Necrotizing fasciitis is a rapidly progressive, life-threatening soft tissue infection characterized by widespread necrosis of the subcutaneous tissue and fascia, often caused by group A Streptococcus or polymicrobial organisms.

Exam domain	One-glance essentials
Definition / diagnosis	Rapidly progressive infection primarily involving the superficial fascia with secondary skin/soft tissue necrosis due to microvascular thrombosis ^[1]^[2] *Type I = polymicrobial (post-surgical, Fournier's); Type II = monomicrobial (GAS); Type III = marine Vibrio/Aeromonas (HK high-yield); Gas gangrene = C. perfringens* ^[3]^[4] Progression from Stage 1 (pain, erythema) → Stage 2 (bullae, hyposensitivity) → Stage 3 (necrosis, anaesthesia) can occur within 24–72 hours — a surgical emergency ^[1]^[5]
Epidemiology / risks	*DM (57%), alcohol abuse, renal insufficiency, liver disease, immunocompromised, NSAIDs* are major risk factors ^[4]^[2] HK high-yield: *Vibrio vulnificus* in patients handling raw seafood / saltwater exposure with liver cirrhosis (HBV/HCV endemic) — mortality 50–70% ^[3]^[6] Type II GAS NF can affect previously healthy young adults with no comorbidities — do not rely on risk factors to exclude diagnosis ^[2]^[7]
Core mechanism	Bacteria spread along superficial fascia → release exotoxins/superantigens → thrombosis of perforating vessels → ischaemic necrosis of overlying skin ^[1]^[2] Nerve destruction along necrotic fascia explains the classic progression: *pain (Stage 1) → hyposensitivity (Stage 2) → anaesthesia (Stage 3)* ^[5]^[4] GAS superantigens trigger massive cytokine release → streptococcal toxic shock syndrome (STSS) → septic shock and MODS ^[2]^[6]
Clinical picture	Cardinal feature: *pain out of proportion to clinical findings* — the most important red flag distinguishing NF from cellulitis ^[1]^[4] Stage 2 signs: haemorrhagic bullae, crepitus, dishwater discharge, skin hyposensitivity with systemic toxicity disproportionate to skin findings ^[5]^[2] NF misdiagnosed as cellulitis in up to 50–70% of cases; failure to respond to IV antibiotics + escalating pain should trigger urgent reassessment ^[2]^[7] Fournier's gangrene: perineal NF presenting with scrotal/perineal pain, swelling, crepitus — DDx includes testicular torsion, epididymitis ^[8]^[9]
Investigations	*NF is a clinical diagnosis* — disproportionate pain, rapidly progressive skin change and systemic toxicity are key; thrombocytopenia suggests severe sepsis/DIC, not a required criterion; never delay surgery for tests ^[10]^[1] Finger probe test (bedside gold standard): *lack of bleeding, foul-smelling dishwater pus, minimal tissue resistance to finger dissection* ^[5]^[2] LRINEC score ≥ 8 = high risk (CRP, Hb, WCC, Na, Cr, glucose; max 13) — but a low score does NOT rule out NF; clinical suspicion overrides ^[2]^[11] CT preferred emergently (gas, non-enhancing fascia); XR may show *linear streak of gas* in gas gangrene; gas gangrene shows *no neutrophilia on CBC* ^[12]^[11]
Management	*Aggressive surgical debridement + IV broad-spectrum antibiotics* — antibiotics alone will NOT cure NF; surgical exploration is both diagnostic and therapeutic ^[13]^[14] *Empiric therapy must be broad* (MRSA agent + piperacillin–tazobactam/carbapenem, or ceftriaxone + metronidazole); add exposure-directed cover. Confirmed GAS or clostridial infection → penicillin + clindamycin ^[10]^[15] Add linezolid/clindamycin to suppress superantigen and exotoxin production — shuts off the toxin factory, not just kills bacteria ^[13]^[16] Amputation indications (3D): Dead, Damage, *Danger (NF, gangrene, ascending sepsis)* — unremitting shock + myonecrosis + PVD + rapidly progressive → amputation ^[2]^[17]
Complications	Sepsis → septic shock → MODS (leading cause of death); STSS from GAS superantigens; DIC (*low platelet count); rhabdomyolysis → AKI + hyperkalaemia ^[2]^[16] Amputation in 46% of HK patients; mortality 20–75%* — time to debridement is the most important modifiable prognostic factor ^[4]^[7] Amputation complications — early: *phantom limb pain* (Mx: amitriptyline/gabapentin), *skin necrosis; late: stump neuroma, osteomyelitis, fixed flexion deformity* ^[18]^[17] Late sequelae: disfiguring scars, contractures, chronic wounds, psychological morbidity (PTSD, depression); Fournier's → urethral fistula, colostomy ^[2]^[19]
Prevention / follow-up	Avoid NSAIDs in suspected soft tissue infection — mask pain/fever, may worsen NF and contribute to AKI ^[2]^[16] HK-specific: wear protective gloves when handling raw seafood; immediate wound care after saltwater/marine exposure, especially in patients with liver disease ^[3]^[6] Post-amputation rehabilitation: *BKA is the most common amputation; 90% walk again; AKA: 50% walk again* — early prosthetic fitting and physiotherapy are essential ^[18]^[19]
Exam traps	#1 exam trap: NF misdiagnosed as cellulitis — key differentiators are pain out of proportion, haemorrhagic bullae, skin anaesthesia, and systemic toxicity disproportionate to skin findings ^[2]^[7] A low LRINEC score does NOT exclude NF — clinical suspicion always overrides; never delay surgery for imaging, HBOT, or scoring ^[11]^[16] NF vs gas gangrene: fascia vs muscle involvement; NF shows leucocytosis vs *no neutrophilia* in gas gangrene; dishwater pus vs brownish liquid with gas bubbles ^[12]^[11] Pain out of proportion DDx: NF, gas gangrene, compartment syndrome, acute limb ischaemia, mesenteric ischaemia — do not anchor on one diagnosis ^[20]^[21]

References

^[1] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p39, p40, p44, p46) ^[2] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p36–46) ^[3] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p3, p36, p37, p39, p42, p44, p54) ^[4] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p36, p39, p41, p42, p44, p54) ^[5] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p39, p41, p44, p54) ^[6] Senior notes: maxim.md (NF table entries, gas gangrene, LRINEC score) ^[7] Senior notes: maxim.md (Necrotizing fasciitis table entries, LRINEC score, gas gangrene) ^[8] Senior notes: felixlai.md (Testicular torsion - DDx of scrotal pain including Fournier's gangrene) ^[9] Senior notes: felixlai.md (Fournier's gangrene reference under testicular torsion differential) ^[10] Guideline: IDSA. Diagnosis and Management of Skin and Soft Tissue Infections (2014), necrotizing fasciitis recommendations ^[11] Senior notes: maxim.md (NF table entries, LRINEC score, gas gangrene CBC/XR findings) ^[12] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p51, Gas Gangrene) ^[13] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p45, management principles/antibiotic table) ^[14] Senior notes: maxim.md (NF management — aggressive debridement + IV broad-spectrum antibiotics) ^[15] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p45, antibiotic regimens) ^[16] Senior notes: maxim.md (NF management — aggressive debridement + IV broad-spectrum antibiotics; LRINEC score; gas gangrene CBC findings) ^[17] Senior notes: maxim.md (Amputation indications, complications, rehabilitation) ^[18] Senior notes: felixlai.md (Amputation types, complications — early and late) ^[19] Senior notes: maxim.md (Amputation indications, principles, complications, rehabilitation) ^[20] Senior notes: felixlai.md (Rhabdomyolysis mechanism and management; compartment syndrome) ^[21] Senior notes: felixlai.md (Acute limb ischaemia 6Ps, DVT/phlegmasia cerulea dolens, compartment syndrome)

Necrotizing Fasciitis

Necrotizing fasciitis (NF) — let's break the name down:

"Necrotizing" (Greek nekros = death) → causing tissue death
"Fasciitis" (fascia = band/sheet of connective tissue + -itis = inflammation) → inflammation of the fascia

So the name literally tells you: an infection that causes inflammation and necrosis of the fascia.

More precisely, NF is a rapidly progressive, life-threatening soft tissue infection that primarily involves the superficial fascia ^[1], with secondary necrosis of the overlying subcutaneous fat and skin due to thrombosis of nutrient blood vessels running through the fascial planes. It can extend to involve deep fascia and muscle (myonecrosis). The hallmark is that the degree of deep tissue destruction is far greater than what the overlying skin initially suggests — hence the classic teaching point of "pain out of proportion to clinical findings" ^[2]^[3].

Key Concept

Necrotizing fasciitis is not primarily a skin infection — it is a fascial infection. The skin changes you see are secondary to vascular compromise in the fascial plane beneath. This is why early clinical findings can be deceptively mild ("the tip of the iceberg"), and why surgical exploration is the definitive diagnostic and therapeutic manoeuvre.

When NF involves the male perineum, it is called Fournier's gangrene ^[2] (named after Jean-Alfred Fournier, a French venereologist who described it in 1883).

2. Epidemiology

Overall incidence: approximately 0.3–5 per 100,000 population per year globally
Higher in tropical/subtropical climates (relevant to Hong Kong) due to warm, humid environments favouring Vibrio species and Aeromonas species
In Hong Kong specifically, Vibrio vulnificus NF is a particular concern due to high seafood consumption and marine/freshwater exposure ^[1]^[4]

Can affect any age, but more common in adults > 50 years
Male-to-female ratio approximately 2–3:1
Fournier's gangrene has an even stronger male predominance

Despite modern treatment, mortality remains 20–40%, and higher (up to 70–80%) with delayed surgical intervention, multiorgan failure, or extensive truncal involvement
Mortality is directly related to time from symptom onset to first surgical debridement — every hour of delay increases mortality

HK sees a disproportionate number of Vibrio vulnificus cases compared to Western populations, particularly in:
- Fishermen, fishmongers, and individuals handling raw seafood
- Patients with underlying liver disease (hepatitis B/C-related cirrhosis is highly prevalent in HK) — V. vulnificus has a particular tropism for iron-overloaded states and cirrhotic patients
- Exposure to seawater or freshwater wounds ^[4]

Risk factors for developing necrotizing fasciitis ^[1]:

Risk Factor	Mechanism / Why It Matters
Diabetes mellitus (present in 57% of patients) ^[1]	Microangiopathy impairs tissue perfusion and immune cell delivery; hyperglycaemia impairs neutrophil chemotaxis, phagocytosis, and oxidative burst; peripheral neuropathy delays symptom recognition
Alcohol abuse ^[1]	Impaired hepatic function → decreased complement and opsonin production; malnutrition; immune suppression
Renal insufficiency ^[1]	Uraemia impairs T-cell and neutrophil function; fluid overload may impair tissue perfusion
Liver diseases ^[1]	Decreased synthesis of complement, acute-phase reactants, and opsonins; portal hypertension → iron overload → V. vulnificus thrives in high-iron environments; HK context: HBV/HCV cirrhosis
Immunocompromised ^[1]	Any cause: HIV, malignancy, chemotherapy, chronic steroids, transplant recipients — impaired ability to contain local infection
Exposure to NSAIDs ^[1]	Inhibit prostaglandins which alter the inflammatory response to microorganisms ^[1]; may mask early symptoms (fever, pain) causing delayed presentation; Develop acute renal failure ^[1] which further impairs immune function
Intra-abdominal/gynaecological/perineal surgery ^[4]	Provides portal of entry for enteric organisms; disruption of mucosal barriers
IVDU (intravenous drug use) ^[2]	Non-sterile injection → direct inoculation of bacteria into soft tissue; often polymicrobial
Peripheral vascular disease	Impaired tissue oxygenation → anaerobic bacteria thrive; poor wound healing
Obesity	Poorly vascularised subcutaneous fat provides a favourable environment for bacterial proliferation

NSAID Warning

A common exam pitfall: NSAIDs are frequently prescribed for early NF that is misdiagnosed as cellulitis. They do two harmful things: (1) mask pain and fever → delayed diagnosis, and (2) inhibit prostaglandins altering the inflammatory response + cause acute renal failure ^[1]. Always consider NF in a patient with "cellulitis" not responding to antibiotics, especially if they are on NSAIDs.

