Superior vena cava (SVC) syndrome (SVCS) is a constellation of symptoms that result from obstruction of the SVC (see the images from a single case, below).
The superior vena cava (SVC) is formed in the upper middle part of the mediastinum by the junction of the brachiocephalic veins. It is 6-8 cm long and drains into the right atrium at approximately the level of the right mainstem bronchus. The azygous vein loops over the right mainstem bronchus and connects to the posterolateral wall of the SVC. The SVC lies in a relatively confined space and is surrounded by several lymph node groups that predispose it to compression, invasion, or involvement in inflammatory conditions. [1, 2]
Causes of SVC
Obstruction of the SVC can be caused by malignant or benign disease. In one series, malignancy was, by far, the most common etiology. [3, 4] The most frequent malignancies are bronchogenic carcinoma (in order of decreasing frequency: small cell carcinoma, squamous cell carcinoma, adenocarcinoma, large cell carcinoma), followed by non-Hodgkin lymphoma. Many other malignancies have been reported; essentially, any mediastinal mass may compress or invade the SVC.
Benign causes include central venous catheters (increasing in frequency), pacemaker wires, fibrosing mediastinitis, thoracic aortic aneurysms, and a multitude of unusual conditions. 
The examination of a patient suspected of having superior vena cava syndrome depends on the patient's prior medical history. All patients should undergo chest radiography and Doppler ultrasonographic evaluation of the central veins. If normal venous waveforms are seen in the brachiocephalic, subclavian, and internal jugular veins, the presence of a significant SVC stenosis is unlikely. [6, 7, 8, 9, 2, 10, 11, 12, 5]
CT and MRI
In patients with suspected malignancy, CT of the thorax should be performed. [13, 14, 1, 15] Magnetic resonance venography may also be used to image the central veins for patients with allergies to contrast material.  If the findings of noninvasive imaging studies or if the clinical diagnosis of SVCS is uncertain, venography with contrast medium or carbon dioxide, as well as pressure measurements, are extremely useful.
The presence or absence of venous thrombosis must be determined. Acute onset or a change in symptoms suggests acute thrombosis. Ultrasonography is useful for excluding thrombus in the upper extremity, axillary, subclavian, and brachiocephalic veins in most patients. The SVC cannot be directly imaged because of the lack of an adequate acoustic window. SVC patency may be indirectly determined with normal waveforms in the subclavian and brachiocephalic veins.
Contrast-enhanced venography may be required to exclude central venous thrombosis when ultrasonographic findings are suboptimal or inconclusive. Transesophageal echocardiography may also be used to image the SVC and right atrium. 
Obtaining a tissue diagnosis is the first step in the treatment of patients with SVC syndrome (SVCS) caused by a mediastinal mass. The workup of patients varies according to the patient's age, medical history, and imaging findings. Mediastinal masses may be examined by means of percutaneous biopsy under CT guidance and, occasionally, under ultrasonographic guidance. In certain circumstances, mediastinoscopy is required. Large core samples are needed for patients with suspected lymphoma. Pleural effusions may be aspirated and sent for cytology.