CHAPTER 8 The widened mediastinum
This is the film of a patient with a descending thoracic aortic aneurysm. The mediastinum is widened to the left of the midline throughout its length, with increased convexity around the aortic arch.
Always look carefully at the mediastinum. If you think that it is widened then relate this finding to the clinical history. Try to find some old films and see if the mediastinum has got larger. Be aware that a normal chest X-ray does not exclude a significant aortic event, such as a dissection, and in the presence of clinical suspicion an urgent CT may be an appropriate investigation.
2. Decide whether the enlargement is at the top, middle or bottom of the mediastinum. If at the top it is likely to be thyroid, thymus or innominate artery. If in the middle or bottom of the mediastinum it could be lymphadenopathy, aortic widening, dilatation of the oesophagus or a hiatal hernia.
3. If the shadowing is at the top then look at the position of the trachea. An enlarged thyroid will displace or narrow the trachea. This will not happen with a tortuous innominate artery – a common finding in the elderly.
4. Look at the right side of the trachea. The white edge of the trachea should be less than 2–3 mm wide on an erect film. An increase in its width suggests either an enlarged superior vena cava or a paratracheal mass. This rule does not apply to supine films.
5. If you suspect an enlarged thyroid then look at the outline of the shadow. A thyroid has a well-defined outline that tends to become less clear as one moves up the neck.
6. If you suspect widening of the aorta then try and follow its outline, remembering that the root of the aorta is not visible. You may be able to detect a continuous edge which widens to form the edge of the enlarged mediastinum. This would suggest that the widening is due to dilatation of the aorta.
7. The commonest cause of an abnormal whiteness of the mediastinum in the elderly will be unfolding of the aorta. You may be able to trace the margin of the ascending aorta around the arch and to the descending aorta. Some calcification in the wall of the aortic knuckle is a common feature. If the line of calcium is separated from the edge of the aortic shadow this strongly suggests a dissection.
