Pneumomediastinum

Pneumomediastinum is the presence of free air in the mediastinum. It is spontaneous when no precipitating illness or injury is found (Hamman syndrome), and secondary when it follows trauma, oesophageal or airway rupture, procedures or mechanical ventilation.

Spontaneous pneumomediastinum affects mainly tall, thin young men between 15 and 30 years, with an incidence of about 1 in 7000 to 30,000 emergency attendances. Triggers include asthma, vomiting, coughing, Valsalva manoeuvres (weightlifting, childbirth, straining), inhaled drugs (cannabis, cocaine, methamphetamine), smoking, diabetic ketoacidosis, infections and interstitial lung disease. In COVID-19 pneumonia it occurs with barotrauma and alveolar rupture.

Pathophysiology. Raised intra-alveolar pressure ruptures marginal alveoli, and air tracks along the bronchovascular sheaths (Macklin effect) to the hilum and mediastinum. It can spread to the neck and subcutaneous tissue, the pleural space, pericardium or retroperitoneum. Secondary causes include rupture of the oesophagus (Boerhaave syndrome, instrumentation, ingestion), trachea or bronchus (trauma, intubation), and gas from infection.

Retrosternal chest pain (often pleuritic and radiating to the back, neck or shoulders), dyspnoea, cough, sore throat, dysphagia and a change in voice. Subcutaneous emphysema in the neck and chest wall is felt as crepitus. Hamman sign (a crunching sound synchronous with the heartbeat) is heard in some. Most patients are haemodynamically stable. Fever, vomiting or severe pain suggests oesophageal perforation.

Fever, severe pain, vomiting, haematemesis, shock, tension features or marked subcutaneous emphysema need urgent assessment for oesophageal or airway rupture.

• Chest radiograph (posteroanterior and lateral): linear streaks of air outlining the mediastinal structures, the continuous diaphragm sign, the ring around the artery sign, Naclerio V sign (air outlining the descending aorta and left hemidiaphragm), and subcutaneous emphysema. Sensitivity is about 70 to 90%, so the radiograph may miss it.
• CT chest: the most sensitive test, showing air and any cause (pneumothorax, airway tear, oesophageal leak, mediastinitis, bullae, infection).
• Water soluble contrast swallow followed by barium swallow, or CT oesophagography, for suspected oesophageal rupture (vomiting, fever, pain on swallowing, pleural effusion).
• Bronchoscopy for suspected airway injury (haemoptysis, persistent air leak, severe emphysema).
• Blood tests: white cell count and C-reactive protein, arterial gas in respiratory compromise, and urine toxicology when drug use is suspected.
• Electrocardiogram, since pain may be cardiac.

Differential diagnosis.

• Oesophageal rupture
• Tracheobronchial injury
• Pneumothorax
• Acute coronary syndrome
• Pulmonary embolism
• Pericarditis and pneumopericardium
• Aortic dissection

• Spontaneous, stable, with no sign of perforation: conservative management with rest, oxygen if required, and analgesia (paracetamol 1 g every 6 hours, ibuprofen 400 mg every 8 hours with food, and a short course of an opioid if needed). Admit for observation for 24 to 48 hours, repeat the radiograph, and allow oral intake when no oesophageal injury is suspected.
• Avoid forced expiration against a closed airway (Valsalva), heavy lifting, and playing wind instruments or diving until resolved, usually for 1 to 2 weeks. Avoid positive pressure ventilation where possible, and lower pressures in ventilated patients.
• Treat the trigger: bronchodilators and steroids in asthma (salbutamol 2.5 to 5 mg nebulised, prednisolone 40 mg daily for 5 days), antiemetic for vomiting (ondansetron 4 to 8 mg), cough suppression, and cessation of inhaled drugs and smoking.
• Oesophageal perforation: nil by mouth, broad spectrum antibiotics (piperacillin-tazobactam 4.5 g intravenously every 6 hours), antifungal cover (fluconazole 400 mg daily), proton pump inhibitor (omeprazole 40 mg intravenously daily), and early surgical or endoscopic repair or stenting, with surgical drainage when mediastinitis develops.
• Airway injury: bronchoscopy, intubation beyond the tear, and surgical repair in selected cases.
• Tension pneumomediastinum or cardiac tamponade (rare): decompression by mediastinal drainage.
• Discharge when pain is controlled, the patient eats normally and the radiograph is stable, with advice to return for chest pain, fever, breathlessness, vomiting or swelling.

Complications and prognosis. Spontaneous disease resolves within 7 to 10 days in nearly all, and recurrence is rare. Secondary disease carries the risk of the underlying cause, and oesophageal rupture has a high mortality if delayed beyond 24 hours.

Monitoring. Repeat radiograph at 24 hours and before discharge, and follow up at 1 to 2 weeks with chest radiograph, with spirometry in those with asthma.

Referral and escalation. Refer to thoracic or upper gastrointestinal surgery the same day for suspected perforation, and to intensive care for instability.