Pathology and aetiology

This condition can be classified microbiologically into 4 groups :

• type I - polymicrobial infection, 70%-80% of cases
  • also known as synergistic NF
  • typically a slow process that evolves over days
  • affects immunocompromised patients or those with an underlying abdominal pathology
  • a mixture of aerobic and anaerobic organisms can be seen, common pathogens include pseudomonas, haemolytic staphylococcus, bacteroides, coliforms (2)
  • more indolent better prognosis and easier to recognize clinically
• type II - monomicrobial, 20%-30% of cases
  • progresses more rapidly
  • approximately 50% of type II NF cases are associated with the exotoxin driven disease - toxic shock syndrome
  • generally due to gram positive organism e.g. - group A streptococci (most common), Clostridium perfringens, Staphylococcus aureus (1)
• type III
  • commoner in Asia
  • caused by gram negative organism (often marine related organism) e.g - Vibrio spp such as V. damselae and V. vulnificus
• type IV
  • caused by fungal infection
  • usually it is associated with traumatic wound and burns (1)

Some known risk factors of NF include (1,2,3):

• diabetes and other chronic medical diseases
• immunosuppressive drugs
• malnutrition
• advanced age e.g. - age >60 years
• IV drug use
• peripheral vascular disease
• obesity
• underlying malignancy

Clinicians should obtain specific history form patient to gather information about any precipitating events which might have caused NF. e.g. -

• traumatic event - surgery, IV drug use, penetrating injury
• non traumatic - soft tissue infections, burns, childbirth etc (1)

Notes:

• the underlying pathogenesis reflects the evolution of clinical signs over time:
  • subcutaneous infection spreads from either a breach in the soft tissue or haematogenous spread
    • causes erythema and swelling (mimics features occurring in a soft tissue infection)
      
      
  • pathogen then spreads along the horizontal planes
    • causes infarction of the nutrient vessels and nerves
    • and then consequent induration and disproportionate pain
      
      
  • in the final stage the infarction leads to oedematous changes in compartments, forming haemorrhagic bullae and then the appearance of gas gangrene

Reference:

• (1) Machado NO. Necrotizing fasciitis: The importance of early diagnosis, prompt surgical debridement and adjuvant therapy. North Am J Med Sci 2011; 3: 107-118
• (2) Puvanendran R, Huey JC, Pasupathy S.Necrotizing fasciitis. Can Fam Physician. 2009;55(10):981-7
• (3) Diab J et al. Necrotising fasciitis. BMJ 2020;369:m1428.