful substances from circulating systemically through the bloodstream. When a person experiences a respiratory infection, these nodes become highly active, which is why they may enlarge temporarily as they perform their job.
Defining Normal Size and Appearance
Having mediastinal lymph nodes is not only normal but also necessary for maintaining immune surveillance in the chest. The concern arises when their size or appearance suggests an underlying pathological process rather than routine immune activity. Radiologists typically use a short-axis measurement to define the size of a lymph node on imaging studies, such as a computed tomography (CT) scan.
A mediastinal lymph node is generally considered normal or non-enlarged if its short-axis diameter is less than 10 millimeters (1.0 cm). Beyond this measurement, the node is classified as enlarged, or having lymphadenopathy, which warrants closer attention. Normal nodes also tend to have a smooth, oval, or reniform (kidney-like) shape and often display a visible fatty hilum, the central area where blood vessels enter and leave.
Common Causes of Enlargement
When a mediastinal lymph node exceeds the normal size threshold, it is a sign that the node is reacting to a stimulus, but this reaction is often benign. The majority of cases of temporary enlargement are due to infectious or inflammatory processes. Common respiratory infections, such as pneumonia or severe viral illnesses, can cause a reactive swelling as the nodes work to clear the infection.
Chronic inflammatory conditions also frequently lead to persistent, though often benign, enlargement. These include diseases like sarcoidosis, which causes abnormal collections of inflammatory cells, or granulomatous infections such as tuberculosis and histoplasmosis. In these situations, the node is enlarged because it is actively containing a chronic pathogen or responding to systemic inflammation.
A more serious, though less frequent, cause of lymph node enlargement is malignancy. This can involve primary cancers of the lymphatic system, such such as lymphoma, or, more commonly, metastatic disease where cancer cells spread from a primary tumor (e.g., lung cancer) to the nearby mediastinal nodes. The presence of cancer cells or aggressive infection causes the nodes to grow, often resulting in a more rounded shape and loss of the normal fatty hilum seen on imaging.
Diagnostic Tools and Biopsy Procedures
Evaluation of an enlarged mediastinal lymph node begins with advanced imaging to assess its characteristics and metabolic activity. Computed tomography (CT) scans provide detailed images of the node’s size, shape, and structure. A positron emission tomography (PET) scan is often used next, involving injecting a radioactive sugar tracer absorbed by highly active cells. Malignant or highly inflammatory cells absorb this tracer more intensely, appearing as increased metabolic activity on the scan, which helps differentiate between benign and malignant causes.
If imaging suggests a concerning finding, a tissue sample, or biopsy, is required for a definitive diagnosis. Endobronchial Ultrasound with Transbronchial Needle Aspiration (EBUS-TBNA) is a widely used, minimally invasive technique. This procedure involves inserting a bronchoscope with an ultrasound probe through the mouth and into the airways. The ultrasound allows the physician to visualize the lymph nodes adjacent to the trachea and bronchi in real-time, guiding a fine needle through the airway wall to collect a tissue sample.
For certain node locations or when EBUS is not suitable, a surgical procedure called mediastinoscopy may be performed. This involves a small incision above the sternum to insert an instrument for direct visualization and sampling of the nodes. While more invasive than EBUS, this technique is highly accurate for obtaining sufficient tissue for pathological analysis, confirming whether the enlargement is due to infection, inflammation, or malignancy.