What is the mechanism behind pneumomediastinum vs pneumothorax in regards to alveolar architecture, particularly in patients with a history of trauma, lung disease, or recent medical procedures, and how does fistulization affect the condition?

Mechanism of Pneumomediastinum vs Pneumothorax: Alveolar Architecture and Fistulization

Pneumomediastinum occurs when alveolar rupture allows air to dissect along bronchovascular sheaths toward the mediastinum (Macklin effect), while pneumothorax results from air entering the pleural space directly through visceral pleural breach—the key difference is whether the visceral pleura remains intact. 1, 2

Fundamental Pathophysiology

Pneumomediastinum Mechanism

• Air escapes from ruptured alveoli but remains contained within the interstitial tissue planes because the visceral pleura stays intact, allowing air to track centrally along perivascular and peribronchial sheaths into the mediastinum 1, 2
• The process begins with alveolar rupture from increased intra-alveolar pressure (from coughing, Valsalva, mechanical ventilation, or airway obstruction), which creates a pressure gradient that drives air into the pulmonary interstitium 1, 3
• Once in the interstitium, air follows the path of least resistance along bronchovascular bundles toward the hilum and into the mediastinum rather than crossing into the pleural space 1, 2
• The mediastinal air can then dissect into surrounding tissues including the neck (causing subcutaneous emphysema in 100% of cases), retroperitoneum, and rarely the peritoneum 4, 3

Pneumothorax Mechanism

• Air enters the pleural space through a breach in the visceral pleura, whether from direct trauma, spontaneous rupture of subpleural blebs, or iatrogenic injury 5
• In spontaneous pneumothorax, rupture of apical subpleural blebs or bullae allows direct communication between the alveolar space and pleural cavity 5
• The pleural space normally maintains negative pressure (-4 mmHg at rest), so any communication with atmospheric pressure or positive alveolar pressure causes lung collapse 5

Fistulization Patterns

Bronchopleural Fistula (Leading to Pneumothorax)

• Creates persistent communication between the bronchial tree and pleural space, most commonly seen in mechanically ventilated patients, post-surgical complications, or necrotizing infections 5, 6
• Positive pressure ventilation maintains the air leak and prevents spontaneous resolution, requiring tube thoracostomy in all cases 6
• The fistula acts as a one-way valve in tension pneumothorax, allowing air entry during inspiration but preventing escape during expiration, causing progressive pressure buildup that shifts the mediastinum and impairs venous return 5, 6

Tracheoesophageal or Esophageal Perforation (Leading to Pneumomediastinum)

• Esophageal rupture introduces air directly into the mediastinum without necessarily violating the pleural space initially 7, 1
• CT scan can identify patients at high risk for aerodigestive injury with 100% sensitivity and 85% specificity when pneumomediastinum is present 7
• Only 10 of 136 patients (7%) with traumatic pneumomediastinum had major aerodigestive injuries requiring operative intervention (5 laryngeal, 3 tracheal, 2 esophageal) 7

Progression from Pneumomediastinum to Pneumothorax

• Mediastinal air can rupture through the mediastinal pleura into the pleural space, converting pneumomediastinum to pneumothorax, particularly under positive pressure ventilation 4, 2
• In rare cases, mediastinal air can track through the diaphragmatic hiatus into the retroperitoneum and peritoneum, potentially causing tension pneumoperitoneum and abdominal compartment syndrome 4

Clinical Differentiation

Pneumomediastinum Presentation

• Central chest pain is the predominant symptom (present in most cases), often with subcutaneous emphysema detectable in 100% of cases on examination 2, 3
• Hamman's crunch (crunching sound with cardiac cycle) is pathognomonic but only audible in a minority of patients 2, 3
• Radiographic signs include air outlining mediastinal structures: continuous diaphragm sign (air between pericardium and diaphragm), "ring around the artery" sign, and thymic sail sign 1
• CT scan detects pneumomediastinum in 100% of cases versus only 15% detection rate on plain radiograph 7

Pneumothorax Presentation

• Dyspnea and pleuritic chest pain with decreased breath sounds on the affected side are cardinal features 5
• Arterial hypoxemia (PaO₂ <10.9 kPa) occurs in 75% of patients, with severity correlating to pneumothorax size and underlying lung disease 5
• Tension pneumothorax presents with rapid labored respiration, cyanosis, tachycardia, and hemodynamic instability requiring immediate needle decompression before radiographic confirmation 6, 8

Management Implications

Pneumomediastinum

• Conservative management is appropriate for most cases as mediastinal tissues slowly resorb air over 4-13 days (mean 7.8 days hospitalization) 2, 3
• Prophylactic antibiotics and NPO status should be considered when esophageal origin cannot be excluded, though emergency endoscopy is usually unnecessary 3
• High suspicion for progression to tension pneumoperitoneum is needed in mechanically ventilated patients, requiring immediate decompression if abdominal compartment syndrome develops 4

Pneumothorax with Fistula

• Patients on positive pressure ventilation always require tube thoracostomy (small-bore ≤14F or 16-22F chest tube) as positive pressure maintains the air leak 6
• Large chest tubes (24-36F) are reserved only for large bronchopleural fistulas or when positive pressure ventilation is required 6
• Persistent air leak indicates ongoing fistulization requiring prolonged drainage until the fistula closes or surgical intervention 5