Radiological and CT anatomy
On a standard chest X-ray (CXR), lymph node enlargement can be suspected based on indirect signs dependent on their location:
- Right paratracheal group: causes uniform or lobulated widening of the right paratracheal stripe (right paratracheal stripe).
- Azygos nodes (azygos nodes): displace the azygos vein laterally and increase its shadow size to more than 10 mm on the short axis.
- Subaortic nodes: bulge beyond the aortopulmonary window, creating a local protrusion between the aortic arch and pulmonary trunk.
- Hilar lymph nodes (hilar lymph nodes): lead to widening and lobulation of the contours of the lung root shadows.
- Subcarinal (bifurcation) nodes: widen the tracheal bifurcation angle and smooth the azygo-esophageal line. On a lateral view, the combination of these nodes with hilar nodes around the bronchi produces the doughnut sign (doughnut sign).
- Posterior mediastinal nodes: displace the paraspinal and paraesophageal lines.
Contrast-enhanced computed tomography is the method of choice, allowing clear differentiation of lymph nodes from blood vessels. MRI is used less frequently due to scanning duration, poor detection of calcifications, and non-specificity of the T2 signal.
Assessment of density and contrast enhancement
Analysis of lymph node structure on CT narrows the differential diagnosis:
- Necrotic nodes (decreased density): Have a low-density center (low attenuation) with peripheral ring enhancement (peripheral enhancement). This is a typical sign of active tuberculosis, fungal granulomatous infections (histoplasmosis), as well as metastases (especially testicular tumors) and lymphomas in immunodeficiency.
- Negative Hounsfield units: A drop in density below that of water indicates fatty replacement of inflamed nodes or Whipple disease (Whipple disease).
- Intense contrast enhancement (enhancing nodes): Characteristic of Castleman disease (angiofollicular hyperplasia, which produces massive vascularized conglomerates), hypervascular metastases (melanoma, renal cell carcinoma, papillary thyroid cancer, carcinoid), and Kaposi sarcoma.
- High-density nodes (calcifications): Dystrophic calcification often remains after prior histoplasmosis or tuberculosis. Calcifications are also found in metastases of mucinous adenocarcinomas, osteosarcoma, and in sarcoidosis (egg-shell calcification).
Etiology of intrathoracic lymphadenopathy
Causes of lymph node enlargement are divided into infectious-inflammatory and neoplastic:
- Sarcoidosis: Common. Involvement is symmetrical, hilar lymph nodes are enlarged in nearly 100% of cases, and tracheobronchial and aortopulmonary nodes are frequently involved.
- Tuberculosis and histoplasmosis: Involvement is more often ipsilateral (on the side of pulmonary consolidation), but can be massive and bilateral. Upon healing, nodes may shrink and become densely calcified. Massive conglomerates can compress the superior vena cava (SVC).
- Malignant tumors: Mediastinal metastases are caused by head/neck tumors, breast cancer, melanoma, and the genitourinary system (specifically, the testes—seminoma and choriocarcinoma). Primary lymphomas (e.g., diffuse large B-cell lymphoma) form large infiltrative anterior mediastinal masses that displace the heart and cause pronounced SVC syndrome (facial edema, dysphonia, collateral circulation).
- HIV/AIDS: If an infected patient has lymphadenopathy without pulmonary parenchymal involvement, it is highly likely tuberculosis, Mycobacterium avium-intracellulare (MAC) infection, or cryptococcosis.
- Reactive hyperplasia: Moderate enlargement of nodes draining an area of routine infection (usually visible only on CT).
Foregut duplication cysts
Sometimes a mediastinal mass turns out to be not a lymph node, but a congenital foregut duplication cyst (foregut duplication cyst). This group includes bronchogenic and enterogenous cysts. Due to prior infections or hemorrhages, their epithelial lining is often destroyed, making them extremely difficult to differentiate using imaging and even histological methods.
Briefly: answers to topical questions
What is superior vena cava syndrome in mediastinal lymphadenopathy, and what symptoms does it manifest with?
Superior vena cava syndrome is a mediastinal compression syndrome resulting from compression of the superior vena cava by enlarged lymph nodes (sometimes along with surrounding fibrosis) or a tumor, leading to its obstruction.
Clinically, the syndrome manifests with the following signs:
- Facial swelling (facial swelling) — localized edema of the face, neck, and upper extremities.
- Dyspnea (dyspnoea) — breathing impairment.
- Dysphonia (dysphonia) — hoarseness of voice.
- Venous collaterals (collateral venous flow) — engorgement of neck veins and dilation of subcutaneous chest veins ("caput medusae").
- Peripheral cyanosis — bluish discoloration of the face, neck, and occasionally upper extremities.
- Headache — an accompanying symptom in cases of pronounced compression.
On the lateral radiograph, enlarged subcarinal nodes together with hilar nodes form the doughnut sign.
Class and exam questions
Why is the short axis used when measuring a lymph node on CT?
The long-axis dimension is heavily distorted depending on the spatial orientation of the node on the tomographic slice. The short-axis reflects the true diameter and is the standard.
What does ring-like contrast enhancement with a low-density center indicate?
This is a sign of necrosis. It most commonly occurs in active tuberculosis, histoplasmosis, immunodeficiencies, as well as in testicular tumor metastases and lymphomas.
How to differentiate sarcoidosis from tuberculosis based on lymph node imaging features?
A key sign of sarcoidosis is marked symmetry of involvement, especially in the hila. Ipsilateral lymphadenopathy (on the side of pulmonary involvement) with signs of central necrosis is more typical of tuberculosis.
What else to study on this topic
- Castleman disease: histological subtypes and CT differentiation
- Mass effect and superior vena cava syndrome in mediastinal lymphomas
- Metastatic choriocarcinoma and seminoma: imaging features of complications
- Differential diagnosis of congenital mediastinal cysts
- Lymphadenopathy in AIDS: tuberculosis vs atypical mycobacterioses
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