4. Anatomy and Function

Understanding the anatomy is essential to understanding why NF behaves the way it does.

Layers of soft tissue and associated disease ^[1]:

Layer	Associated Infection
Epidermis	Impetigo
Dermis	Erysipelas, Folliculitis, Furunculosis
Superficial fatty fascia	—
Deep fascia	Necrotizing fasciitis
Perimysium / Muscle	Myonecrosis

Superficial fascia (Scarpa's fascia in the abdomen, Colles' fascia in the perineum): a loose areolar tissue layer between subcutaneous fat and deep fascia. It is relatively avascular and has limited immune surveillance — making it an ideal highway for bacterial spread.
Deep fascia: a dense, tough layer investing muscles (e.g., fascia lata in the thigh). NF primarily spreads along the superficial fascial plane, often between subcutaneous fat and the deep fascia.
Blood supply: The overlying skin receives its blood supply from perforating vessels that travel through the fascia. When bacteria destroy the fascia, these perforating vessels undergo thrombotic occlusion → ischaemic necrosis of the overlying skin. This explains why:
- Early skin changes are minimal despite extensive fascial necrosis below
- Late skin changes include dusky discolouration, haemorrhagic bullae, and frank gangrene
Nerve supply: Cutaneous sensory nerves also traverse the fascia. Their destruction causes the paradoxical progression from disproportionate pain → hyposensitivity → anaesthesia ^[1] as the disease advances.

Extremities (most common overall, especially lower limbs) — the leg and thigh are classic
Perineum (Fournier's gangrene)
Abdominal wall (post-surgical)
Head and neck (odontogenic infections, particularly in diabetic patients)

5. Etiology (Focus on Hong Kong)

NF is classically divided into types based on the causative organisms:

Type	Organisms	Typical Setting / Portal of Entry	HK Relevance
Type I (Polymicrobial / Synergistic)	Polymicrobial: Enterobacteriaceae, streptococci, anaerobes ^[4]	Following intra-abdominal, gynaecological or perineal surgery ^[4]; Fournier's gangrene; diabetic foot	Very common — high rates of DM and abdominal surgery
Type II (Monomicrobial)	Group A Streptococcus (S. pyogenes) ^[1]^[3]; occasionally other streptococci, S. aureus (including MRSA)	Following cuts and abrasion; recent chickenpox; IVDU; healthy adults ^[4]	Common; can affect young, previously healthy individuals
Type III (Marine Vibrio / Gas gangrene)	Vibrio vulnificus, Vibrio species ^[1]^[3]; Aeromonas hydrophila, A. caviae	Following exposure to freshwater, seawater or seafood ^[4]; contact with marine environment	Highly relevant in Hong Kong — seafood culture, subtropical waters
Gas gangrene (sometimes classified separately)	Most commonly caused by C. perfringens ^[5]	Recent trauma / GI surgery ^[3]	Post-traumatic, post-surgical

Hong Kong High Yield

In Hong Kong exams, always think of Vibrio vulnificus when the vignette mentions:

Handling raw seafood / shellfish
Saltwater or brackish water exposure
Underlying liver cirrhosis (HBV/HCV endemic area)

V. vulnificus NF in a cirrhotic patient has mortality approaching 50–70% — it is a true emergency.

Scenario	Likely Organisms	First-Line Antibiotics	Alternative
Following exposure to freshwater, seawater or seafood	Aeromonas hydrophila, A. caviae; Vibrio vulnificus	IV fluoroquinolone + IV amoxicillin-clavulanate ^[4]	—
Following cuts/abrasion; recent chickenpox; IVDU; healthy adults	Group A streptococci	IV penicillin G + IV linezolid ^[4]	—
Following intra-abdominal/gynaecological/perineal surgery	Polymicrobial: Enterobacteriaceae, streptococci, anaerobes	IV imipenem or IV meropenem ^[4]	IV amoxicillin-clavulanate + IV levofloxacin ^[4]

Why linezolid with penicillin G for GAS? Penicillin kills the bacteria, but in high-inoculum infections, many organisms are in stationary phase (Eagle effect) where penicillin (which requires active cell wall synthesis) is less effective. Linezolid (a protein synthesis inhibitor) directly suppresses production of streptococcal superantigens and exotoxins — it acts as a "toxin shut-off valve."

6. Pathophysiology

Pathogenesis ^[1]:

Let me walk through this step by step:

Step 1 — Portal of Entry ^[1]

A skin or mucosal breach — this can be as trivial as an insect bite, needle puncture, laceration, or surgical wound ^[1]
In some cases (especially GAS NF), no identifiable portal of entry is found — haematogenous seeding to a site of blunt trauma is postulated

Step 2 — Bacterial Proliferation in the Fascial Plane

The superficial fascia is relatively hypovascular → limited immune surveillance
Bacteria proliferate and produce:
- Hyaluronidase ("spreading factor") — breaks down hyaluronic acid in connective tissue, facilitating spread along fascial planes
- Lipases — destroy subcutaneous fat
- Streptococcal pyrogenic exotoxins (SPE) — act as superantigens, triggering massive T-cell activation and cytokine storm (TNF-α, IL-1, IL-6)
- Streptolysin O and S — directly lyse host cells
- C. perfringens produces alpha-toxin (lecithinase/phospholipase C) — destroys cell membranes, causing myonecrosis
- V. vulnificus produces metalloproteinases and cytolysins — particularly virulent in iron-overloaded states (cirrhosis)

Step 3 — Vascular Endothelial Damage ^[1]

Enzymes and exotoxins damage vascular endothelium ^[1]
This triggers the coagulation cascade locally

Step 4 — Microvascular Thrombosis ^[1]

Platelet-leukocyte aggregates occlude capillaries ^[1]
Progressive thrombosis of perforating vessels → tissue ischaemia
This creates a vicious cycle: ischaemia → anaerobic environment → anaerobic bacteria thrive → more tissue destruction

Step 5 — Tissue Necrosis ^[1]

Deeper venules and arterioles are occluded ^[1]
Necrosis affects all tissue layers ^[1]
Overlying skin undergoes ischaemic necrosis → bullae formation → haemorrhagic bullae → frank gangrene
Nerve destruction → anaesthesia (the transition from severe pain to numbness is an ominous sign)

Step 6 — Systemic Response

Bacterial toxins and necrotic tissue products enter the systemic circulation
SIRS → Sepsis → Septic shock → Multiorgan dysfunction syndrome (MODS)
Streptococcal toxic shock syndrome (STSS) can occur with GAS infections

Why Pain Out of Proportion?

In the early stage, bacteria are destroying fascia and occluding small vessels, causing tissue ischaemia — but the overlying skin still looks relatively normal. The ischaemic fascia and subcutaneous tissue generate intense pain via nociceptor stimulation and local inflammatory mediators, yet the skin shows only mild erythema or swelling. This mismatch between symptoms (severe pain) and signs (minimal skin changes) is the hallmark "pain out of proportion to clinical findings" ^[3]^[2].

Gas gangrene ^[5]:

Most commonly caused by C. perfringens ^[5]
Rapid clinical onset, crepitus is characteristic ^[5]
The alpha-toxin (phospholipase C) cleaves lecithin in cell membranes → massive cell lysis → myonecrosis
C. perfringens inhibits the inflammatory response → no neutrophilia on CBC ^[3] — this is a distinctive feature
Gas production occurs via fermentation of tissue glucose and amino acids → crepitus and gas on X-ray ^[3]
Sudden onset of pain, rapidly progressive soft tissue infection, development of blisters containing foul smelling brownish liquid with gas bubbles, soft tissue induration and discolouration may also be present ^[5]

7. Classification

Classification	Synonyms	Organisms	Key Features
Type I	Polymicrobial / Synergistic necrotizing fasciitis	Mixed aerobic + anaerobic: Enterobacteriaceae, Streptococcus spp., Bacteroides, Clostridium, Peptostreptococcus	Most common type (55–75%); typically post-surgical or perineal; Fournier's gangrene; gas formation common
Type II	Monomicrobial	Group A Streptococcus (most common monomicrobial cause); S. aureus (including MRSA)	Can occur in young, healthy patients; associated with streptococcal toxic shock syndrome
Type III	Marine / Vibrio	Vibrio vulnificus, Aeromonas hydrophila	Seawater/freshwater/seafood exposure; fulminant in liver disease patients; very relevant in Hong Kong
Gas gangrene	Clostridial myonecrosis	Clostridium perfringens (most common), C. septicum	Post-traumatic; rapid onset; crepitus; absent neutrophilia

Location	Special Name	Common Organisms	Notes
Perineum / genitalia	Fournier's gangrene	Polymicrobial (Type I)	Often from anorectal or urogenital source; rapidly fatal if not debrided
Head and neck	Cervical necrotizing fasciitis	Mixed aerobic-anaerobic; often odontogenic	Dental infection is the most common source
Abdominal wall	Post-surgical NF	Polymicrobial	After abdominal/gynaecological surgery
Extremities	NF of extremity	Any type	Most common overall location; lower limb > upper limb

Stage	Clinical Features
Stage 1 (Early)	Tenderness, Erythema, Oedema, Warm skin, Fever ^[1]
Stage 2 (Intermediate)	Blisters and bullae formation, Hyposensitivity, Tissue crepitation, Haemorrhagic bullae ^[1]
Stage 3 (Late)	Tissue necrosis, Anaesthesia ^[1] — "the skin has died"

Clinical Staging — High Yield

The progression from Stage 1 to Stage 3 can occur within 24–72 hours. This is why NF is a surgical emergency. By Stage 3, the patient is often septic with multiorgan failure. The transition from pain (Stage 1) → hyposensitivity (Stage 2) → anaesthesia (Stage 3) reflects progressive destruction of cutaneous nerves traversing the necrotic fascia ^[1].

8. Clinical Features

Symptom	Pathophysiological Basis
Disproportionate pain ("pain out of proportion to clinical findings") ^[1]^[2]^[3]	Fascial ischaemia and necrosis stimulate deep nociceptors intensely, but overlying skin initially appears relatively normal because skin necrosis lags behind fascial destruction
Rapid progression of symptoms (hours)	Bacteria spread rapidly along relatively avascular fascial planes with minimal immune resistance; enzymatic tissue destruction (hyaluronidase, lipases) facilitates spread
History of minor trauma to extremities ^[1]	Even trivial wounds (insect bite, needle puncture, laceration) serve as the portal of entry ^[1]
Systemic toxicity (fever, rigors, malaise, confusion)	Bacterial toxins (superantigens, endotoxins) and necrotic tissue products trigger SIRS → sepsis → septic shock
Progression from pain to numbness	Cutaneous nerve destruction from fascial necrosis → transition from pain to anaesthesia (ominous sign indicating Stage 3)

Sign	Stage	Pathophysiological Basis
Tenderness ^[1]	Stage 1	Local inflammatory mediators and tissue ischaemia stimulate nociceptors
Erythema ^[1]	Stage 1	Vasodilation from local inflammatory response (prostaglandins, histamine)
Oedema / Swelling ^[1]	Stage 1	Increased vascular permeability from endothelial damage → interstitial fluid accumulation
Warm skin ^[1]	Stage 1	Increased blood flow from inflammatory vasodilation
Fever ^[1]	Stage 1	Pyrogenic cytokines (IL-1, IL-6, TNF-α) act on hypothalamic thermoregulatory centre
Blisters and bullae formation ^[1]	Stage 2	Subepidermal oedema from microvascular thrombosis and vascular leak → fluid accumulation separates epidermis from dermis
Haemorrhagic bullae ^[1]^[3]	Stage 2	Thrombosis of dermal vessels → red blood cell extravasation into bullae; a hallmark sign
Hyposensitivity ^[1]	Stage 2	Early cutaneous nerve ischaemia from fascial vessel thrombosis
Tissue crepitation ^[1]	Stage 2	Gas production by bacteria (especially Clostridium spp., mixed anaerobes) through fermentation of tissue substrates; palpable as a crackling sensation
Tissue necrosis (dusky, grey-black skin) ^[1]	Stage 3	Complete thrombotic occlusion of all nutrient vessels → full-thickness tissue death
Anaesthesia ^[1]	Stage 3	Complete destruction of cutaneous sensory nerves — an ominous sign indicating irreversible tissue death
Generalised erythematous rash and toxic appearance ^[1]	Stage 2–3	Streptococcal superantigens trigger widespread immune activation (toxic shock syndrome)
Low platelet count ^[1]	Any	DIC from systemic activation of coagulation; platelet consumption in microvascular thrombi
Dirty "dishwater" discharge ^[3]	Stage 2–3	Necrotic fascial tissue produces thin, grey, foul-smelling exudate (not frank pus — because the immune cells cannot reach the necrotic tissue due to vascular occlusion)
Skin changes extending beyond apparent margins of erythema	Any	Fascial necrosis extends far beyond what is visible on the skin surface — "the iceberg sign"
Rapid extension of erythema borders (can mark with pen and observe)	Any	Unimpeded bacterial spread along fascial planes
Systemic signs: tachycardia, hypotension, confusion, oliguria	Late	Sepsis → septic shock → multiorgan dysfunction

