Necrotising fasciitis

Overview

Necrotising fasciitis (NF) is a rapidly progressive, life-threatening soft tissue infection spreading along fascial planes. Mortality is 20-35%, rising sharply if surgery is delayed beyond 12-24 hours.

Risk Factors

•Diabetes mellitus, obesity, immunosuppression, liver disease, malignancy, peripheral vascular disease, IVDU

•Portal of entry: surgical wound, trauma, perianal abscess, injection site - no clear portal in up to 20%

⚠️
Up to 20% of cases occur in previously healthy individuals with no identifiable risk factors - do not be falsely reassured by the absence of comorbidity.

Presentation

•Disproportionate pain - intense pain out of keeping with degree of skin change; the single most important early sign

•Rapid systemic toxicity - high fever, tachycardia, hypotension, confusion; severity disproportionate to skin findings

•Erythema, warmth, swelling - early; mimics cellulitis with poorly defined borders

•Crepitus - palpable subcutaneous gas; pathognomonic when present

•Wooden-hard texture on deep palpation - reflects fascial involvement beneath near-normal skin

•Late signs - dusky discolouration, haemorrhagic bullae, skin necrosis, anaesthesia of overlying skin (nerve destruction - ominous)

🎯
Classic triad: disproportionate pain (early) + rapid systemic deterioration + skin findings that lag behind the true extent of deep tissue destruction. If thinking 'this looks like cellulitis but the patient is far too sick' - think NF.

Investigations

🥇 First-line

•Bloods (FBC, U&E, LFTs, CRP, lactate, blood cultures, clotting, glucose) - baseline organ function, feeds LRINEC score

•Imaging of choice: CT of affected area - fascial thickening, fluid tracking along fascial planes, gas; helps delineate extent

•Plain X-ray - quick; may show subcutaneous gas (low sensitivity)

•LRINEC score - calculated from CRP, WBC, haemoglobin, sodium, creatinine, glucose; score >6 moderate risk, >8 high risk; useful adjunct, not a substitute for clinical judgement

🏆 Gold standard

•Surgical exploration ('finger test') - blunt finger dissection through small incision; lack of resistance, grey necrotic tissue, lack of bleeding confirms diagnosis with near 100% specificity

🚨
A normal CT does NOT exclude NF - sensitivity is approximately 80%. If clinical suspicion is high, proceed directly to surgical exploration regardless of imaging.

Differential Diagnosis

NF vs key differentials
FeatureNecrotising fasciitisSevere cellulitisGas gangrene
PainDisproportionate to skin findingsProportionate to erythemaSevere, very rapid onset
Systemic toxicitySevere, out of proportionMild to moderateProfound
Crepitus/gasPresent (Type I)AbsentPresent
Tissue involvedFascia and subcutaneous tissueDermisMuscle primarily
Response to antibioticsNo - surgery requiredYesNo - surgery required

Feature	Necrotising fasciitis	Severe cellulitis	Gas gangrene
Pain	Disproportionate to skin findings	Proportionate to erythema	Severe, very rapid onset
Systemic toxicity	Severe, out of proportion	Mild to moderate	Profound
Crepitus/gas	Present (Type I)	Absent	Present
Tissue involved	Fascia and subcutaneous tissue	Dermis	Muscle primarily
Response to antibiotics	No - surgery required	Yes	No - surgery required

Management

NF is a surgical emergency requiring immediate multidisciplinary involvement: surgery, ICU, microbiology, and plastic surgery. Three pillars - surgical debridement, broad-spectrum antibiotics, resuscitation - in that order of priority.

Immediate
1Aggressive IV fluid resuscitation - treat septic shock per Sepsis-6
2Urgent surgical referral - do not wait for imaging if high suspicion
3Blood cultures before antibiotics but do not delay treatment
↓
Antibiotics - empirical broad-spectrum
1Piperacillin-tazobactam - broad Gram-positive, Gram-negative, and anaerobic cover
2Clindamycin - added for anti-toxin effect (inhibits bacterial protein/toxin synthesis); essential in Group A Streptococcus
3Intraoperative tissue cultures to guide de-escalation
↓
Surgery - cornerstone of treatment
1Urgent surgical debridement: all necrotic tissue excised until healthy, bleeding tissue is reached
2Second-look operation at 24-48 hours - reassess for further spread and re-debride
3Amputation if limb salvage is impossible
4Reconstruction (skin grafting) after infection controlled
↓
ICU
1Organ support: vasopressors, renal replacement therapy if required
2Hyperbaric oxygen - adjunct in specialist centres; not first-line

💡
Clindamycin must be added empirically because it inhibits bacterial toxin production - critical in Group A Streptococcus where superantigen-mediated streptococcal toxic shock syndrome (mortality ~50%) may coexist.

Complications

•Streptococcal toxic shock syndrome - GAS superantigen-mediated; mortality approaching 50%

•Septic shock and multi-organ failure - AKI, ARDS, DIC, hepatic failure

•Amputation - when limb salvage is impossible

•Death - overall mortality 20-35%; exceeds 70% with toxic shock or delayed surgery

📌
Fournier's gangrene is NF of the perineum and genitalia - same pathophysiology and management principles, named separately due to anatomical location.