Tuberculosis and Histoplasmosis.
Lymph node enlargement caused by tuberculous or fungal infection may affect any of the nodal groups in the hila or mediastinum. One or more lymph nodes may be visibly enlarged and an associated area of pulmonary consolidation may be present. Lymph node enlargement is usually seen ipsilateral to the side of lung disease but involvement of the contralateral nodes may be present. Occasionally, widespread massive mediastinal and hilar node enlargement is seen. With healing, the nodes usually become smaller, often returning
to normal size. Dense nodal calcification is frequent whether the nodes stay enlarged or shrink. The enlarged nodes, together with surrounding fibrosis, may compress the SVCor pulmonary veins and cause obstruction. Rim enhancement with a low-density centre may be seen with tuberculosis on contrast medium-enhanced CT.
Metastatic Carcinoma.
Mediastinal lymph node metastases can occur from primary bronchogenic carcinoma or from extrathoracic primary carcinomas. The extrathoracic tumours likely to metastasise to the mediastinum are head and neck cancer, breast cancer, genitourinary cancers and melanoma. In one large series, half the cases of mediastinal lymph node enlargement from extrathoracic primary carcinomas arose from tumours of the genitourinary tract, particularly the kidney and testis. Most metastatic tumours cause lymph node enlargement without distinguishing characteristics. Calcified lymph node metastases are typical of mucinous adenocarcinoma or thyroid carcinoma.
Reactive Hyperplasia.
Reactive hyperplasia in nodes draining infection/inflammation may cause mild enlargement, recognisable on CT but rarely on CXR.
Thoracic Lymphadenopathy in AIDS.
Mediastinal lyphadenopathy is seen in 35–40% of HIV-infected patients and may raise concern for infection or malignancy. Tuberculous and non-tuberculous mycobacterial disease and bacterial pneumonia are the primary infectious causes. Lymphoma and Kaposi's sarcoma are the major causes of malignancy. Lymphadenopathy without parenchymal lung disease may occur in patients with tuberculosis, Mycobacterium avium-intracellulare or cryptococcal infection.
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