Bedside procedure — the Finger Probe Test ^[1]^[3]:

Performed at the bedside or in the operating theatre
A small incision (2 cm) is made down to deep fascia under local anaesthesia
Positive if: minimal resistance to blunt dissection ^[3] — a finger or blunt probe passes easily along the fascial plane with no resistance, and the tissue does not bleed (avascular necrotic fascia)
Lack of bleeding, foul smelling dishwater pus, minimal tissue resistance to finger dissection ^[1]
This test is both diagnostic (confirms NF) and should prompt immediate surgical debridement

The Finger Probe Test

If you suspect NF but aren't sure, the finger probe test can be done at the bedside. Normal fascia is adherent and vascular — you cannot easily separate it from overlying tissue. In NF, the necrotic fascia separates easily with no bleeding and releases grey, foul-smelling ("dishwater") fluid. A positive test = go to theatre NOW.

Feature	Cellulitis	Necrotizing Fasciitis
Pain	Proportionate to skin changes	Disproportionate — far worse than skin appearance suggests
Skin colour	Bright red, well-demarcated	Dusky, grey, purplish; may have patchy areas
Bullae	Absent	Present (serous → haemorrhagic)
Crepitus	Absent	May be present
Systemic toxicity	Mild–moderate	Severe, out of proportion
Skin anaesthesia	Absent	Present (ominous)
Response to IV antibiotics	Improves within 24–48 hours	Does not improve or worsens despite antibiotics
Margins	Well-defined, advancing border	Ill-defined, spreading beyond visible erythema

Must-Know Exam Point

Cellulitis that fails to respond to IV antibiotics within 24–48 hours should raise suspicion for NF. Do not keep re-dosing antibiotics hoping the patient will improve. Consider surgical exploration/finger probe test early. The single most important determinant of survival is time to first debridement.

NF of the male perineum, scrotum, and penis
Can also occur in females (vulva, perineum) but much rarer
Source is typically:
- Anorectal (perianal abscess, anorectal surgery)
- Urogenital (urethral instrumentation, indwelling catheter)
- Cutaneous (scrotal skin infection)
Polymicrobial (Type I) in most cases
Rapidly fatal — the perineal fascial planes communicate widely (Colles' fascia → Scarpa's fascia → anterior abdominal wall)

Factors favouring limb salvage vs amputation ^[1]:

Limb Salvage Surgery ^[1]	Amputation ^[1]
Good past health	Concurrent medical disease with high anaesthetic risk from multiple operations (e.g., poorly controlled diabetes mellitus, valvular heart disease)
Not life-threatening state	Myonecrosis
Multiple sites (can debride serially)	Unremitting shock
Responsive to inotropic support	Concurrent peripheral vascular insufficiency
	Rapidly progressive infection
	Large area of tissue necrosis (heel pad and sole skin loss)

Amputation Decision

The decision to amputate is essentially a life over limb decision. When the patient is in unremitting shock, has myonecrosis, peripheral vascular disease preventing healing, or the infection is spreading faster than you can debride — amputation removes the source of sepsis and saves the patient's life.

Although sometimes grouped under NF, gas gangrene (clostridial myonecrosis) has distinct features worth separating:

Orthopaedic emergency ^[5]
Most commonly caused by C. perfringens ^[5]
Rapid clinical onset, crepitus is characteristic ^[5]
Sudden onset of pain, rapidly progressive soft tissue infection, development of blisters containing foul smelling brownish liquid with gas bubbles, soft tissue induration and discolouration may also be present ^[5]
No neutrophilia on CBC — inflammatory response inhibited by C. perfringens ^[3]
- Why? Alpha-toxin directly kills neutrophils and inhibits their migration
XR: linear streak of gas ^[3]
Management: Resuscitation → Urgent surgical debridement with fasciotomy → IV antibiotics ^[3]

High Yield Summary

Definition: Necrotizing fasciitis is a rapidly progressive infection primarily involving the superficial fascia with secondary skin/soft tissue necrosis due to microvascular thrombosis.

Risk Factors (exam favourite): DM (57%), alcohol abuse, renal insufficiency, liver disease, immunocompromised, NSAIDs ^[1]

Classification:

Type I = Polymicrobial (post-surgical, Fournier's)
Type II = Monomicrobial (GAS in healthy patients)
Type III = Marine Vibrio / Aeromonas (HK high yield — seafood/seawater + liver disease)
Gas gangrene = C. perfringens (no neutrophilia, gas on XR)

Cardinal Clinical Feature: Pain out of proportion to clinical findings ^[1]^[2]^[3]

Clinical Stages ^[1]:

Stage 1: Tenderness, erythema, oedema, warm skin, fever
Stage 2: Bullae, haemorrhagic bullae, hyposensitivity, crepitation
Stage 3: Tissue necrosis, anaesthesia

Key Diagnostic Test: Finger probe test — lack of bleeding, dishwater pus, minimal tissue resistance ^[1]

Scoring: LRINEC score > 8 = high risk ^[3]

Treatment Principle: Aggressive debridement + IV broad-spectrum antibiotics ^[3] — antibiotics alone will NOT cure NF

Amputation: When life-threatening — unremitting shock, myonecrosis, PVD, rapidly progressive ^[1]

Active Recall - Necrotizing Fasciitis

^[1] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p36–46) ^[2] Senior notes: felixlai.md (Fournier's gangrene reference under testicular torsion differential) ^[3] Senior notes: maxim.md (Necrotizing fasciitis table entries, LRINEC score, gas gangrene) ^[4] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p45, management principles/antibiotic table) ^[5] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p51, gas gangrene)

Differential Diagnosis of Necrotizing Fasciitis

Necrotizing fasciitis is a surgical emergency where every hour of delay to debridement increases mortality. The problem? In its early stage (Stage 1), NF can look identical to several far more common and far less dangerous conditions — particularly cellulitis. The differential diagnosis of NF is essentially the differential of "an acutely painful, red, swollen limb (or trunk/perineum) with systemic toxicity." The goal is to distinguish NF from its mimics as early as possible so you don't waste time treating cellulitis with antibiotics while the fascia is dying underneath.

As the lecture slides emphasise: early recognition and a high index of suspicion are important if life-threatening infections ^[1]. You need to be highly suspicious ^[1] — it is better to take a patient to theatre unnecessarily for exploration than to miss NF.

The key organising principle here is the layers of soft tissue and associated disease framework from the lecture slides ^[1]. Each layer of tissue has its own characteristic infection, and the DDx of NF essentially involves working through these layers:

Condition	Layer Involved	Key Clinical Features	How to Distinguish from NF	Why It Can Mimic NF
Cellulitis	Dermis ^[1]	Spreading erythema, warmth, tenderness, fever; well-demarcated borders; pain proportionate to skin changes	Responds to IV antibiotics within 24–48 h; no bullae, no crepitus, no skin necrosis; pain is proportionate	Both present with red, warm, swollen, tender skin with fever — the overlap is greatest in Stage 1 NF
Erysipelas	Dermis (upper) ^[1]	Sharply demarcated, raised, bright red "peau d'orange" plaque; usually face or lower limb; GAS	Very well-demarcated raised border (unlike NF which has ill-defined margins); superficial; responds rapidly to penicillin	Red, hot, swollen skin with fever can look like early NF
Abscess (soft tissue)	Deep dermis / subcutaneous ^[1]^[3]	Raised, tender, fluctuant nodule with central purulence; localised; S. aureus	Localised and well-circumscribed; fluctuant (not woody hard); no disproportionate pain; no systemic toxicity (unless very large)	Can be the portal of entry for NF; a deep abscess with surrounding induration may progress to NF
Gas gangrene (Clostridial myonecrosis) ^[5]	Perimysium / Muscle ^[1]	Most commonly caused by C. perfringens ^[5]; rapid clinical onset, crepitus is characteristic ^[5]; blisters containing foul smelling brownish liquid with gas bubbles ^[5]; no neutrophilia ^[3] (alpha-toxin kills neutrophils)	Crepitus is more prominent and earlier; XR shows linear streak of gas ^[3]; CBC shows no neutrophilia ^[3]; muscle involvement predominates (vs. fascia in NF); often post-traumatic	Both present with pain out of proportion, rapid progression, systemic toxicity, and tissue necrosis — they exist on a spectrum and can coexist
Pyomyositis	Muscle	Deep muscular pain, fever; usually S. aureus; common in tropics and immunocompromised (HIV)	More insidious onset than NF; localised to a muscle group; no bullae or skin necrosis early; MRI shows intramuscular abscess	Deep pain with fever and toxicity can mimic NF, especially when overlying skin is secondarily involved
Infective tenosynovitis ^[1]	Tendon sheath	Kanavel signs: fusiform swelling, flexed posture, tenderness along sheath, pain with passive extension; typically hand/fingers	Follows tendon sheath anatomy (linear); fusiform swelling of finger; Kanavel signs present; localised	Can track along tendon sheaths rapidly, mimicking the fascial plane spread of NF; can progress to NF
Acute limb ischaemia ^[6]	Vascular	6Ps: Pain, Paraesthesia, Pallor, Pulseless, Perishingly cold, Paralysed ^[6]; sudden onset; cold pale limb	Pulseless (NF usually has palpable pulses until late); cold (NF skin is initially warm); no erythema (pale/mottled instead); no bullae early	Both can present with severe pain in a limb with colour changes; late NF with vascular occlusion can cause a cool, mottled limb mimicking ischaemia
DVT / Phlegmasia cerulea dolens ^[6]	Venous	Painful blue oedema ^[6]; massive swelling; cyanotic limb; may have palpable pulses (venous, not arterial occlusion)	Swelling is the dominant feature (not erythema); limb is cyanotic/blue (not red/warm initially); Doppler shows venous occlusion; venous pressure increased impairs perfusion ^[6]	Swollen, painful limb with colour changes and systemic toxicity
Compartment syndrome ^[6]	Fascial compartment (enclosed space)	Pain with passive stretch of compartment muscles; tense, wooden compartment; often post-fracture or post-ischaemia-reperfusion	Typically post-traumatic context; pain specifically with passive stretch (pathognomonic); no skin colour changes early; compartment pressure > 30 mmHg diagnostic	Severe pain "out of proportion" in a swollen limb — the most commonly confused condition with NF in post-traumatic settings
Calciphylaxis	Subcutaneous / dermal vasculature	Extremely painful, violaceous, retiform (net-like) purpura → necrotic eschar; CKD on dialysis with Ca/PO4 derangement	Chronic renal failure context; net-like livedo pattern (not spreading erythema); calcification on imaging; biopsy shows small vessel calcification	Painful skin necrosis with systemic illness in a renal patient
Warfarin skin necrosis	Dermal microvasculature	Painful purpuric skin → necrosis; occurs days 3–6 after warfarin initiation; protein C/S deficiency predisposes	Temporal relationship with warfarin initiation; well-demarcated necrotic patches; no systemic sepsis; typically fatty areas (breasts, buttocks, thighs)	Painful, rapidly progressive skin necrosis
Toxic epidermal necrolysis (TEN)	Epidermis / dermal-epidermal junction	Widespread erythema → flaccid bullae → epidermal detachment (Nikolsky sign +); drug reaction; mucosal involvement	Drug history (sulfonamides, allopurinol, anticonvulsants); mucosal involvement; Nikolsky sign; widespread and symmetrical (not localised); biopsy shows full-thickness epidermal necrosis	Widespread bullae and skin necrosis with systemic toxicity
Contact dermatitis / Burns	Epidermis/dermis	Clear exposure history; distribution matches contactant/burn; pain proportionate; no systemic toxicity unless extensive	History of exposure; distribution pattern; no fascia involved; no crepitus	Bullae and skin erythema can look alarming

