pathology and aetiology

Last edited 04/2020

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.