The Middle Mediastinum

Signs and Symptoms

Although often asymptomatic, a variety of signs and symptoms may be present in patients with diseases of the middle mediastinum. Individual presentation depends on the size and location of the lesion and whether it is benign, malignant, inflammatory, or infectious. The most common symptoms include cough, dyspnea, and chest pain. In the presence of malignancy, direct invasion can lead to diaphragmatic paralysis or chylothorax. Compression of the superior vena cava can obstruct blood return from the upper body. Rarely, hormonal or endocrine substrates lead to systemic manifestations, leading to workup and diagnosis of middle mediastinal pathology.

Imaging

The conventional chest radiograph often provides the first indication of an abnormality in the middle mediastinum. Narrowing or deviation of the tracheobronchial tree, enlargement of the pericardial silhouette, and calcification or enlargement of the great vessels are detectable radiographically ( Fig. 42-2 A ). The right paratracheal stripe is formed by the lateral trachea, mediastinal tissue, and paratracheal pleura, and it should be visible on posterior-anterior chest radiograph (CXR). Obliteration of this stripe can indicate the presence of lymphadenopathy in station 4R, a right-sided aortic arch, or a paratracheal mass. Similarly, the aortopulmonary window should appear as a concavity at the interface between the aortic knob and left pulmonary artery on CXR. Fullness in this region is considered abnormal and may reflect lymphadenopathy in the aortopulmonary window. However, because of its limited resolution, CXR often does not provide adequate information about specific mediastinal pathology. Further radiographic assessment by other modalities is necessary for a more accurate assessment.

Computed tomography (CT), often with intravenous or oral contrast, is indicated when mediastinal pathology is suspected (see Fig. 42-2 B ). High-resolution spiral CT, with cross-sectional imaging of structures at intervals as narrow as 1 mm, is the radiologic modality of choice for imaging the middle mediastinum. All structures of the middle mediastinum are visible on CT scan, and their relationship to adjacent structures is often easily delineated. Three-dimensional reconstructions can be created, and they are particularly useful for detailed assessment of structures, such as the trachea and aortic arch. The presence of calcified mediastinal lymph nodes and pulmonary granulomas may suggest a subacute, benign process, such as mediastinal histoplasmosis.

Magnetic resonance imaging (MRI) may provide additional information in the assessment of middle mediastinal pathology. Advantages of MRI include its ability to differentiate between vascular, solid, and fluid elements in a given mass. MRI has been shown to be superior to CT for assessing tumor invasion into vascular structures. T1-weighted images are preferable for delineating anatomy, whereas T2-weighted images are preferable for characterizing solid from cystic structures.

Positron emission tomography (PET) using fluorine-18 fluorodeoxyglucose (FDG) is a valuable tool for evaluating middle mediastinal pathology. It has become an integral component in the clinical staging of lung and esophageal cancers, as well as in the follow-up of mediastinal lymphomas. The fusion of PET and CT technology increases diagnostic accuracy by allowing precise localization of hypermetabolism. The degree of FDG uptake is used as a surrogate marker for the degree of metabolic activity in a given tissue. Areas of increased activity are often targeted for biopsy to provide the most high yield results. However, the utility of PET scanning is limited because it is difficult to distinguish between hypermetabolism caused by malignancy and that caused by inflammation. For example, acute mediastinal lymphadenopathy related to pneumonia commonly demonstrates FDG avidity.

Outside of echocardiography, transthoracic ultrasonography is of limited value in the evaluation of the middle mediastinum. However, endobronchial ultrasonography (EBUS) and transesophageal ultrasonography (EUS) have emerged as valuable tools for the evaluation of the mediastinum. Both techniques can be performed using moderate sedation on an outpatient basis. Using EBUS, lymph nodes in stations 2, 4, 7, 10, and 11 can be visualized and sampled. The addition of EUS allows access to lymph nodes in stations 8 and 9. Both modalities can differentiate solid from cystic masses, detect lymph nodes as small as 3 × 5 mm, and allow for reliable ultrasound-guided fine-needle aspiration.

Other modalities, such as leukocyte scintigraphy, lymphoscintigraphy, and metaiodobenzylguanidine (MIBG) scanning, have limited and very specific indications in the evaluation of pathology in the middle mediastinum.

Invasive Techniques