When NF involves the perineum, the DDx also includes ^[2]:

Condition	How to Distinguish from Fournier's Gangrene
Testicular torsion	Acute scrotal pain; high-riding testis; absent cremasteric reflex; Doppler shows absent testicular blood flow; no skin necrosis or crepitus
Epididymitis / Orchitis	Gradual onset; tenderness localised to epididymis; Prehn sign positive (relief with elevation); positive urine culture; no skin changes
Incarcerated inguinal hernia	Groin swelling that is irreducible; bowel obstruction symptoms (vomiting, distension); hernia is palpable above the testis
Perianal / ischiorectal abscess	Localised fluctuant perianal swelling; no spreading necrosis; can be the source that progresses to Fournier's if untreated
HSP scrotal involvement	Purpuric rash on scrotum and lower limbs; joint pain; abdominal pain; IgA vasculitis; paediatric age group; no necrosis

The following features, when present, should make you think "this is NOT just cellulitis — this could be NF":

Red Flags for NF (vs. Cellulitis / Other DDx):

Pain out of proportion to clinical findings ^[1]^[3]

Haemorrhagic bullae ^[3]

Skin colour changes: dusky, grey, purplish (not bright red)

Tissue crepitation ^[1]

Rapid progression despite IV antibiotics

Systemic toxicity out of proportion — low platelet count, hypotension, confusion ^[1]

Skin anaesthesia / hyposensitivity ^[1] — cellulitis is always tender; NF progresses from pain to numbness

Dirty "dishwater" discharge ^[3] from wound or incision

Lack of response to antibiotics within 24–48 hours

Wood-hard induration of subcutaneous tissue (vs. the soft, pitting oedema of cellulitis)

"Pain out of proportion to clinical signs" is not unique to NF. It narrows the differential but doesn't clinch it. The conditions sharing this feature are:

Condition	Why Pain is Disproportionate
Necrotizing fasciitis	Deep fascial ischaemia/necrosis but minimal overlying skin changes early
Gas gangrene ^[3]^[5]	Muscle necrosis from alpha-toxin; tissue destruction outpaces visible changes
Compartment syndrome	Ischaemia of muscles within a tight fascial compartment
Acute mesenteric ischaemia (abdominal equivalent)	Bowel ischaemia but peritoneal signs lag behind
Ischaemic limb	Tissue ischaemia

Exam Pitfall — DDx of Pain Out of Proportion

When an exam vignette describes "pain out of proportion to clinical findings," do NOT immediately jump to NF. Consider the context: post-traumatic → think compartment syndrome; abdominal → think mesenteric ischaemia; limb with absent pulses → think acute limb ischaemia; limb with crepitus and no neutrophilia → think gas gangrene. The context narrows the DDx for you.

These two conditions often coexist and overlap, but there are key distinguishing features ^[3]^[5]:

Feature	Necrotizing Fasciitis	Gas Gangrene
Primary layer	Deep fascia ^[1]	Muscle (perimysium) ^[1]
Common organisms	S. pyogenes, V. vulnificus, polymicrobial ^[3]	C. perfringens ^[5]
Crepitus	May or may not be present	Characteristic ^[5] — prominent and early
WBC	Leucocytosis (often > 15,000)	No neutrophilia ^[3] (C. perfringens inhibits inflammatory response)
XR	May show soft tissue gas (inconsistent)	Linear streak of gas ^[3] — consistent finding
Discharge	Dirty "dishwater" pus ^[3]	Foul smelling brownish liquid with gas bubbles ^[5]
Bullae content	Serous → haemorrhagic	Brownish with gas bubbles
Context	Any; seafood exposure; post-surgical; immunocompromised	Recent trauma / GI surgery ^[3]
Onset	Hours to days	Rapid — often within hours ^[5]

If not treated promptly ^[1]:

Amputation — HK figure: radical debridements (amputations and disarticulations) were performed in 46% of 24 patients ^[1]
Mortality ranges from 20 to 75% ^[1]

This underscores why the DDx matters: missing NF and treating it as cellulitis leads to catastrophic outcomes.

High Yield Summary — Differential Diagnosis

The #1 DDx pitfall: NF is misdiagnosed as cellulitis in up to 50–70% of initial presentations. Key differentiators: NF has pain out of proportion, haemorrhagic bullae, skin anaesthesia, failure to respond to antibiotics, and systemic toxicity disproportionate to skin findings.

Red flags that distinguish NF from cellulitis: Pain out of proportion, haemorrhagic bullae, crepitus, dishwater discharge, skin hyposensitivity/anaesthesia, failure to improve on IV antibiotics, systemic toxicity.

"Pain out of proportion" DDx: NF, gas gangrene, compartment syndrome, acute limb ischaemia, mesenteric ischaemia.

NF vs Gas Gangrene: Fascia vs muscle; polymicrobial/GAS/Vibrio vs C. perfringens; leucocytosis vs no neutrophilia; dishwater pus vs brownish liquid with gas bubbles.

Perineal NF (Fournier's): DDx includes testicular torsion, epididymitis, incarcerated hernia, perianal abscess, HSP.

Mortality 20–75% if not treated promptly; 46% of HK patients required amputation ^[1].

Active Recall - DDx of Necrotizing Fasciitis

References

^[1] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p3, p36, p37, p39, p42, p44, p54) ^[2] Senior notes: felixlai.md (Testicular torsion - DDx of scrotal pain including Fournier's gangrene) ^[3] Senior notes: maxim.md (NF table entries, gas gangrene, LRINEC score) ^[5] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p51, Gas Gangrene) ^[6] Senior notes: felixlai.md (Acute limb ischaemia 6Ps, DVT/phlegmasia cerulea dolens, compartment syndrome)

Diagnosis of Necrotizing Fasciitis — Criteria, Algorithm & Investigations

Let me be very direct here: necrotizing fasciitis is a clinical and surgical diagnosis. No single blood test, scoring system, or imaging modality can definitively rule it in or out. The lecture slides make this clear — the slide is titled "Clinical diagnosis" ^[1], not "laboratory diagnosis" or "radiological diagnosis." The reason is simple: by the time imaging or lab results confirm NF, the fascia has been dying for hours and the patient may be unsalvageable.

That said, investigations play crucial supporting roles:

Raising suspicion when clinical features are equivocal (LRINEC score)
Assessing physiological derangement (sepsis workup)
Guiding antibiotic therapy (cultures)
Delineating extent of disease (CT/MRI — but never delay surgery for imaging)

Early recognition and a high index of suspicion are important if life-threatening infections ^[1]. Empirical antibiotics and early surgical intervention are necessary in case of life-threatening conditions such as septic arthritis, NF and gas gangrene ^[1].

Diagnostic Criteria

There is no universally accepted "diagnostic criteria" for NF the way there is for, say, rheumatic fever (Jones criteria) or SLE (ACR criteria). Instead, diagnosis rests on a combination of:

Clinical diagnosis ^[1] is based on:

Clinical Feature	Source	Explanation
History of minor trauma to extremities	^[1]	Portal of entry — even trivial (lacerations, insect bite, needle puncture) ^[1]
Disproportionate pain	^[1]^[3]	Pain out of proportion to clinical findings — the hallmark; fascial ischaemia produces severe pain while overlying skin appears deceptively normal
Generalised erythematous rash and toxic appearance	^[1]	Superantigen-mediated systemic immune activation (streptococcal toxic shock); indicates systemic disease, not just local infection
Low platelet count	^[1]	Reflects disseminated intravascular coagulation (DIC) — platelet consumption in microvascular thrombi throughout the necrotic fascial bed
Rapid progression despite IV antibiotics	—	Antibiotics cannot penetrate avascular necrotic tissue; failure to improve within 24–48 h of appropriate IV antibiotics is a red flag
Clinical stages progression ^[1]	^[1]	Stage 1 → Stage 2 → Stage 3 (tenderness/erythema → bullae/crepitus → necrosis/anaesthesia)

Bedside procedure ^[1]:

This is a critical bedside test that is both diagnostic and therapeutic (it commits you to debridement if positive).

Technique:

Under local anaesthesia, make a 2 cm incision over the area of maximal concern, down to the deep fascia
Insert a finger or blunt probe along the fascial plane

Positive result — Lack of bleeding, foul smelling dishwater pus, minimal tissue resistance to finger dissection ^[1]^[3]:

Lack of bleeding: Normal fascia is vascular; necrotic fascia has undergone thrombotic vessel occlusion → avascular → no bleeding
Foul smelling dishwater pus: Thin, grey, watery, malodorous discharge from necrotic tissue (not thick, creamy pus like an abscess — because immune cells cannot reach the avascular zone)
Minimal tissue resistance: Normal fascia is adherent and tough; necrotic fascia has lost its structural integrity → the finger passes through it with no resistance ("the tissue falls apart")

Finger probe test: positive if minimal resistance to blunt dissection ^[3]

Finger Probe Test

Think of the finger probe test as testing the structural integrity and vascularity of the fascia. Healthy fascia = tough + bleeds. Necrotic fascia = falls apart + doesn't bleed + smells terrible. If positive → you are already making your initial debridement incision. Go to theatre immediately.

The LRINEC score (Laboratory Risk Indicator for Necrotizing Fasciitis) was developed by Wong et al. (2004) to help distinguish NF from other soft tissue infections using routine blood tests ^[3].

The name breaks down: Laboratory Risk Indicator for NEcrotizing fasCiitis.

LRINEC score ^[3]:

Variable	Value	Score	Pathophysiological Rationale
C-reactive protein	≥ 150 mg/L	4	Massive acute-phase response from extensive tissue necrosis and bacterial sepsis; CRP is the single most heavily weighted variable because NF produces disproportionately high CRP
	< 150 mg/L	0
Haemoglobin (g/dL)	> 13.5	0	Haemolysis from bacterial toxins (e.g., C. perfringens alpha-toxin, streptolysins) and haemodilution from aggressive fluid resuscitation lower Hb
	11–13.5	1
	< 11	2
Total leucocyte count (×10³/μL)	< 15	0	Severe infection triggers marked leucocytosis; very high WCC ( > 25) reflects overwhelming sepsis
	15–25	1
	> 25	2
Serum sodium (mmol/L)	≥ 135	0	Hyponatraemia results from: (1) dilutional — SIADH from sepsis/stress; (2) sodium shift into necrotic cells; (3) third-spacing of fluid
	< 135	2
Serum creatinine (μmol/L)	< 141	0	Renal impairment from sepsis-induced acute kidney injury, hypovolaemia from third-spacing, and direct nephrotoxicity from myoglobin (if myonecrosis)
	≥ 141	2
Blood glucose (mg/dL)	< 180	0	Hyperglycaemia from stress response (cortisol, catecholamines) and underlying diabetes (present in 57% of NF patients ^[1]); also reflects severity of systemic inflammatory response
	≥ 180	1

Maximum score: 13

Interpretation:

LRINEC Score	Risk Category	Interpretation
≤ 5	Low risk	< 50% probability of NF; consider other diagnoses but do NOT rule out NF if clinical suspicion is high
6–7	Intermediate risk	Moderate probability; warrants close monitoring and consideration of surgical exploration
≥ 8	High risk	High risk of NF ^[3]; strong indication for surgical exploration

LRINEC Score Limitations

The LRINEC score is a screening tool, not a diagnostic gold standard. Key limitations:

Sensitivity is only ~70–80% — meaning 20–30% of true NF cases will have a LRINEC score < 8 (false negatives)
It was developed to distinguish NF from cellulitis/abscess, not from other necrotizing infections
It uses admission lab values — NF can evolve rapidly, and labs may be normal very early
A low LRINEC score does NOT rule out NF — if clinical suspicion is high, proceed to surgical exploration regardless

Bottom line: Clinical judgement trumps the LRINEC score. Never let a "low" LRINEC score stop you from operating on a patient you think has NF.

