A bulla is an air-filled space within the lung parenchyma that is greater than 1 cm in diameter and has a thin wall (<1 mm). It results from the destruction of alveolar walls due to processes such as emphysema or other parenchymal damage. Bullae are commonly subpleural and are considered a form of emphysematous change.

 

Basal Bulla

Bulla Masquerading as a Deep Sulcus Sign

 

Ruptured Bulla and Tension Pneumothorax

 

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Radiological Features

1. Chest X-Ray (CXR):
   • Appearance:
     • A hyperlucent, air-filled space with no visible lung markings inside.
     • Thin, imperceptible walls (if visible, suggests a thicker-walled cavity).
     • Typically located in the upper lobes or subpleural regions.
   • Complications:
     • Large bullae may mimic a pneumothorax (known as a “vanishing lung” phenomenon).
2. Computed Tomography (CT):
   • Appearance:
     • Thin-walled air-filled space (>1 cm).
     • Often subpleural, but can be central or paraseptal.
     • Adjacent lung parenchyma may show emphysematous changes (e.g., centrilobular emphysema).
   • Size and Location:
     • Variable sizes; bullae may be solitary or multiple.
     • Large bullae (>5 cm) are sometimes referred to as giant bullae.
   • Associated Findings:
     • Evidence of adjacent parenchymal destruction or fibrosis.
     • Possible compression of adjacent lung tissue or mediastinal structures in giant bullae.
3. Magnetic Resonance Imaging (MRI):
   • Rarely used but can confirm the presence of air-filled spaces.
4. Ultrasound:
   • Rarely used in routine evaluation.
   • May show absence of normal lung sliding if a bulla is suspected near the pleura.

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Common Causes

Bullae are typically associated with conditions that result in alveolar wall destruction:

1. Primary Causes:
   • Emphysema:
     • Centrilobular or paraseptal emphysema commonly leads to bullae formation.
   • Congenital Bullae:
     • Occur in rare conditions such as congenital lobar emphysema.
2. Secondary Causes:
   • Chronic Obstructive Pulmonary Disease (COPD):
     • Strongly linked to bullae, especially in smokers.
   • Infections:
     • Post-tubercular or fungal infections may leave bullae as residual lesions.
   • Trauma:
     • Barotrauma from mechanical ventilation or blunt chest trauma.
   • Fibrosis-Related Diseases:
     • Paraseptal bullae near fibrotic lung tissue (e.g., in idiopathic pulmonary fibrosis).
3. Rare Associations:
   • Birt-Hogg-Dubé Syndrome:
     • Autosomal dominant disorder associated with bullae and spontaneous pneumothorax.
   • Marfan Syndrome:
     • Connective tissue disorders predisposing to bullae formation.

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Differential Diagnosis

Bullae must be distinguished from other radiological entities:

1. Cysts:
   • Have thicker walls and may contain fluid or solid components.
2. Cavities:
   • Often irregular with thicker walls, associated with necrosis or infection (e.g., tuberculosis, abscess).
3. Pneumatoceles:
   • Transient, thin-walled air spaces seen in trauma or infections like staphylococcal pneumonia.
4. Pneumothorax:
   • Hyperlucency without a wall and no visible lung markings, often with pleural line displacement.

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Complications

1. Spontaneous Pneumothorax:
   • Rupture of a bulla can lead to air escaping into the pleural space.
2. Infection:
   • Superinfection of a bulla can result in an abscess or empyema.
3. Respiratory Compromise:
   • Large bullae can compress adjacent lung tissue, reducing functional lung capacity.

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Management

1. Observation:
   • Small, asymptomatic bullae usually require no intervention.
2. Medical Therapy:
   • Smoking cessation and bronchodilators in cases of underlying COPD or emphysema.
3. Surgical Treatment:
   • Bullectomy:
     • Indicated for symptomatic or complicated bullae (e.g., recurrent pneumothorax or compression).
   • Lung Volume Reduction Surgery (LVRS):
     • For giant bullae causing severe respiratory compromise.

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Clinical Relevance

• Bullae are often incidental findings on imaging but may have significant clinical implications if they rupture or cause compression.
• Identifying and distinguishing bullae from other similar-appearing lesions on imaging is critical for appropriate management.