Key Facts

Necrotising fasciitis is one of the most feared surgical emergencies. Recognition and immediate surgical intervention are paramount — delays of even hours significantly increase mortality.

Epidemiology

Incidence ~0.4-1 per 100,000/year in UK. Type I more common than Type II. Fournier's gangrene (perineal necrotising fasciitis): M:F ~10:1. Mortality ~20-40% overall; >60% if surgery delayed >24 hours.

Aetiology

Type I (Polymicrobial — 70-80%):

• Mixed aerobic and anaerobic organisms (Bacteroides, Clostridium, E. coli, Streptococcus, Staphylococcus)
• Risk factors: Diabetes, immunosuppression, obesity, peripheral vascular disease, alcohol, chronic renal failure
• Often follows surgery, trauma, or spreads from perianal/genitourinary infection

Type II (Monomicrobial — 20-30%):

• Usually Group A Streptococcus (GAS) ± Staphylococcus aureus
• Can affect young, previously healthy individuals
• May follow minor trauma, varicella, NSAID use (controversial)

Pathophysiology

• Bacteria spread along fascial planes (low resistance) → thrombosis of perforating blood vessels → tissue ischaemia → further bacterial proliferation
• Toxin production (streptococcal pyrogenic exotoxins, alpha toxin) → massive inflammatory response → septic shock, multi-organ failure
• Skin initially appears normal because disease spreads beneath it — skin necrosis is a LATE sign

Early Signs (Critical to Recognise)

• Pain out of proportion to clinical findings — most important early sign
• Erythema spreading rapidly (mark the boundary to track progression)
• Swelling, warmth, tenderness out of keeping with apparent cellulitis
• Systemic toxicity: Fever, tachycardia, hypotension disproportionate to apparent infection

Late Signs

• Skin necrosis: Dusky, purple-black discolouration
• Bullae/blisters: Haemorrhagic or serous
• Crepitus: Subcutaneous gas (especially Type I) — may be palpable or visible on imaging
• Anaesthesia: Loss of sensation over involved skin (cutaneous nerve destruction)
• Dishwater grey discharge: Thin, grey, foul-smelling fluid on surgical exploration

Red Flags

• Rapidly spreading cellulitis not responding to IV antibiotics
• Pain out of proportion
• Systemic sepsis with soft tissue infection
• Crepitus or subcutaneous gas on imaging
• Skin necrosis, bullae
• LRINEC score ≥6

Bloods

• FBC: WCC (often >15 × 10⁹/L or <4 × 10⁹/L in severe sepsis)
• CRP: Markedly elevated (often >150)
• U&Es: AKI, raised creatinine
• Glucose: Often elevated (diabetes, stress response)
• Sodium: Often low (<135)
• Lactate: Elevated (tissue hypoperfusion)
• CK: May be elevated (muscle involvement)
• Blood cultures: Before antibiotics
• Clotting: DIC screen

LRINEC Score

Imaging (Do NOT Delay Surgery)

• CT: May show fascial thickening, gas tracking, fluid collections
• MRI: Most sensitive (hyperintense fascial planes on T2) — but time-consuming
• Plain X-ray: May show subcutaneous gas
• Imaging is supportive only — clinical diagnosis and early surgery are paramount

Immediate (Emergency)

1. Resuscitation: Sepsis 6 pathway — O₂, cultures, antibiotics, fluids, lactate, urine output
2. IV antibiotics: Empirical broad-spectrum:
   • Meropenem 1g IV TDS + clindamycin 600mg IV QDS (anti-toxin effect) + flucloxacillin 2g IV QDS (if MRSA not suspected)
   • Adjust based on cultures
3. Emergency surgical debridement: ALL necrotic tissue must be excised; extent often far greater than skin changes suggest
   • 'Finger test': If tissue separates easily from fascia with finger dissection, it confirms necrotising fasciitis
4. ICU referral: Organ support (ventilation, vasopressors, renal replacement)

Post-Operative

• Planned re-look at 24-48 hours: Further debridement until tissue is healthy
• Multiple returns to theatre may be needed
• IVIG: Consider for streptococcal toxic shock syndrome (controversial)
• Wound management: NPWT (VAC) → eventual reconstruction (skin grafting, flaps)
• HDU/ICU care: Multi-organ support

Referral Criteria

• Any suspected necrotising fasciitis — immediate senior surgical review
• Do NOT delay for imaging or score calculation if clinical suspicion is high

• Overall mortality: ~20-40%
• Mortality if surgery delayed >24 hours: >60%
• Fournier's gangrene: Mortality ~20-40% (higher in elderly and diabetics)
• Type II (GAS): Can be rapidly fatal in previously healthy individuals
• Survivors: May require extensive reconstruction; significant functional and psychological morbidity
• Amputation: May be necessary for limb involvement
• Long-term: Chronic wounds, disfigurement, PTSD

Type I vs Type II Necrotising Fasciitis