In an adult with unilateral pulmonary trunk enlargement on chest X‑ray, what are the most likely causes and the appropriate next diagnostic and management steps?

Unilateral Pulmonary Trunk Enlargement on Chest X-Ray

When you see unilateral enlargement of the pulmonary trunk on chest X-ray, you are most likely looking at pulmonary hypertension, congenital heart disease with left-to-right shunting, or unilateral absence of the contralateral pulmonary artery—proceed immediately to CT angiography of the chest with contrast to define the vascular anatomy and guide further management. 1, 2

Initial Radiographic Assessment

Measure the right descending pulmonary artery diameter at the hilum on the posterior-anterior view:

• >15 mm in women or >16 mm in men indicates pulmonary hypertension with 93% sensitivity and 88% specificity 1, 2
• Main pulmonary artery enlargement >35 mm from midline to left lateral border occurs in 96% of pulmonary hypertension cases 1, 3
• Look for central pulmonary artery enlargement with rapid peripheral tapering ("pruning" pattern), which is characteristic of pulmonary hypertension 2, 4
• Chest X-ray has 96.9% sensitivity and 99.1% specificity for moderate to severe pulmonary hypertension 1, 2

Critical caveat: Never rely on chest X-ray alone—sensitivity is inadequate and up to 72% of lesions may be missed 2. Normal chest radiographs do not exclude significant pathology 1, 2.

Most Likely Causes of Unilateral Pulmonary Trunk Dilation

Pulmonary Hypertension (Most Common)

• Defined as mean pulmonary arterial pressure ≥25 mmHg at rest by right heart catheterization 3
• Chronic pulmonary hypertension from any cause leads to vessel wall remodeling and dilation 3
• Associated findings include right heart chamber enlargement and pulmonary vascular pruning 1, 4

Congenital Heart Disease with Left-to-Right Shunts

• Atrial septal defects cause massive dilation of pulmonary arterial trunk and branches 3
• Ventricular septal defects, especially moderate-to-large defects, lead to pulmonary trunk dilation 5, 3
• Patent ductus arteriosus presents with prominent proximal pulmonary artery segment indicating elevated pulmonary artery pressure 5

Pulmonary Valve Stenosis

• Paradoxically causes post-stenotic dilation of the pulmonary trunk 5, 3
• Dilation of the pulmonary trunk and left pulmonary artery may occur independent of hemodynamics due to intrinsic wall abnormalities 5
• The right pulmonary artery is generally less affected 5

Unilateral Absence of Pulmonary Artery (Rare but Important)

• Rare congenital abnormality from malformation of the sixth aortic arch during embryonic development 6, 7
• Presents with hemoptysis (41.5%), exertional dyspnea (41.5%), or recurrent respiratory infection (35.4%) 7
• The contralateral pulmonary artery becomes dilated due to increased flow and secondary pulmonary hypertension 6, 7
• Associated with hypertrophic bronchial, phrenic, internal thoracic, and intercostal arteries as collateral circulation 7

Mandatory Next Steps

Immediate Advanced Imaging

Order CT chest with IV contrast—this is the gold standard for definitive diagnosis 1, 2, 8:

• Provides superior detection of vascular anatomy and spatial localization 2
• Evaluates the entire thorax including heart, mediastinal structures, and lungs 8
• Identifies associated findings such as bronchiectasis (30.2%), interstitial changes (14.0%), or multiple bullae (14.0%) in unilateral absence of pulmonary artery 7

Functional Assessment

Perform transthoracic Doppler echocardiography when pulmonary hypertension is suspected 1, 2:

• Sensitivity of 85% and specificity of 74% for moderate to severe disease 1, 2
• Measures pulmonary trunk diameter and estimates pulmonary pressures 3
• Assesses right ventricular size and function 5

Hemodynamic Confirmation

Right heart catheterization is required to confirm pulmonary hypertension and measure pulmonary capillary wedge pressure 3:

• Differentiates pre-capillary from post-capillary pulmonary hypertension 3
• Necessary for preoperative risk assessment in patients with chronic thromboembolic pulmonary hypertension 5
• Determines pulmonary vascular resistance and reactivity to vasodilator therapy 5

Key Pitfalls to Avoid

• Do not assume the diagnosis without CT confirmation—chest X-ray is insensitive for mild pulmonary hypertension and cannot determine the underlying cause 1, 2, 8
• Do not delay echocardiography if clinical suspicion persists despite normal-appearing hilar vessels—chest X-ray can miss early pulmonary hypertension 1
• Consider malignancy in the differential, particularly in patients with >20 pack-year smoking history, as hilar masses can mimic vascular enlargement 2
• Remember that infectious causes (tuberculosis, sarcoidosis) can present as hilar masses requiring histological confirmation 2