Here is the systematic approach to a patient with suspected NF:

The Algorithm in One Sentence

If you think it's NF → operate. If you're not sure → operate. If you're sure it's NOT NF → watch very closely and operate at the first sign of deterioration. The threshold for surgical exploration should be LOW. You will never be criticised for exploring a patient with suspected NF who turns out to have cellulitis. You WILL be criticised for watching a patient with NF die because you were waiting for imaging.

Investigation Modalities — Detailed Breakdown

Investigation	Key Findings in NF	Interpretation / Why
Complete Blood Count (CBC)	Leucocytosis ( > 15 × 10³/μL, often > 25); low platelet count ^[1]; anaemia (Hb < 11)	Leucocytosis = systemic infection; thrombocytopaenia = DIC from microvascular thrombosis in necrotic tissue; anaemia = haemolysis from toxins + haemodilution. Exception: Gas gangrene → no neutrophilia ^[3] because C. perfringens alpha-toxin kills neutrophils
CRP	Markedly elevated ( > 150 mg/L)	Intense acute-phase response from massive tissue necrosis; most heavily weighted LRINEC variable; serially rising CRP despite antibiotics is a red flag
Renal function (Cr, BUN)	Elevated creatinine (≥ 141 μmol/L)	Sepsis-induced AKI; hypovolaemia from third-spacing; myoglobin nephrotoxicity if myonecrosis present; also relevant for LRINEC scoring
Electrolytes	Hyponatraemia ( < 135 mmol/L)	SIADH from sepsis; sodium shifts into damaged cells; third-space fluid loss → dilutional effect; LRINEC variable
Blood glucose	Hyperglycaemia (≥ 180 mg/dL or ≥ 10 mmol/L)	Stress response + pre-existing diabetes mellitus (57% of patients) ^[1]; LRINEC variable
Coagulation (PT, aPTT, fibrinogen, D-dimer)	Prolonged PT/aPTT; low fibrinogen; elevated D-dimer	DIC — consumption of clotting factors and platelets in microvascular thrombi; elevated D-dimer from fibrinolysis of the myriad microthrombi in necrotic tissue
Lactate	Elevated ( > 2 mmol/L)	Tissue hypoperfusion from sepsis-induced vasodilation and microvascular thrombosis; anaerobic metabolism in ischaemic tissue; correlates with mortality
Arterial blood gas (ABG)	Metabolic acidosis (± respiratory compensation)	Lactic acidosis from tissue hypoperfusion; renal failure → uraemic acidosis; metabolic acidosis is a marker of severity
Creatine kinase (CK)	Markedly elevated if myonecrosis present	Muscle destruction releases CK; if CK is very high, consider concomitant myonecrosis / gas gangrene involving muscle layer
Liver function tests (LFT)	May show transaminitis, hypoalbuminaemia	Pre-existing liver disease ^[1] (HBV/HCV cirrhosis in HK context); sepsis-induced hepatic dysfunction; hypoalbuminaemia from acute phase response and capillary leak
Procalcitonin	Elevated ( > 2 ng/mL suggests severe bacterial sepsis)	More specific for bacterial infection than CRP; helps distinguish bacterial from viral/inflammatory causes; rising trend suggests uncontrolled infection

Investigation	Timing / Source	Key Points
Blood cultures (×2 sets)	Before antibiotics (ALWAYS)	Positive in 20–40% of NF cases; identifies the causative organism for targeted therapy; essential for detecting bacteraemia/sepsis
Wound swab / aspirate	From bullae fluid, wound discharge, or tissue during finger probe	Gram stain gives rapid preliminary organism identification (Gram +ve cocci in chains = streptococci; Gram +ve rods = Clostridia; Gram -ve rods = Vibrio/enterics); culture and sensitivity (C/ST) for definitive identification
Deep tissue specimens at surgery	Multiple samples from debridement	Most important microbiological specimens — superficial swabs are unreliable due to contamination; send for: Gram stain, aerobic C/ST, anaerobic C/ST, and histopathology
Histopathology of debrided tissue	Intra-operative	Definitive diagnostic confirmation — shows: necrosis of superficial fascia, polymorphonuclear infiltration, fibrinoid thrombi in blood vessels, bacterial colonisation along fascial planes; fat necrosis; absence of muscle involvement (if pure NF without myonecrosis)

Why Deep Tissue Cultures Beat Wound Swabs

Superficial wound swabs grow skin commensals and colonisers that may not represent the true causative pathogen. NF is a fascial infection — the organisms are deep in the tissue. Tissue specimens obtained at debridement give you the accurate bacteriology. Always send multiple tissue specimens from different areas of the wound, including both the advancing margin and the necrotic centre.

The critical rule: Imaging must NEVER delay surgical intervention when clinical suspicion is high. Imaging is most useful when the diagnosis is uncertain, the patient is clinically stable enough to tolerate the delay, and a negative result would genuinely change management.

Modality	Key Findings	Advantages	Limitations
Plain X-ray	Soft tissue gas (linear streaks of gas ^[3]) — seen as dark lucencies along fascial planes; soft tissue swelling	Fast, cheap, readily available; can demonstrate subcutaneous gas if present	Low sensitivity (~25% for NF overall) — gas is present in only a minority of NF cases; more reliably seen in gas gangrene (C. perfringens); absence of gas does NOT rule out NF
CT scan (with IV contrast)	Fascial thickening and enhancement; subcutaneous fat stranding; fluid tracking along fascial planes; soft tissue gas (more sensitive than XR); asymmetric fascial thickening; non-enhancing fascia (suggests avascular necrosis — this is the most specific CT finding)	Much more sensitive than XR for gas and fascial abnormalities; rapid acquisition; can delineate extent of disease and guide surgical planning; can identify concurrent abscess	Can still miss early NF; false negatives occur; should not delay surgery; IV contrast requires adequate renal function (many NF patients have AKI)
MRI	Best soft tissue resolution: fascial thickening with high T2/STIR signal (oedema); fascial non-enhancement post-gadolinium (avascular necrosis); deep fascial involvement; muscle oedema (if myonecrosis); can track extent along fascial planes	Most sensitive imaging modality for fascial pathology; excellent for delineating extent; helps surgical planning	Time-consuming (30–60 min); not always available emergently; patient may be too unstable for MRI; high sensitivity but moderate specificity (fascial oedema also seen in severe cellulitis); should never delay surgery
Ultrasound	Subcutaneous thickening; fascial oedema and fluid along fascial planes; subcutaneous air (hyperechoic foci with dirty shadowing)	Bedside, no radiation, rapid; can detect subcutaneous fluid and gas	Operator-dependent; limited by oedema/pain; cannot reliably differentiate NF from severe cellulitis; poor for deep fascial assessment

CT vs MRI — When to Use Which?

CT: preferred in the emergency setting when the diagnosis is uncertain. It is fast, widely available, and can detect gas and fascial abnormalities. A CT that shows fascial gas + non-enhancing fascia in a clinically suspicious patient is essentially diagnostic.
MRI: preferred when you have time (patient stable, diagnosis uncertain) and need to delineate the extent of disease for surgical planning. Best sensitivity for fascial pathology. But it takes too long for the acutely septic patient.
Neither: if clinical diagnosis is clear (hard signs of NF), skip imaging and go straight to theatre.

Surgical exploration is both the definitive diagnostic test and the definitive treatment. Intra-operative findings that confirm NF include:

Finding	Significance
Grey, necrotic fascia	Direct visualisation of fascial necrosis — the hallmark finding
"Dishwater" pus	Thin, grey, malodorous fluid (not thick creamy pus) — reflects the avascular, poorly immune-surveilled necrotic environment
No bleeding from fascia	Confirms vascular thrombosis of fascial vessels
Fascia separates easily from underlying muscle	Loss of normal fascial adherence from necrotic dissolution — the surgical equivalent of the finger probe test
Subcutaneous fat necrosis	Dusky, non-viable fat overlying the necrotic fascia
Extent greater than skin appearance suggests	Necrotic fascia extends well beyond the margins of overlying skin changes — confirming the "tip of the iceberg" phenomenon
Foul smell	Anaerobic bacterial metabolism produces sulphur-containing compounds (hydrogen sulphide, mercaptans)
Viable underlying muscle (in pure NF)	In pure NF without myonecrosis, the muscle beneath the deep fascia is healthy; if muscle is necrotic, consider concomitant gas gangrene/myonecrosis

Phase	Actions	Purpose
1. Clinical Assessment	History (trauma, risk factors, pain character); examination (clinical stages ^[1], disproportionate pain ^[1], haemorrhagic bullae ^[3], crepitus, anaesthesia)	Primary diagnostic method — clinical diagnosis ^[1]
2. Bedside Test	Finger probe test ^[1]^[3] — lack of bleeding, dishwater pus, minimal tissue resistance	Rapid confirmation at the bedside; positive test = go to theatre
3. Blood Tests	CBC, CRP, Na, Cr, glucose, lactate, coagulation, CK, blood cultures, ABG → calculate LRINEC score ^[3]	Risk stratification; assess physiological derangement; guide resuscitation; microbiological sampling
4. Imaging (if indicated)	XR (gas?); CT with contrast (if diagnosis uncertain and patient stable); MRI (if time allows and extent needs delineation)	Support diagnosis when equivocal; delineate extent; never delay surgery
5. Surgical Exploration	Aggressive debridement ^[3] with tissue specimens for Gram stain, C/ST (aerobic + anaerobic), histopathology	Definitive diagnosis and treatment — nothing else will save the patient

Gas gangrene: XR: linear streak of gas ^[3]; CBC: no neutrophilia ^[3] — these two findings are classic differentiators from other NF types.

High Yield Summary — Diagnosis of NF

NF is a CLINICAL DIAGNOSIS — clinical diagnosis ^[1] based on: disproportionate pain, toxic appearance, low platelet count, and clinical stage progression ^[1].
Finger probe test = bedside gold standard: lack of bleeding, foul smelling dishwater pus, minimal tissue resistance to finger dissection ^[1]^[3].
LRINEC score ≥ 8 = high risk ^[3]. Variables: CRP (4 pts), Hb (2 pts), WCC (2 pts), Na (2 pts), Cr (2 pts), glucose (1 pt). Maximum 13. But a low score does NOT rule out NF — clinical suspicion overrides.
Imaging: CT is preferred emergently (gas, non-enhancing fascia); MRI is most sensitive but time-consuming. XR can show linear streak of gas ^[3] in gas gangrene. Never delay surgery for imaging.
Surgical exploration is the definitive diagnostic AND therapeutic intervention. Send deep tissue for Gram stain, C/ST, and histopathology.
Gas gangrene clues: crepitus, no neutrophilia on CBC ^[3], linear gas on XR ^[3], foul brownish liquid with gas bubbles ^[5].

Active Recall - Diagnosis of Necrotizing Fasciitis

References

^[1] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p36, p39, p41, p42, p44, p54) ^[3] Senior notes: maxim.md (NF table entries, LRINEC score, gas gangrene CBC/XR findings) ^[5] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p51, Gas Gangrene)

Management of Necrotizing Fasciitis

Let me state the single most important management principle upfront: Aggressive debridement + IV broad-spectrum antibiotics ^[3]. That's it. Everything else — ICU support, wound care, reconstruction — is adjunctive to these two pillars. Antibiotics alone cannot cure NF because the necrotic, avascular tissue creates a pharmacological sanctuary where antibiotics cannot reach therapeutic concentrations. Surgery alone is insufficient because bacteria will have disseminated systemically. You need both, simultaneously, urgently.

Empirical antibiotics and early surgical intervention are necessary in case of life-threatening conditions such as septic arthritis, NF and gas gangrene ^[1]

The management can be conceptualised in five sequential phases:

Resuscitation (keep the patient alive)
Empirical antibiotics (start killing bacteria systemically)
Emergency surgical debridement (remove the source)
Ongoing critical care + serial debridements (control the damage)
Reconstruction and rehabilitation (restore function)

NF patients are often septic or in septic shock by the time they present. Resuscitation is not a "nice to do" — it is essential to keep the patient alive long enough to get to theatre.

