What are the causes of pruning of blood vessels in a chest X-ray?

Vascular Pruning on Chest X-Ray: Causes

Vascular pruning on chest X-ray—characterized by enlarged central pulmonary arteries contrasting with attenuated peripheral vascular markings—is primarily caused by pulmonary arterial hypertension (PAH) and related pulmonary vascular diseases. 1

Primary Cause: Pulmonary Arterial Hypertension

• Pulmonary arterial hypertension is the most important cause of vascular pruning, manifesting as central pulmonary arterial dilatation with loss of peripheral blood vessels 1
• This radiographic finding reflects underlying pulmonary vascular pathology with elevated pulmonary vascular resistance (PVR >3 Wood units and mean PAP >25 mm Hg) 1
• The pruning pattern represents histologic intimal and medial thickening in small pulmonary arteries, eventually resulting in loss of peripheral vasculature 2

Specific PAH Subtypes That Cause Pruning:

• Idiopathic PAH (IPAH): Most patients with IPAH show decreased peripheral lung vascular markings with hilar pulmonary artery prominence on chest X-ray 1
• PAH associated with connective tissue disease (particularly scleroderma spectrum): These patients commonly demonstrate pruning and require evaluation even when other risk factors are present 1
• Chronic thromboembolic pulmonary hypertension (CTEPH): Presents with cardiomegaly (86%), right ventricular enlargement (68%), and mosaic oligemia (68%), along with vascular pruning 1
• Pulmonary veno-occlusive disease (PVOD): Should be considered when pruning is accompanied by digital clubbing 1

Secondary Causes: Lung Parenchymal Diseases

• Severe asthma: Vascular pruning is associated with asthma severity, poor control, and exacerbations, correlating with lower FEV1 and greater eosinophilia 3
• Interstitial lung abnormalities (ILA): More severe vascular pruning on CT is associated with greater odds of ILA and ILA progression 4
• COPD with pulmonary hypertension: Hypoxic pulmonary hypertension from COPD can produce pruning, though this is typically accompanied by hyperinflation and other emphysematous changes 1, 5

Hemodynamic Causes Without Primary Vascular Disease

• High cardiac output states (exercise, anemia, pregnancy, sepsis, portopulmonary syndrome, thyrotoxicosis): These can cause elevated pulmonary artery pressure but typically have normal PVR and the pulmonary vascular bed is anatomically normal 1
• However, a subset of patients with portopulmonary syndrome may develop true pulmonary vascular disease with pruning 1

Critical Diagnostic Distinctions

It is essential to distinguish vascular pruning from increased pulmonary vascular markings:

• Increased vascular markings occur with left-to-right shunts or congenital heart disease with increased pulmonary blood flow, showing prominent vessels throughout the lung fields 6
• Pulmonary venous congestion from left heart disease shows increased pulmonary vascular markings with redistribution to upper lobes, not pruning 1
• The artery-to-vein ratio helps differentiate: arterial PH shows increased ratio while venous PH shows decreased ratio 1

Diagnostic Workup When Pruning Is Identified

• Transthoracic Doppler echocardiography should be performed immediately when pruning suggests pulmonary hypertension (sensitivity 85%, specificity 74% for moderate-severe disease) 7
• Right heart catheterization is required to confirm PAH diagnosis and define hemodynamics accurately, as echocardiographic estimates can be unreliable 1
• Ventilation-perfusion scan is more sensitive than CT for excluding CTEPH, which has substantial therapeutic implications 1
• Pulmonary function tests with DLCO help identify underlying lung disease; DLCO <45% predicted is associated with poor outcomes in PAH 1

Important Clinical Caveat

The absence of pruning does not exclude pulmonary hypertension—many patients with confirmed PAH have normal chest radiographs, particularly in early or mild disease 1. The degree of radiographic abnormalities does not correlate with the severity of pulmonary hypertension 1. Therefore, if clinical suspicion persists despite normal chest X-ray findings, proceed directly to echocardiography and consider advanced imaging 7.