Mediastinitis

Mediastinitis is infection and inflammation of the mediastinal tissues, nearly always bacterial. It is a surgical emergency, with an overall mortality of 10 to 40%, depending on the cause and speed of treatment.

The main causes are oesophageal perforation (Boerhaave syndrome, endoscopic instrumentation, foreign body ingestion, trauma, malignancy), median sternotomy (deep sternal wound infection and mediastinitis in 1 to 3% of cardiac surgery), and descending necrotising mediastinitis from odontogenic, peritonsillar, retropharyngeal or deep neck infection. Less common are tracheobronchial rupture, oesophageal anastomotic leak, haematogenous seeding and extension from pneumonia or empyema.

Pathophysiology. Contamination of the mediastinum by oral, gastric or skin flora (mixed anaerobes, streptococci, Staphylococcus aureus, Gram negative bacilli, Candida) in a loose connective tissue space spreads along fascial planes with little barrier. Necrosis, abscess, vascular erosion, sepsis and pleural or pericardial extension follow quickly.

Fever, chills, severe retrosternal or interscapular chest pain, dysphagia and odynophagia, tachycardia, tachypnoea and sepsis. Neck swelling, trismus, subcutaneous emphysema and crepitus in descending disease. After sternotomy: wound drainage, sternal instability or click, erythema, and fever 1 to 3 weeks after surgery. Mackler triad of vomiting, chest pain and subcutaneous emphysema suggests Boerhaave syndrome.

Septic shock, spreading neck or chest swelling, airway compromise or sternal dehiscence needs immediate resuscitation and surgical contact.

• Clinical suspicion with imaging, and early CT chest and neck with intravenous contrast: mediastinal fat stranding, fluid or gas collections, abscess with rim enhancement, pleural and pericardial effusion, and the cause (oesophageal wall defect, extravasated oral contrast).
• Water soluble contrast swallow or CT with oral contrast, for oesophageal perforation. Endoscopy or bronchoscopy for assessment of the defect.
• Blood tests: white cell count, C-reactive protein, procalcitonin, lactate, renal and liver function, coagulation, and blood cultures.
• Cultures of surgical specimens, aspirates and wound fluid, including anaerobic and fungal cultures.
• Chest radiograph: widened mediastinum, air fluid level, pleural effusion, pneumomediastinum, subcutaneous emphysema (late and insensitive).
• Post-sternotomy: diagnosis by wound culture, and CT for retrosternal collections, with the criteria of positive mediastinal culture, or mediastinal abscess, or sternal instability with fever.

Differential diagnosis.

• Aortic dissection and acute coronary syndrome
• Pericarditis and pericardial tamponade
• Pneumonia, empyema and lung abscess
• Pulmonary embolism
• Fibrosing mediastinitis (chronic)
• Pneumomediastinum without infection

• Resuscitate: oxygen, intravenous fluid, vasopressors when needed, and airway protection (early intubation for neck swelling), within the first hour.
• Broad spectrum antibiotics immediately: piperacillin-tazobactam 4.5 g intravenously every 6 hours (or meropenem 1 g every 8 hours) with vancomycin 15 to 20 mg/kg every 8 to 12 hours (adjusted to trough 15 to 20 mg/L), plus metronidazole 500 mg every 8 hours if piperacillin-tazobactam is not used. Add fluconazole 400 mg daily (or an echinocandin) for oesophageal perforation. Duration is 2 to 6 weeks, directed by culture and source control.
• Source control, which is essential: surgical drainage and debridement by thoracotomy, video assisted thoracoscopy, cervical drainage with mediastinal drainage (descending disease), or sternal re-exploration with debridement.
• Oesophageal perforation: nil by mouth, primary repair with buttressing or exclusion within 24 hours, or endoscopic stent or clip for small contained leaks, with enteral feeding by jejunostomy and proton pump inhibitor (omeprazole 40 mg intravenously daily).
• Post-sternotomy: sternal debridement, vacuum assisted closure, and closure with pectoralis or omental flaps, with prolonged antibiotics.
• Odontogenic and deep neck source: extraction or drainage of the source, and tracheostomy when the airway is at risk.
• Intensive care support, enteral or parenteral nutrition, and glycaemic control (glucose 7.8 to 10 mmol/L).

Complications and prognosis. Septic shock, multi-organ failure, empyema, pericarditis, vessel erosion with haemorrhage, and chronic sternal osteomyelitis. Mortality is 10 to 20% for post-sternotomy, 20 to 40% for descending necrotising mediastinitis, and highest in oesophageal rupture treated beyond 24 hours.

Monitoring. Daily inflammatory markers, haemodynamics and drain output, repeat CT at 48 to 72 hours or if no improvement, and trough levels for vancomycin with renal function.

Referral and escalation. Refer immediately to cardiothoracic surgery, with ear, nose and throat or maxillofacial surgery for deep neck sepsis, and to intensive care.