The Common Vein

AKA

Mediastinal emphysema

What is it?

Pneumomediastinum is a condition characterized by the abnormal presence of free air within the mediastinal space. It can occur spontaneously or secondary to trauma, medical procedures, or other underlying conditions.

Caused by:

• Most common causes
  • Spontaneous pneumomediastinum: Often due to alveolar rupture with air dissecting along the bronchovascular sheath into the mediastinum (e.g., Valsalva maneuver, severe coughing, vomiting).
  • Traumatic pneumomediastinum: Blunt or penetrating trauma to the chest, esophageal rupture (Boerhaave syndrome).
• Other causes include:
  • Infections: Necrotizing infections (e.g., gas-forming organisms such as Clostridium).
  • Inflammation/Immune: Rarely associated with connective tissue diseases.
  • Mechanical Trauma: Tracheal or bronchial injury, barotrauma (e.g., from mechanical ventilation).
  • Iatrogenic: Post-procedural complications (e.g., endoscopy, tracheostomy, chest tube insertion).
  • Metabolic: Rare cases in hyperventilation syndromes.
  • Inherited/Congenital: Associated with conditions such as Marfan syndrome.

Resulting in:

• Compression of mediastinal structures (rare in mild cases).
• Potentially life-threatening complications, such as tension pneumomediastinum or mediastinitis.

Structural changes:

• Air accumulation within the mediastinum, potentially dissecting along fascial planes into the neck, pericardium, or retroperitoneum.

Pathophysiology:

• Alveolar rupture increases intra-alveolar pressure (Macklin effect), allowing air to escape into the interstitium and track along bronchovascular bundles into the mediastinum.

Pathology:

• Microscopic evidence of ruptured alveoli and air dissecting through connective tissue planes.
• Gross evidence of free air in the mediastinal space.

Diagnosis:

• Clinical:
  • Symptoms: Chest pain, dyspnea, neck pain, dysphagia, or subcutaneous emphysema.
  • Signs: Subcutaneous crepitus, Hamman?s sign (crunching sound synchronous with heartbeat).
• Radiology:
  • CXR: Mediastinal air outlining structures (e.g., aorta, heart, trachea).
  • CT: More sensitive, showing mediastinal air and the site of origin (e.g., alveolar rupture, esophageal perforation).
• Labs:
  • Elevated white blood cell count in cases with infection or mediastinitis.
  • Arterial blood gas: May show hypoxemia in severe cases.

Management:

• Conservative management (most cases):
  • Observation, analgesia, and supplemental oxygen to enhance reabsorption of air.
• Surgical or procedural intervention:
  • Indicated for tension pneumomediastinum, significant esophageal rupture, or tracheobronchial injury.

Radiology Detail:

• CXR:
  • Findings: Mediastinal air outlining the heart, aorta, or trachea.
  • Associated Findings: Subcutaneous emphysema or pneumothorax.
• CT:
  • Parts: Mediastinal space, pleural spaces, lung parenchyma.
  • Size: Varies with the volume of air.
  • Shape: Irregular collections of air outlining mediastinal structures.
  • Position: Along the mediastinal compartment, tracking into neck or retroperitoneum.
  • Character: Free air with no mass effect unless severe.
  • Time: Air may persist for hours to days.
  • Associated Findings: Pneumothorax, tracheal or esophageal rupture.
  • Specific Signs:
    • Continuous Diaphragm Sign: Air outlines the inferior heart border, making the central diaphragm visible.
    • Naclerio?s V-sign: V-shaped lucency formed by air outlining the descending aorta and diaphragm.
    • Thymic Sail Sign (in children): Elevated thymus caused by mediastinal air.
• Other relevant Imaging Modalities:
  • MRI: Rarely used; may demonstrate associated soft tissue changes.
  • Ultrasound: Can detect subcutaneous emphysema or tension pneumothorax.

Pulmonary Function Tests (PFTs):

• Rarely performed; not typically indicated in acute cases.

Recommendations:

• Identify and treat the underlying cause (e.g., infection, trauma).
• Monitor for complications such as tension pneumomediastinum or mediastinitis.
• Ensure appropriate oxygenation and pain management.

Key Points and Pearls:

• Spontaneous pneumomediastinum often resolves with conservative management and has a benign prognosis.
• CXR and CT are crucial for diagnosis, with CT being more sensitive in detecting small volumes of air.
• The Macklin effect explains the pathogenesis of spontaneous pneumomediastinum.
• Rarely, pneumomediastinum can progress to life-threatening complications like tension pneumomediastinum or mediastinitis.