Component	Details	Rationale
Airway & Breathing	Assess, secure if needed; supplemental O2; intubate if GCS decreased or impending respiratory failure	Septic encephalopathy can compromise airway; metabolic acidosis drives tachypnoea → respiratory exhaustion
Circulation	Two large-bore IV cannulae (14–16G); rapid IV crystalloid bolus (30 mL/kg in first hour for septic shock); arterial line for continuous BP monitoring	Massive third-spacing and capillary leak from systemic inflammation → hypovolaemia; vasopressors (noradrenaline first-line) if MAP < 65 mmHg despite fluids
Monitoring	Indwelling urinary catheter (target UO > 0.5 mL/kg/h); continuous ECG; SpO2; consider central venous catheter	Urine output is a real-time indicator of organ perfusion; electrolyte derangements (hyperkalaemia from tissue necrosis) can cause arrhythmias
Blood investigations	CBC, CRP, RFT, LFT, coagulation profile, lactate, glucose, blood cultures ×2, group & save/crossmatch, CK, ABG	Assess organ dysfunction; LRINEC scoring; guide resuscitation; anticipate massive transfusion if needed
Correct coagulopathy	FFP, cryoprecipitate, platelets as guided by coagulation results	DIC is common — consumption coagulopathy from microvascular thrombosis; must correct before surgery to reduce operative bleeding
Analgesia	IV opioids titrated to effect (morphine, fentanyl)	Severe pain; avoid NSAIDs — NSAIDs inhibit prostaglandins which alter the inflammatory response to microorganisms and develop acute renal failure ^[1]

NSAID Warning — Again

Do NOT give NSAIDs for pain control in suspected NF. Use IV opioids instead. NSAIDs mask the cardinal symptom (disproportionate pain ^[1]), delay diagnosis, impair immune response, and worsen renal function in an already septic patient ^[1].

Phase 2: Empirical Intravenous Antibiotics

Antibiotics must be started immediately — ideally within the first hour of recognition — and should NOT be delayed waiting for culture results. The regimen depends on the suspected source of infection.

Clinical Scenario	Likely Organisms	First-Line Regimen	Alternative
1. Following exposure to freshwater, seawater or seafood ^[4]	Aeromonas hydrophila, A. caviae; Vibrio vulnificus	IV fluoroquinolone + IV amoxicillin-clavulanate ^[4]	—
2. Following cuts and abrasion; recent chickenpox; IVDU; healthy adults ^[4]	Group A streptococci	IV penicillin G + IV linezolid ^[4]	—
3. Following intra-abdominal, gynaecological or perineal surgery ^[4]	Polymicrobial: Enterobacteriaceae, streptococci, anaerobes	IV imipenem or IV meropenem ^[4]	IV amoxicillin-clavulanate + IV levofloxacin ^[4]

Empirical antibiotic should cover Clostridia and Gram +ve cocci ^[5]
Augmentin + Flagyl ^[5]
- Augmentin (amoxicillin-clavulanate) = broad-spectrum β-lactam covering Gram +ve and some Gram -ve
- Flagyl (metronidazole) = specifically targets anaerobes including Clostridium spp. — works by forming toxic free radicals within anaerobic cells that damage DNA

Drug	Mechanism	Why Used in NF
Penicillin G	β-lactam → inhibits transpeptidase (PBP) → prevents cell wall synthesis → osmotic lysis of growing bacteria	Highly effective against GAS (S. pyogenes); excellent tissue penetration; bactericidal
Linezolid	Oxazolidinone → binds 50S ribosomal subunit → inhibits protein synthesis → bacteriostatic	Suppresses superantigen/exotoxin production by GAS — this is the key reason it is added. In high-inoculum GAS infections, many organisms are in stationary phase (Eagle effect) where penicillin alone is less effective. Linezolid shuts off toxin production even without killing the bacteria directly
Fluoroquinolone (e.g., levofloxacin, ciprofloxacin)	Inhibits DNA gyrase (topoisomerase II) and topoisomerase IV → prevents DNA replication → bactericidal	Excellent activity against Gram -ve rods including Vibrio spp. and Aeromonas; good tissue penetration; oral bioavailability allows step-down
Amoxicillin-clavulanate (Augmentin)	Amoxicillin = β-lactam (cell wall synthesis inhibitor); clavulanate = β-lactamase inhibitor (protects amoxicillin from enzymatic destruction)	Broad-spectrum coverage: Gram +ve, Gram -ve, and anaerobes; the β-lactamase inhibitor is crucial because many organisms produce β-lactamases
Imipenem / Meropenem	Carbapenems → broadest-spectrum β-lactams; resistant to most β-lactamases; cover Gram +ve, Gram -ve, and anaerobes	"Big guns" for polymicrobial NF — covers essentially everything except MRSA and some resistant pseudomonas; reserve for complex/post-surgical cases
Metronidazole (Flagyl)	Prodrug activated inside anaerobic organisms → generates cytotoxic nitroso radicals → DNA damage	Specifically targets anaerobes (Clostridium, Bacteroides); synergistic with β-lactams in polymicrobial infections
Clindamycin	Lincosamide → binds 50S ribosomal subunit → inhibits protein synthesis	Like linezolid, suppresses toxin production (both streptococcal and clostridial exotoxins); excellent tissue penetration including into abscesses; covers Gram +ve and anaerobes

Why Add a Protein Synthesis Inhibitor?

In severe GAS or clostridial NF, cell-wall-active antibiotics (penicillin, carbapenems) kill bacteria by preventing cell wall formation. But in high-inoculum infections, many bacteria are in stationary phase and not actively dividing — so cell-wall antibiotics are less effective (the "Eagle effect"). Adding a protein synthesis inhibitor (linezolid, clindamycin) directly suppresses the production of exotoxins and superantigens that are driving tissue destruction and systemic toxicity. It's not just about killing bacteria — it's about shutting off the toxin factory.

De-escalate once culture and sensitivity results are available (typically 48–72 h)
Escalate if patient deteriorating clinically despite current regimen — consider MRSA coverage (IV vancomycin), antifungal coverage (if immunosuppressed), or broader Gram -ve coverage
Duration: typically 2–4 weeks of IV antibiotics, transitioning to oral when clinically improving and wound healing

Phase 3: Emergency Surgical Debridement

This is the cornerstone of management — nothing else will save the patient. Aggressive debridement ^[3] means radical excision of all non-viable tissue.

Principle	Explanation
Timing	Early aggressive debridement ^[5] — should occur within 6–12 hours of presentation (ideally sooner); every hour of delay increases mortality
Extent	ALL necrotic fascia, subcutaneous fat, and skin must be excised back to bleeding, viable tissue; the wound margin should show: (1) healthy pink tissue, (2) active bleeding, (3) fascia adherent to underlying structures
Do not close the wound	Leave wound open with wet dressings; primary closure traps residual bacteria and necrotic tissue → guaranteed recurrence
Specimens	Send multiple deep tissue samples for Gram stain, aerobic and anaerobic C/ST, and histopathology — these guide de-escalation
Explore beyond visible margins	NF extends well beyond what the skin shows ("iceberg phenomenon"); the surgeon must explore and debride the fascial plane until normal, adherent, bleeding fascia is encountered in all directions
Mark the wound margins	Use a skin marker to draw the margins of erythema pre-operatively; compare post-operatively to assess for progression

Factors favouring limb salvage vs amputation ^[1]:

Limb Salvage Surgery ^[1]	Amputation ^[1]
Good past health	Concurrent medical disease with high anaesthetic risk from multiple operations (e.g., poorly controlled diabetes mellitus, valvular heart disease)
Not life-threatening state	Myonecrosis
Multiple sites (serial debridement feasible)	Unremitting shock
Responsive to inotropic support	Concurrent peripheral vascular insufficiency
	Rapidly progressive infection
	Large area of tissue necrosis (heel pad and sole skin loss)

The decision framework is essentially the "3D" indications for amputation ^[6]^[7]:

Dead: Ischaemia and unsalvageable ^[6]
Damage: Trauma / Burns ^[6]
Danger: Gangrene / Necrotising fasciitis / Osteomyelitis / Ascending sepsis / Malignancy ^[6]

When amputation is indicated, the surgical principles are ^[6]:

Remove all infected tissues ^[6]
Ensure adequacy of blood supply to heal the amputation ^[6]
Preserve as much length of extremity as possible since this improves patient's opportunity for rehabilitation ^[6]

Level	Indication	Functional Outcome
Digital / Ray amputation	Isolated toe/finger gangrene	Good — minimal functional loss
Transmetatarsal	Multiple toes involved	Moderate — can still walk with modified shoe
Below-knee amputation (BKA)	Most common level; aim to preserve knee joint	90% of patients will be able to walk again ^[6]; most common type of amputation performed ^[6]; energy expenditure increased by 40% (unilateral) ^[6]
Above-knee amputation (AKA)	When BKA not feasible (e.g., extensive proximal infection, fixed flexion deformity of knee)	50% of patients will be able to walk again ^[6]; energy expenditure increased by 100% (unilateral) ^[6]; heals more easily than BKA

BKA is contraindicated if there is a fixed flexion deformity of the knee — because a prosthesis cannot be fitted properly ^[7].

Life Over Limb

The amputation decision in NF is a life over limb decision. An unstable patient in unremitting shock with myonecrosis and rapidly progressive infection will die if you attempt serial limb-sparing debridements. A single definitive amputation removes the septic source and saves the patient's life. This is not a failure — it is life-saving surgery.

Amputation — HK figure: radical debridements (amputations and disarticulations) were performed in 46% of 24 patients ^[1]
Mortality ranges from 20 to 75% ^[1]

Phase 4: ICU Care and Serial Debridements

After the first debridement, NF patients require intensive care support and planned relook debridements.

Component	Details	Rationale
Haemodynamic support	Vasopressors (noradrenaline ± vasopressin), inotropes (dobutamine) if needed; invasive monitoring (arterial line, CVP)	Septic shock → vasodilation + myocardial depression; target MAP ≥ 65 mmHg
Ventilatory support	Mechanical ventilation if ARDS or respiratory failure	Sepsis → ARDS via pulmonary capillary leak; respiratory muscle fatigue from metabolic acidosis
Renal replacement therapy	CVVHDF if AKI with oliguria/anuria, refractory hyperkalaemia, acidosis, or fluid overload	Sepsis-induced AKI; myoglobin nephrotoxicity; renal insufficiency is both a risk factor ^[1] and a consequence
Blood product support	Packed RBC (Hb < 7 g/dL target for most; < 9 g/dL if cardiac disease); FFP and platelets for DIC; cryoprecipitate if fibrinogen < 1.5 g/L	Massive tissue necrosis → ongoing blood loss at wound; DIC → consumptive coagulopathy
Nutrition	Early enteral nutrition (within 24–48 h if possible); high-protein, high-calorie diet	NF creates a massive catabolic state; wound healing requires protein (collagen synthesis), calories, and micronutrients (zinc, vitamin C)
Glycaemic control	Insulin infusion targeting glucose 6–10 mmol/L	Hyperglycaemia impairs neutrophil function; DM present in 57% of patients ^[1]; tight control improves outcomes in critical illness
DVT prophylaxis	LMWH (enoxaparin) once haemostasis adequate; mechanical prophylaxis (TED stockings, pneumatic compression)	Immobilised, septic patients at very high VTE risk

Planned return to theatre every 24–48 hours for wound inspection and further debridement
Continue until:
- No further necrotic tissue is found
- Wound base is clean with healthy granulation tissue
- Tissue cultures are negative or flora improving
Typically requires 2–5 debridements (sometimes more)
Each relook should include tissue sampling for culture to monitor microbiological response

Method	Description	When to Use
Wet-to-dry dressings	Saline-soaked gauze packed into wound → dries → mechanically debrides when removed	Simple, widely available; good between early debridements
Negative-pressure wound therapy (NPWT / VAC)	Foam dressing applied to wound, sealed with adhesive, connected to continuous suction	After wound is clean and no further debridement needed; promotes granulation tissue formation, reduces oedema, removes exudate; bridges to definitive closure

