The Common Vein

Etymology

• The term “batwing” describes the bilateral perihilar distribution of lung opacities resembling the shape of a bat?s wings on frontal chest X-rays.

AKA

• Butterfly pattern

Definition

What is it?

• The batwing pattern refers to bilateral, symmetric, perihilar opacities seen on chest imaging.
• It is associated with conditions causing alveolar or interstitial filling, often sparing the lung periphery.

Caused by:

• Pulmonary edema (most common, especially cardiogenic edema).
• Other causes include infections, neoplastic conditions, and pulmonary hemorrhage.

Resulting in:

• Diffuse opacification around the hilum while preserving the peripheral lung fields.
• Symptoms depend on the underlying cause, commonly presenting as dyspnea or hypoxia.

Structural Changes:

• Alveoli become filled with fluid, pus, blood, or tumor cells, creating a dense, perihilar opacity.

Pathophysiology:

• The central regions of the lungs, or perihilar areas, are particularly prone to fluid accumulation in conditions like pulmonary edema due to their vascular and lymphatic dynamics:
  • Hydrostatic Pressure Gradient: The central regions of the lung, closer to the pulmonary hilum, experience higher hydrostatic pressure in pulmonary veins during left heart failure or pulmonary venous congestion. This increased pressure facilitates fluid leakage from capillaries into the interstitial and alveolar spaces around the hilum.
  • Lymphatic Drainage Dynamics:
    • The peripheral lung regions, with their larger surface area, provide extensive lymphatic drainage pathways. However, fluid from these regions often drains toward the central lymphatic system. This creates a relative convergence of fluid into the perihilar lymph nodes and vessels, where drainage may become overwhelmed during pathological states.
  • Gravitational and Positional Effects: Fluid redistribution occurs with gravity, particularly in supine patients, favoring central accumulation. This contributes to the batwing appearance seen on imaging.
  • Distribution of Capillaries: Although peripheral areas have a larger cumulative surface area for capillary exchange, the central regions house more densely packed capillaries and larger vessels, making them more susceptible to congestion and leakage.

This combination of factors explains why fluid, even when originating in peripheral regions, often manifests prominently in the perihilar regions during conditions like pulmonary edema, contributing to the classic “batwing” radiographic appearance.

Pathology:

• Changes in alveolar structure and surrounding lung tissue depending on the underlying cause:
  • Pulmonary Edema: Alveolar flooding with transudate or exudate.
  • Infection: Filling of alveoli with inflammatory cells and exudates.
  • Pulmonary Hemorrhage: Alveolar spaces fill with blood, often appearing as perihilar ground-glass opacities or consolidations.

Diagnosis

Clinical:

• Dyspnea, orthopnea, and pulmonary congestion in cardiogenic edema.
• Fever and productive cough in infections.
• Hemoptysis in pulmonary hemorrhage.

Radiology:

• CXR: Bilateral perihilar opacities resembling wings of a bat, with peripheral sparing.
• CT: More detailed evaluation of the underlying cause, often showing perihilar ground-glass opacities or consolidations.

Labs:

• BNP or echocardiography for heart failure.
• Infection markers (e.g., WBC, CRP) in pneumonia.
• Hematologic and coagulation profiles for hemorrhage.

Treatment

• Directed at the underlying cause:
  • Diuretics and oxygen therapy for pulmonary edema.
  • Antibiotics for infections.
  • Supportive care or interventions for hemorrhage.

Radiology in Detail

CXR

Findings:

• Bilateral, symmetric perihilar opacities.
• Often associated with signs of heart failure, such as cardiomegaly or pleural effusion.

Associated Findings:

• Kerley B lines, pleural effusions, or cephalization of pulmonary vessels in pulmonary edema.
• Air bronchograms or patchy consolidations in infectious causes.

CT

Parts:

• Perihilar lung regions with ground-glass opacities or consolidations.

Size:

• Variable, depending on the extent of alveolar involvement.

Shape:

• Wing-like distribution around the hilum.

Position:

• Typically central and perihilar, with relative sparing of peripheral lung fields.

Character:

• Ground-glass opacities, consolidations, or interstitial thickening.

Time:

• Speed of Onset:
  • Rapid onset (minutes to hours) in cardiogenic pulmonary edema or acute pulmonary hemorrhage can help distinguish these causes from slower processes.
  • Gradual onset (days to weeks) suggests conditions such as infections, neoplastic infiltration, or chronic interstitial diseases.
• Resolution:
  • Rapid resolution (within hours to days) is typical of cardiogenic pulmonary edema when appropriately treated with diuretics and afterload reduction.
    Pulmonary hemorrhage can also resolve within 1-2 days
  • Slower resolution over weeks is more consistent with infections or inflammatory causes, such as organizing pneumonia.
  • Lack of resolution may raise concern for neoplastic or chronic interstitial processes.
• The time course of onset and resolution provides critical clues to differentiate acute from chronic conditions.

Associated Findings:

• Pulmonary venous congestion, pleural effusions, or signs of infection.

Other Imaging Modalities

MRI/PET CT/NM/US/Angio:

• MRI: Limited role; may show edema with contrast enhancement.
• PET-CT: Useful in distinguishing infectious or neoplastic causes.
• Ultrasound: Detects pleural effusions or interstitial thickening.

Key Points and Pearls

• The batwing pattern is a descriptive term, most commonly seen in cardiogenic pulmonary edema.
• Central lung involvement reflects anatomical and physiological factors such as vascular distribution, lymphatic drainage, and gravitational effects.
• Differentiating the underlying cause requires clinical correlation, lab tests, and advanced imaging.
• CT is invaluable for distinguishing among infectious, neoplastic, and hemorrhagic causes.
• The speed of onset and resolution, particularly in pulmonary hemorrhage and pulmonary edema, provides essential diagnostic clues to distinguish acute from chronic conditions.