Mediastinitis

Background

• Infection of the structures in the mediastinum
  • The mediastinum refers to the space in the chest between the lungs, which contains the heart, thymus, esophagus, and trachea
• Confusing nomenclature, and includes mediastinal infections caused by extension of a cervical infection (necrotizing mediastinitis or descending mediastinitis) and mediastinal infection after sternotomy (sternal infection or deep sternal wound infection)

Microbiology

• Post-cardiac surgery: often mixed
  • Staphylococcus aureus and coagulase-negative staphylococci are the most common
  • Can also involve other Gram-positive cocci and Gram-negative bacilli (including SPICE organisms and Pseudomonas aeruginosa)
• Esophageal rupture: Viridans group streptococci, Neisseria, Haemophilus, Prevotella, Fusobacterium
• Descending head and neck infections: streptococci, Staphylococcus aureus, Peptostreptococcus, Haemophilus influenzae, Pseudomonas aeruginosa, Escherichia coli, Moraxella catarrhalis, Fusobacterium, Prevotella, Actinomyces
• Can also occur from contiguous extension of pneumonia, pancreatitis, subphrenic abscess, or epidural abscess, with the microbiology determined by the source of infection

Etiologies

• Acute (discussed herein)
  • Complication of cardiovascular or thoracic surgery with median sternotomy (deep sternal wound infection [DSWI])
  • Esophageal perforation
    • Iatrogenic, from a procedure
    • Swallowed foreign body
    • Trauma (penetration or blunt)
    • Other, including esophageal carcinoma, emesis (Boerhaave syndrome), cricoid pressure from induction, heavy lifting, defecation, parturition, ingestion of caustic or corrosive liquids
  • Continguous spread of infection
    • Head and neck infections, including odontogenic infections, Ludwig angina, pharyngitis, tonsillitis, parotitis, epiglottitis, Lemierre syndrome
    • Lymph nodes, from anthrax or tuberculosis
    • Pneumonia, empyema, subphrenic abscess, pancreatitis, cellulitis, osteomyelitis, or hematogenous spread
• Chronic (fibrosing/sclerosing/granulomatous), caused by Histoplasma capsulatum

Epidemiology

• Post-surgical deep sternal wound infections occur after about 1% of cardiac surgeries
  • Incidence increases with the complexity of the operation, and is highest with CABG = valve replacement or aortic surgery
  • Risk increases with BMI, diabetes, and number of platelet transfusions
• Complicates less than 0.5% of upper endoscopies

Clinical Manifestations

Deep Sternal Wound Infection

• Usually presents within 30 days of surgery involving thoracotomy
• Fever, chest pain, drainage from surgical site
• See CDC definition, below

Prognosis

• Mortality is about 15% in the short term, but is also higher than those without infection in the years following surgery
• Mortality is higher when Gram-negatives are isolated

Investigations

• CT chest can show evidence, including sternal wire displacement, sternal disruption, free gas, or fluid collections
• Surgical exploration, with fluid sent for culture

Diagnosis

CDC Definition of DSWI

• One of the following:
  • Organisms cultured from mediastinal tissue or fluid
  • Gross anatomical or histopathologic evidence of mediastinitis
  • Fever, chest pain, or sternal instability
• Plus one of the following:
  • Purulence from the mediastinal area
  • Mediastinal widening on imaging

Management

DSWI

• In general, surgical exploration with debridement should always be performed as quickly as possible
• Empiric antibiotics should be broad, including coverage for Staphylococcus aureus, coagulase-negative staphylococci, and resistant Gram-negatives
  • Piperacillin-tazobactam plus vancomycin is a reasonable choice
• Duration of therapy unclear but depends on adequacy of source control, presence of sternal osteitis, and presence of metal or other foreign bodies
  • Probably somewhere between 3 and 12 weeks
    • Usual duration 14 to 21 days
    • As long as 6 weeks when foreign bodies (such as sternotomy wires) are present
  • Following sternal resection and pectoral flap: maybe 2 to 3 weeks
  • After debridement without resection, or with residual bone infection: probably 4 to 6 weeks

Esophageal Perforation

• Empiric antibiotics should target upper GI pathogens, including anaerobes
  • Piperacillin-tazobactam or ceftriaxone plus metronidazole
  • Consider empiric antifungals for more complicated infections
    • Risk factors include septic shock, female, prior upper GI surgery, and recent antimicrobials
• Continue for at least 7 days after drainage

Descending Head and Neck Infections

• Empiric antibiotics to cover oropharyngeal pathogens including anaerobes
  • Ceftriaxone plus metronidazole

Further Reading

• Pastene B, Cassir N, Tankel J, Einav S, Fournier P-E, Thomas P, Leone M. Mediastinitis in the intensive care unit patient: a narrative review. Clin Microbiol Infect. 2020;26(1):26-34. doi: 10.1016/j.cmi.2019.07.005