Phase 5: Reconstruction and Rehabilitation

Once the wound is clean with healthy granulation tissue and infection is controlled:

Modality	Indication	Details
Split-thickness skin graft (STSG)	Clean granulating wound bed; most common method	Donor site (usually thigh); heals well over granulation tissue; may require multiple sessions for large defects
Full-thickness skin graft (FTSG)	Smaller defects; areas requiring better cosmetic outcome or durability	Limited by donor site availability
Local / regional / free flaps	Deep defects exposing bone, tendon, or joint; weight-bearing areas; Fournier's gangrene perineal reconstruction	Requires plastic surgery input; more complex but provides better coverage for critical structures
Secondary intention	Small wounds in well-vascularised areas	Slowest but simplest; acceptable for minor defects

Physiotherapy: Early mobilisation, range-of-motion exercises, strengthening; for amputees — pre-prosthetic training, gait retraining
Occupational therapy: ADL retraining, adaptive devices
Prosthetics: For amputees — temporary prosthesis by ~3 weeks, definitive prosthesis once stump matured (3–6 months) ^[7]
Psychological support: NF is devastating — body image issues, PTSD, anxiety, depression are common; psychiatric/psychological input essential

Timing	Complication	Mechanism / Notes
Early	Bleeding / haematoma	Intra-operative vessel injury; coagulopathy from DIC
	Wound infection	Residual contamination; immunosuppressed state
	Phantom limb pain ^[6]	Cortical reorganisation + peripheral nerve sprouting from transected nerves; management: reassurance, amitriptyline, gabapentin ^[7]
	Skin necrosis ^[6]	Secondary to poor perfusion of stump ^[6]; may require revision to higher level
	DVT / PE	Immobility + hypercoagulable septic state
Late	Stump ulceration ^[6]	Pressure from prosthesis on poorly vascularised skin
	Stump neuroma ^[6]	Disorganised nerve regeneration at cut nerve ends
	Osteomyelitis ^[6]	Residual bone infection or secondary contamination
	Osteophyte formation ^[6]	New bone formation at stump end — irritates overlying tissue
	Fixed flexion deformity ^[6]	Muscle imbalance around joint proximal to amputation; prevented by early physiotherapy and prone positioning

Therapy	Mechanism	Evidence & Role
Intravenous immunoglobulin (IVIG)	Pooled IgG neutralises streptococcal superantigens and exotoxins; modulates immune response	Used in GAS necrotizing fasciitis with streptococcal toxic shock syndrome; observational data suggests mortality benefit; not universally recommended; consider in severe GAS cases
Hyperbaric oxygen therapy (HBOT)	Increases tissue oxygen tension → enhances neutrophil oxidative killing; direct bactericidal effect on anaerobes (Clostridium); promotes angiogenesis and wound healing	Theoretical benefit especially in gas gangrene (anaerobic organisms); limited RCT evidence; should never delay surgical debridement; may be considered as adjunct if available
Negative-pressure wound therapy (VAC)	Discussed above — promotes granulation, removes exudate	Good evidence for wound management post-debridement; widely used

HBOT — Don't Fall for the Exam Trap

Hyperbaric oxygen is sometimes suggested as a treatment for NF, especially gas gangrene. While there is theoretical rationale (killing anaerobes, enhancing immune function), it must NEVER delay surgical debridement. It is an adjunct at best, not a substitute. If your exam asks "what is the most important treatment for NF?" the answer is always aggressive surgical debridement, not HBOT.

Phase	Key Actions	Timing
1. Resuscitation	ABC, IV fluids, vasopressors, monitoring, bloods, crossmatch	Immediately on recognition
2. Empirical antibiotics	Scenario-based regimen (see table above); start within 1 hour	Within 1 hour
3. Emergency debridement	Early aggressive debridement +/- amputation ^[5]; radical excision of all necrotic tissue	Within 6–12 hours (sooner is better)
4. ICU + Serial relook	Organ support; return to theatre every 24–48 h; adjust antibiotics based on C/ST	Ongoing days–weeks
5. Reconstruction	STSG, flaps, VAC therapy; rehabilitation	Weeks–months

High Yield Summary — Management of NF

Two pillars: Aggressive debridement + IV broad-spectrum antibiotics ^[3]

Antibiotic regimens ^[4]:

Seawater/seafood → IV fluoroquinolone + IV amoxicillin-clavulanate
Cuts/abrasion/healthy adults → IV penicillin G + IV linezolid
Post-surgical polymicrobial → IV imipenem or IV meropenem
Gas gangrene → Augmentin + Flagyl ^[5]

Why add linezolid/clindamycin? Suppresses superantigen and exotoxin production — shuts off the toxin factory, not just kills bacteria.

Amputation indications (3D): Dead, Damage, Danger (NF, gangrene, ascending sepsis) ^[6]

Limb salvage vs amputation: Good health + not life-threatening + responsive to inotropes → salvage. Unremitting shock + myonecrosis + PVD + rapidly progressive → amputation ^[1].

Sequelae: 46% of HK patients required amputation; mortality 20–75% ^[1].

BKA is the most common amputation ^[6]; 90% walk again. AKA: 50% walk again ^[6].

Never delay surgery for imaging, HBOT, or any other investigation. Never give NSAIDs ^[1].

Active Recall - Management of Necrotizing Fasciitis

References

^[1] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p39, p41, p44, p54) ^[3] Senior notes: maxim.md (NF management — aggressive debridement + IV broad-spectrum antibiotics; LRINEC score; gas gangrene CBC findings) ^[4] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p45, Management principles/antibiotic table) ^[5] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p51–52, Gas Gangrene management) ^[6] Senior notes: felixlai.md (Amputation indications 3D, types, complications, principles) ^[7] Senior notes: maxim.md (Amputation indications, principles, complications, rehabilitation)

Complications of Necrotizing Fasciitis

NF is one of the most devastating infections a patient can experience. Even with optimal management, complications are the rule, not the exception. They arise from three interrelated sources:

The disease itself — direct tissue destruction, systemic toxicity, and the septic cascade
The treatment — radical debridement and amputation carry their own morbidity
The critical illness — prolonged ICU stay, immobility, and organ support

I find it useful to organise complications using a temporal framework (immediate, early, late) combined with a local vs systemic distinction, because this maps onto clinical decision-making: immediate complications kill the patient today, early complications kill them this week, and late complications affect quality of life for years.

Systematic Classification of Complications

These are the complications that will kill your patient if you don't anticipate and treat them.

Complication	Mechanism (First Principles)	Clinical Relevance
Sepsis → Septic shock → Multiorgan dysfunction syndrome (MODS)	Bacterial toxins (endotoxins, superantigens) trigger massive systemic inflammatory response → vasodilation → distributive shock → inadequate organ perfusion → sequential organ failure (lungs → kidneys → liver → brain → heart). Necrosis in multiple tissue layers ^[1] releases damage-associated molecular patterns (DAMPs) that amplify the inflammatory cascade beyond the infectious source.	The leading cause of death in NF. This is why mortality ranges from 20 to 75% ^[1]. Every hour of delayed debridement allows more toxins into the systemic circulation, worsening the septic cascade. The source must be removed surgically — antibiotics alone cannot penetrate avascular necrotic tissue.
Streptococcal Toxic Shock Syndrome (STSS)	Group A Streptococcus produces superantigens (SPE-A, SPE-B, SPE-C) that bypass normal antigen presentation and directly activate up to 20% of T-cells (vs. < 0.01% in normal immune response) → massive cytokine storm (TNF-α, IL-1β, IL-6, IFN-γ) → capillary leak, vasodilation, myocardial depression, DIC.	Occurs specifically in Type II (GAS) NF. Presents with generalised erythematous rash and toxic appearance ^[1], hypotension, and multiorgan failure. This is why IV penicillin G + IV linezolid ^[4] is the regimen — linezolid suppresses superantigen production. IVIG may be considered as adjunct (neutralises circulating superantigens).
Disseminated Intravascular Coagulation (DIC)	Tissue factor release from necrotic tissue + bacterial endotoxins → simultaneous activation of coagulation and fibrinolysis → microvascular thrombosis throughout the body → consumption of clotting factors and platelets → paradoxical bleeding tendency.	Manifests as low platelet count ^[1], prolonged PT/aPTT, low fibrinogen, elevated D-dimer. Clinically: bleeding from wound edges, IV sites, mucosal surfaces; simultaneously, microvascular thrombi cause organ ischaemia (renal cortical necrosis, hepatic ischaemia, peripheral gangrene). Treatment: treat the underlying cause (debridement), replace consumed factors (FFP, cryoprecipitate, platelets).
Acute Kidney Injury (AKI)	Multi-factorial: (1) Pre-renal — hypovolaemia from third-spacing and sepsis-induced vasodilation; (2) Intrinsic — acute tubular necrosis from myoglobin (if myonecrosis/rhabdomyolysis), sepsis-mediated tubular injury, nephrotoxic drugs (aminoglycosides, IV contrast); (3) Renal insufficiency as both a risk factor and consequence ^[1]; NSAIDs develop acute renal failure ^[1].	Monitor urine output (catheterise), serial creatinine. Management: aggressive fluid resuscitation, avoid nephrotoxins, renal replacement therapy (CVVHDF) if severe. Rhabdomyolysis-related AKI requires IV bicarbonate (alkalinise urine to prevent myoglobin precipitation in tubules) and mannitol diuresis ^[8].
Rhabdomyolysis	When infection extends to muscle (myonecrosis — particularly in gas gangrene), widespread muscle cell death releases intracellular contents: creatine kinase (CK), myoglobin, and potassium into the systemic circulation. Myoglobin is freely filtered by the glomerulus but precipitates in acidic urine → obstructive acute tubular necrosis ^[8].	Characterised by massively elevated CK ( > 5× normal), dark "tea-coloured" urine (myoglobinuria), and hyperkalaemia → risk of cardiac arrhythmia. Management: aggressive hydration, IV bicarbonate to alkalinise urine, mannitol for osmotic diuresis, continuous cardiac monitoring for arrhythmias ^[8].
Acute Respiratory Distress Syndrome (ARDS)	Systemic inflammatory response → pulmonary capillary endothelial injury → increased alveolar-capillary permeability → protein-rich pulmonary oedema → surfactant dysfunction → alveolar collapse → refractory hypoxaemia.	Develops in severe sepsis; presents with progressive dyspnoea, bilateral infiltrates on CXR, PaO2/FiO2 < 300. Requires mechanical ventilation with lung-protective strategy (low tidal volumes 6 mL/kg IBW, PEEP, plateau pressure < 30 cmH2O).

Why Sepsis Kills in NF — The Vicious Cycle

NF creates a self-amplifying loop: tissue necrosis → releases toxins and DAMPs → systemic inflammation → vasodilation and capillary leak → hypoperfusion → more tissue ischaemia → more necrosis → more toxins. The only way to break this cycle is to remove the necrotic tissue surgically. Antibiotics cannot reach the avascular necrotic tissue, and supportive care alone cannot outpace the toxin release. This is why aggressive debridement ^[3] is the cornerstone.

These develop during the hospitalisation period, related to ongoing wound management, ICU stay, and the initial surgical intervention.

