Mediastinal haemorrhage is most commonly caused by trauma to the arteries and veins within the mediastinum, with other causes including rupture of an aneurysm, aortic dissection and complications of central venous catheterisation.

Radiologically, haemorrhage produces an increase in the mediastinal diameter, which is maximal at the point of bleeding. Blood may track through the mediastinum, frequently running over the apex of the left lung to produce a smooth and well-defined apical cap. When haemorrhage is severe, blood may rupture into the pleural cavity or dissect into lung along peribronchovascular sheaths, resulting in a radiographic pattern resembling interstitial oedema. On unenhanced CT, acute haemorrhage may appear of relative high attenuation. The appearance of mediastinal haematoma on MRI varies with the age of the haemorrhage.

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