Normal Diaphragm Flattening Size on Chest Radiography
Diaphragm flattening refers to the loss of the normal dome-shaped contour of the hemidiaphragm, assessed on the posteroanterior (PA) chest radiograph by measuring the perpendicular height from the apex of the diaphragmatic dome to a line drawn between its costophrenic and cardiophrenic angles. This measurement is a practical, non-invasive marker of lung volumes and intrathoracic pressure dynamics. Accurate assessment is essential in evaluating patients with suspected obstructive lung disease or acute respiratory compromise.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Posteroanterior | <1.5 cm |
Clinical Significance
A diaphragm dome height of less than 1.5 cm on the PA radiograph indicates significant flattening, reflecting marked hyperinflation of the underlying lung. In normal individuals, the diaphragmatic dome projects well above this threshold, maintaining its characteristic convex superior contour. When flattening is present, it implies elevated residual lung volumes and air trapping, compressing the diaphragm inferiorly and inverting its normal curvature.
Diaphragm flattening is most commonly encountered in advanced obstructive airway disease. Its presence should prompt clinical correlation with spirometry and symptom burden. Bilateral flattening carries greater specificity for diffuse hyperinflation, whereas unilateral flattening may reflect localized pathology such as a large pneumothorax or unilateral air trapping.
- Chronic obstructive pulmonary disease (COPD) / emphysema — most common cause of bilateral flattening
- Severe asthma exacerbation — reversible bilateral flattening during acute attack
- Tension pneumothorax — unilateral diaphragm depression and contralateral mediastinal shift
- Large unilateral pleural effusion — may mimic or obscure diaphragm position
- Bronchiolitis or foreign body aspiration — consider in pediatric patients with unilateral air trapping
Reference: M.D. JC, Stern EJ. Chest Radiology, The Essentials. Lippincott Williams & Wilkins. (2008).
Imaging Notes
Diaphragm flattening is assessed on the posteroanterior (PA) chest radiograph obtained at full inspiration with the patient erect. Measurement technique involves identifying the highest point of the hemidiaphragmatic dome and dropping a perpendicular to the line connecting the costophrenic and cardiophrenic insertion points. A value below 1.5 cm defines flattening. Ensure the radiograph is taken at true full inspiration — a technically limited or rotated film can artificially depress or elevate the diaphragm, leading to measurement error.
On lateral views, flattening is corroborated by loss of the anterior diaphragmatic concavity and an increased retrosternal airspace. Portable AP projections magnify and distort diaphragm position and are less reliable for this measurement; PA technique should be used whenever feasible for consistent, reproducible assessment.