Definition and Classification

      • Necrotising fasciitis is the term used to describe progressive, fulminant bacterial infection of the subcutaneous tissue, that spreads rapidly through the fascial planes and causes extensive tissue necrosis and destruction9.
      • It can affect any part of the body, but most commonly involves the extremities, perineum or trunk10.
      • It is classified into 4 types according to causative organism (see table below)3,4,9.
      • Types I and II cause most of the necrotising fasciitis seen in the UK, whilst types III and IV are the rarest; normally only occurring in those with specific exposures, e.g. contaminated water, extensive burns or immunocompromised patients.
Type % total cases Aetiology Causative Organisms Site of Infection
I 70-80% cases Synergistic polymicrobial infection – often bowel flora derived Mixture of obligate and facultative anaerobes and aerobes e.g. Bacteroides or Peptostreptococcus withan Enterobacteriaceae or non-group A streptococcus Anywhere, but most commonly trunk/perianal region
II 20-30% cases Mono-microbial infection Usually group A β-haemolytic streptococcus, alone or in combination with staphylococcus aureus. Typically affects limbs
III Rare Gram-negative monomicrobial infection Marine organisms such as Vibrio spp. and Aeromonas hydrophila – occur following seawater contamination of wounds or ingestion of raw seafood – mortality very high Limbs, trunk or perineum
IV Rare Fungal infection Zygomycetes (after traumatic wounds or burns) or candidal infection (in immunocompromised patients) Limbs, trunk or perineum

Learning Bite

Type I and II necrotising fasciitis (NF) make up most cases of NF seen in the UK. Type I is polymicrobial infection with mixed aerobes and anaerobes, often affecting the trunk/perineum. By contrast, type 2 NF is caused mostly by group A β haemolytic streptococci and tends to affect the limbs.

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