Expansion
A triangular projection from the upper mediastinum in an infant, resembling a sail
Expansion
The thymus is proportionally large in infancy and lies in the anterior and superior mediastinum, so it is normally visible on a chest radiograph until about 3 years of age.
Features that confirm it is normal thymus rather than a mass:
- Sail sign - a sharply defined triangular projection, usually from the right side
- Wave sign - a rippled lateral border, indented by the anterior ribs
- No mass effect - the trachea and vessels are not displaced or narrowed
- It is soft, so it changes shape with respiration and position
The thymus involutes under stress, including severe illness and steroid therapy, and it can shrink and then rebound. Recognising the normal appearances prevents unnecessary investigation of a healthy infant.