Normal Retrosternal Space Size on Chest Radiography
The retrosternal space is the lucent region on the lateral chest radiograph located anterior to the ascending aorta and posterior to the sternum, representing the contact zone between the anterior lung margins and the chest wall. Accurate measurement of this space is an important indirect marker of lung volume and anterior chest diameter. Its assessment is routinely performed when obstructive lung disease or hyperinflation is clinically suspected.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | <2.5 cm |
Clinical Significance
A retrosternal space measuring 2.5 cm or greater is considered abnormal and is a well-established radiographic sign of pulmonary hyperinflation. Hyperinflation occurs when air trapping causes persistent overexpansion of the lungs, pushing the anterior lung margins forward to occupy space normally free of aerated lung. This finding carries meaningful diagnostic weight when correlated with clinical symptoms and pulmonary function tests.
Enlargement of the retrosternal space is most commonly associated with obstructive lung diseases. It should be interpreted alongside other lateral radiograph signs of hyperinflation, including flattening of the diaphragm and increased AP thoracic diameter. A falsely enlarged measurement may result from patient rotation or a lordotic projection, making strict lateral positioning essential.
- Chronic obstructive pulmonary disease (COPD) — most common cause in adults
- Emphysema — particularly in the setting of bullous disease
- Severe asthma — dynamic hyperinflation during acute exacerbation
- Bronchiectasis — with secondary air trapping
- Constrictive bronchiolitis — especially post-infectious or post-transplant
Reference: Simon G, Pride NB, Jones NL et al. Relation between abnormalities in the chest radiograph and changes in pulmonary function in chronic bronchitis and emphysema. Thorax. 1973;28(1):15-23.
Imaging Notes
The retrosternal space is measured on the lateral chest radiograph as the maximum horizontal distance between the posterior surface of the sternum and the anterior margin of the ascending aorta or nearest soft-tissue structure. The measurement is typically taken at the level where the space appears widest, approximately at the level of the aortic arch. True lateral positioning is critical — even minor patient rotation can artifactually widen or narrow the apparent space, leading to measurement error.
When assessing for hyperinflation, the retrosternal space should be evaluated in conjunction with diaphragm contour, the retrocardiac clear space, and the overall AP chest diameter. Standard inspiratory breath-hold at full inspiration is required for reproducible measurements; expiratory films will overestimate hyperinflation.