Necrotising fasciitis

An infection of the deep soft tissues, that results in progressive destruction of the muscle fascia and overlying subcutaneous fat

Predisposing factors (20% of cases occur in patients with none of these factors)

- Older age
- Obesity
- Advanced diabetes
- Alcoholism
- Cirrhosis
- Chronic debilitation
- Vasculopathy
- IVDU
- Immunosuppression
- Malignancy
- Chemotherapy
- HTN
- COPD
- ESKD
- CCF
- Perianal abscess
- Perforated viscus
- Recent surgery

Pathophysiology

- Rapidly progressive inflammatory infection of deep fascia
- Bacterial exotoxins such as haemolysin, streptolysis and leucocidin
- Infection typically spreads along the muscle fascia due to its relatively poor blood supply, but muscle tissue itself is frequently spared
- There is associated thrombosis of dermal vessels leading to secondary necrosis of overlying subcutaneous tissues
- Skin ischaemia, anaesthesia and necrosis eventually appears, but the skin can appear normal even with quite severe underlying infection
- Results in liquefactive necrosis
- Sepsis from combination of exotoxins, toxic shock syndrome and massive cytokine response

Classification

- Type I (polymicrobial - both aerobic and anaerobic bacteria - Fournier's)
  - Usually seen in elderly or those with underlying illness (diabetes, vascular disease, pressure ulcer, episiotomy, haemorrhoid/fissure)
  - Usually presents with gas in tissue
  - Typically seen with an anaerobic species (Bacteroides, Clostridium, or Peptostreptococcus) in combination with Enterobacteriacaeae (E coli, Enterobacter, Klebsiella, Proteus) and one or more facultative anaerobic streptococci (although not commonly group A strep)
  - The facultative anaerobes lower the local oxygen tension, allowing anaerobic proliferation
  - The anaerobes produce toxins and inhibit host phagocytosis
  - Marked leukaemoid reactions with Clostridial infection
- Type II (monomicrobial)
  - Usually Group A strep (GAS) or Staph aureus
  - Can occur in any age group and without underlying comorbidities
  - Half have a clear portal of entry and half do not (theorised to be haematogenous translocation from the throat to a site of blunt trauma or muscle strain)
  - 'M protein' is an important virulence determinant of GAS - M versions 1 and 3 are associated with streptococcal toxic shock syndrome, where pyrogenic exotoxins are produced, increasing cytokine production and contributing to shock
  - Usually seen with CRP >200, modestly increased WCC with marked left shift, and AKI in the absence of hypotension
- Type III (water-borne bacteria)
  - Vibrio, Aeromonas
  - Cirrhosis and eating contaminated oysters are risk factors
- Type IV (Candida and other fungi)

Differential diagnosis

- Necrotizing cellulitis
  - Typically caused by anaerobes
  - Clostridial vs non-clostridial
    - Crepitus observed in both types
    - Sparing of fascia and muscles
  - Relatively mild systemically
- Cellulitis
  - Normal CK/AST
  - Not usually septic, apart from low-grade
- Pyoderma gangrenosum
  - IBD patients
  - No cellulitis
  - Violaceous ulcer edge
  - Less sepsis
  - Worsens with surgery
  - Normal fascial planes
  - Responds to steroids, does not respond to antibiotics
- Pyomyositis (abscesses) or necrotizing myositis (gangrene)
  - An infection of skeletal muscle typically caused by GAS and other beta-haemolytic streptococci
  - Preceded by skin abrasions, blunt trauma, or heavy exercise
  - Separate to clostridial myonecrosis
- DVT

Diagnosis

- "Necrotising fasciitis is a surgical diagnosis characterised by friability of the superficial fascia, dishwater-grey exudate, and a notable absence of pus."
- Classically:
  - Oedema (75%)
  - Erythema (72%)
  - Severe pain (72%)
  - Tenderness (68%)
  - Fever (60%)
  - Skin bullae or necrosis (38%)
- Differentiating from cellulitis:
  - Recent surgery
  - Pain out of proportion to clinical signs
  - Hypotension
  - Skin necrosis
  - Haemorrhagic bullae
  - Elevated CK and AST suggest injury to deeper tissue
- Scoring systems
  - LRINEC
    - LRINEC: high specificity for severe disease, but PPV is 57-92% with a score of 5.8 or higher
- Bedside exploration
  - LA
  - Area of greatest oedema/necrosis
  - Thin dishwasher-fluid like exudate, not frank pus
  - Non-contractile muscles
  - Send for urgent MCS and histo

Clinical approach

- Indications for immediate exploration in theatre:
  - Crepitus or gas in tissue on imaging
  - Skin discolouration or necrosis
  - Thin, foul-smelling wound discharge
  - Rapid progression clinically
  - Severe pain out of proportion to skin findings
- Suspicious for NF with systemic signs, but not convincing: bedside cut-down, then LRINEC score, imaging and observation/medical management
- Possible diagnosis, but systemically well: LRINEC score, imaging and observation/medical management

Resuscitation

- Blood cultures first
- Bacterial haemolysis often occurs, meaning haematocrit drops before Hb. Base transfusion on haematocrit rather than Hb.
- Need to give a lot of IVF, and albumin along with it
- Antibiotics:
  - First-line: meropenem 1g TDS/tazocin 4.5g TDS + vancomycin 25-30mg/kg loading dose + clindamycin 600mg TDS/lincomycin 600mg TDS
  - Add ciprofloxacin 400mg TDS if there is a risk of water-borne infection
  - Severe penicillin allergy: generally still give meropenem, but 1% risk of cross-reactivity, seek ID advice. They may advise a regime of gentamicin or ciprofloxacin and metronidazole instead.
- IVIg has been shown to reduce 30-day mortality from 33% to 15% in a 2018 meta-analysis, but only in the setting of setreptococcal infection
- Droplet and contact precautions for first 24 hours, if suspicious for GAS

Surgery

- Peri-op factors
  - Faecal diversion?
  - Amputation?
  - Second-look within 48 hours
  - Early consideration of reconstruction - SSG?
  - Remember to ensure they have cross match/G+H
  - Should have second pair of eyes
- Operative debridement
  - Prep and drape widely
  - Debride widely - be brutal - need clear margins
  - Fat that glistens, muscle that twitches, skin that bleeds
  - Send tissue for Gram stain, culture and histopathology

Post-op

- Hyperbaric oxygen treatment MIGHT be useful but jury definitely still out
- If the Gram stain returns as Gram-positive rods, be concerned for Clostridial myonecrosis (gas gangrene) and have a lower threshold for subsequent amputation

Outcome

- Mortality
  - Polymicrobial infection 21%
    - Fournier's 22-40%
    - Cervical necrotizing fasciitis 22%
    - Neonatal NF - 59%
  - Monomicrobial NF 14-34%
- Worse prognostic factors:
  - WCC >30
  - Serum creatinine >177
  - Age >60
  - Streptococcal TSS
  - Clostridial infection
  - Delay in surgery >24 hours
  - Infections involving the head, neck, thorax, or abdomen