Complication	Mechanism	Management
Wound infection / Secondary sepsis	Open wounds after debridement are colonised by nosocomial organisms (MRSA, Pseudomonas, Acinetobacter); immunocompromised state from critical illness; repeated surgical procedures	Serial wound cultures; targeted antibiotics; careful wound care; VAC therapy to maintain clean wound environment
Ongoing tissue necrosis requiring re-debridement	Initial debridement may not capture all affected tissue — NF spreads along fascial planes beyond visible margins ("iceberg phenomenon"); residual bacteria continue to advance	Relook debridement every 24–48 hours until wound is clean; mark wound margins to track progression
Massive blood loss / Transfusion requirements	Repeated surgical debridements of large, raw wound surfaces; DIC-related coagulopathy; loss of haemostasis	Crossmatch and blood product availability; correct coagulopathy; intra-operative attention to haemostasis
Electrolyte derangements	Hyperkalaemia (from rhabdomyolysis / tissue necrosis / AKI); hyponatraemia (SIADH, third-spacing); hypocalcaemia (calcium binds to necrotic fat, consumed in DIC)	Regular monitoring (at least 6–12 hourly in ICU); ECG monitoring for hyperkalaemia; replacement as needed
Nutritional deficiency / Catabolic state	Massive tissue loss + sepsis → hypercatabolic state with accelerated protein breakdown; wound healing demands enormous protein/calorie/micronutrient supply	Early enteral nutrition (high-protein, high-calorie); supplement zinc, vitamin C, vitamin A for wound healing; may require parenteral nutrition if enteral not tolerated
VTE (DVT / Pulmonary embolism)	Virchow's triad: immobility (bedridden in ICU), endothelial injury (sepsis-related), hypercoagulable state (acute-phase response, DIC paradox)	Pharmacological prophylaxis (LMWH) once haemostasis adequate; mechanical prophylaxis (pneumatic compression devices); early mobilisation when possible
Pressure injuries (bed sores)	Prolonged immobility + poor nutrition + tissue oedema → pressure necrosis over bony prominences (sacrum, heels, occiput)	Regular repositioning (every 2 hours); pressure-relieving mattress; nutritional optimisation; skin assessment
ICU-acquired weakness	Prolonged critical illness → critical illness myopathy (direct effect of sepsis, steroids, NMBAs on muscle) and critical illness polyneuropathy (axonal degeneration from systemic inflammation)	Early mobilisation and physiotherapy; minimise sedation and neuromuscular blockade; adequate nutrition
Antibiotic-related complications	Prolonged broad-spectrum antibiotics → Clostridioides difficile colitis; drug-related nephrotoxicity (aminoglycosides); drug allergy/hypersensitivity; bone marrow suppression (linezolid with prolonged use)	Monitor for diarrhoea; de-escalate antibiotics based on cultures; therapeutic drug monitoring where applicable; monitor CBC with prolonged linezolid

These are the complications that affect the patient long after they leave the hospital — and they are often the most devastating to quality of life.

Complication	Mechanism	Clinical Impact
Amputation ^[1] — HK figure: radical debridements (amputations and disarticulations) were performed in 46% of 24 patients ^[1]	When tissue necrosis is too extensive for limb salvage, or when the patient's life is threatened by ongoing sepsis from the limb, amputation is necessary ^[1]. Indications include myonecrosis, unremitting shock, concurrent peripheral vascular insufficiency, rapidly progressive infection, large area of tissue necrosis ^[1].	Life-altering: functional disability, prosthetic dependence, chronic pain, altered body image. BKA: 90% walk again; AKA: 50% walk again ^[6].
Disfiguring scars / Contractures	Extensive tissue loss + healing by secondary intention or skin grafting → scar tissue formation → wound contraction; fascial loss removes normal tissue planes	May require multiple reconstructive surgeries (skin grafts, flaps); physiotherapy to prevent contractures; silicone sheets/pressure garments for scar management
Chronic pain	Neuropathic pain from nerve damage during the disease or surgery; central sensitisation from prolonged pain experience; myofascial pain from altered biomechanics	Gabapentin, pregabalin, amitriptyline for neuropathic pain; multidisciplinary pain team; physiotherapy
Chronic wounds / Non-healing wounds	Underlying risk factors persist (DM, PVD, immunocompromised ^[1]); poor blood supply to grafted areas; repeated infections	VAC therapy, specialist wound care, optimisation of comorbidities (glycaemic control, nutrition, cessation of smoking)
Psychological complications	PTSD (emergency surgery, ICU stay, disfigurement); depression; anxiety; body image disturbance; social isolation	Psychiatric / psychological referral; peer support groups; cognitive behavioural therapy; pharmacotherapy if needed

If the patient undergoes amputation as part of NF management, there is a specific set of complications:

Timing	Complication	Mechanism / Notes
Early	Bleeding and haematoma formation ^[6]	Surgical vessel injury; ongoing coagulopathy from DIC
	Wound infection ^[6]	Contaminated surgical field (NF is by definition an infected wound); immunocompromised host
	Phantom limb pain ^[6]	Cortical reorganisation — the somatosensory cortex retains a "map" of the amputated limb; peripheral nerve sprouting from transected nerve ends generates aberrant signals interpreted as pain from the missing limb. Management: reassurance, amitriptyline, gabapentin ^[7]
	Skin necrosis ^[6] — secondary to poor perfusion of stump ^[6]	Residual vascular disease (concurrent peripheral vascular insufficiency ^[1]); inadequate blood supply at amputation level; may require revision to a higher level
	DVT / PE	Immobility + hypercoagulable septic state; prophylactic heparin ^[7]
Late	Stump ulceration ^[6]	Pressure from prosthesis on poorly vascularised or scarred stump skin
	Stump neuroma ^[6]	Disorganised nerve regeneration at the cut nerve ends forms a painful, palpable nodule; triggered by pressure or tapping (Tinel's sign positive)
	Osteomyelitis ^[6]	Residual bone infection from the original NF or secondary contamination of exposed bone
	Osteophyte formation in underlying bone ^[6]	Reactive new bone growth at the stump end; can irritate overlying soft tissue and cause pain with prosthesis use
	Fixed flexion deformity ^[6]	Muscle imbalance around the joint proximal to the amputation (e.g., hip flexion contracture after AKA, knee flexion contracture after BKA); unopposed action of flexors; prevented by early physiotherapy, prone positioning, and avoidance of prolonged pillow placement under the stump
	Difficult mobilisation ^[6]	Combination of all the above + deconditioning from prolonged illness; rehabilitation is prolonged and requires multidisciplinary team

Phantom Limb Pain — Common Exam Topic

Phantom limb pain affects up to 80% of amputees. It is NOT "in their head" — it has a genuine neurological basis in cortical reorganisation and peripheral nerve sprouting. First-line treatment: reassurance (educate that this is normal and expected), followed by pharmacotherapy: amitriptyline (tricyclic antidepressant — modulates descending pain inhibitory pathways) or gabapentin/pregabalin (bind α2δ subunit of voltage-gated calcium channels → reduce excitatory neurotransmitter release → dampen aberrant nerve signals) ^[7].

Site	Unique Complications
Fournier's gangrene (perineum)	Urethral stricture/fistula; colostomy requirement (if perianal source, faecal diversion may be needed to protect the wound); scrotal skin loss requiring reconstruction (thigh flaps); sexual dysfunction; psychological devastation
Cervical NF (head and neck)	Airway compromise (oedema, direct tissue destruction) → may require emergency tracheostomy; mediastinitis (descending infection along fascial planes into the mediastinum — mortality > 50%); carotid artery erosion and haemorrhage; jugular vein thrombosis
Abdominal wall NF	Hernia (loss of fascial integrity → ventral/incisional hernia); peritonitis (if infection tracks intraperitoneally); stoma requirements; massive wound defects requiring complex reconstruction
Extremity NF	Amputation (46% of HK patients ^[1]); compartment syndrome ^[8]; contractures limiting joint range of motion; chronic lymphoedema from disruption of lymphatic drainage

Factor	Why It Worsens Prognosis
Delayed surgical debridement	Every hour of delay allows further fascial necrosis and systemic toxin release; the single most important modifiable prognostic factor
Truncal / perineal involvement	Harder to debride radically (cannot amputate the trunk); wider fascial planes allow more extensive spread
Polymicrobial infection (Type I)	Synergistic bacterial interactions make the infection more aggressive than monomicrobial forms
Streptococcal toxic shock syndrome	Massive superantigen-driven cytokine storm → refractory shock
Concurrent medical disease ^[1]	DM, liver disease, renal insufficiency, immunocompromised ^[1] — all impair immune response and wound healing
Older age	Reduced physiological reserve; more comorbidities; slower immune response
Higher LRINEC score	Reflects more severe systemic derangement at presentation
Bacteraemia	Indicates systemic dissemination — higher mortality than localised infection

High Yield Summary — Complications of NF

Immediate/life-threatening: Sepsis → septic shock → MODS (leading cause of death); STSS (GAS with superantigens); DIC (low platelet count ^[1]); AKI (pre-renal + intrinsic + NSAID-induced ^[1]); rhabdomyolysis (myonecrosis → myoglobin → AKI + hyperkalaemia); ARDS.

Sequelae of delayed treatment ^[1]: Amputation in 46% of HK patients; mortality 20–75% ^[1].

Amputation complications ^[6]:

Early: bleeding, wound infection, phantom limb pain (Mx: amitriptyline, gabapentin), skin necrosis (poor stump perfusion)
Late: stump ulceration, stump neuroma, osteomyelitis, osteophyte formation, fixed flexion deformity, difficult mobilisation

Late / QoL complications: disfiguring scars, contractures, chronic pain, chronic wounds, psychological (PTSD, depression, body image).

Site-specific: Fournier's → urethral fistula, colostomy; Cervical → mediastinitis, airway compromise; Abdominal → hernia.

Key prognostic factors: Time to debridement (most important modifiable factor), truncal involvement, polymicrobial infection, STSS, comorbidities.

Active Recall - Complications of Necrotizing Fasciitis

References

^[1] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p39, p40, p44, p46) ^[3] Senior notes: maxim.md (NF management — aggressive debridement + IV broad-spectrum antibiotics) ^[4] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025] (1).pdf (p45, antibiotic regimens) ^[6] Senior notes: felixlai.md (Amputation types, complications — early and late) ^[7] Senior notes: maxim.md (Amputation indications, complications, rehabilitation) ^[8] Senior notes: felixlai.md (Rhabdomyolysis mechanism and management; compartment syndrome)

High Yield Summary

Definition: Necrotizing fasciitis is a rapidly progressive infection primarily involving the superficial fascia with secondary skin/soft tissue necrosis due to microvascular thrombosis.

Risk Factors (exam favourite): DM (57%), alcohol abuse, renal insufficiency, liver disease, immunocompromised, NSAIDs ^[1]

Classification:

Type I = Polymicrobial (post-surgical, Fournier's)
Type II = Monomicrobial (GAS in healthy patients)
Type III = Marine Vibrio / Aeromonas (HK high yield — seafood/seawater + liver disease)
Gas gangrene = C. perfringens (no neutrophilia, gas on XR)

Cardinal Clinical Feature: Pain out of proportion to clinical findings ^[1]^[2]^[3]

Clinical Stages ^[1]:

Stage 1: Tenderness, erythema, oedema, warm skin, fever
Stage 2: Bullae, haemorrhagic bullae, hyposensitivity, crepitation
Stage 3: Tissue necrosis, anaesthesia

Key Diagnostic Test: Finger probe test — lack of bleeding, dishwater pus, minimal tissue resistance ^[1]

Scoring: LRINEC score > 8 = high risk ^[3]

Treatment Principle: Aggressive debridement + IV broad-spectrum antibiotics ^[3] — antibiotics alone will NOT cure NF

Amputation: When life-threatening — unremitting shock, myonecrosis, PVD, rapidly progressive ^[1]

High Yield Summary — Differential Diagnosis

"Pain out of proportion" DDx: NF, gas gangrene, compartment syndrome, acute limb ischaemia, mesenteric ischaemia.

NF vs Gas Gangrene: Fascia vs muscle; polymicrobial/GAS/Vibrio vs C. perfringens; leucocytosis vs no neutrophilia; dishwater pus vs brownish liquid with gas bubbles.

Perineal NF (Fournier's): DDx includes testicular torsion, epididymitis, incarcerated hernia, perianal abscess, HSP.

Mortality 20–75% if not treated promptly; 46% of HK patients required amputation ^[1].

High Yield Summary — Diagnosis of NF

NF is a CLINICAL DIAGNOSIS — clinical diagnosis ^[1] based on: disproportionate pain, toxic appearance, low platelet count, and clinical stage progression ^[1].
Finger probe test = bedside gold standard: lack of bleeding, foul smelling dishwater pus, minimal tissue resistance to finger dissection ^[1]^[3].
LRINEC score ≥ 8 = high risk ^[3]. Variables: CRP (4 pts), Hb (2 pts), WCC (2 pts), Na (2 pts), Cr (2 pts), glucose (1 pt). Maximum 13. But a low score does NOT rule out NF — clinical suspicion overrides.
Imaging: CT is preferred emergently (gas, non-enhancing fascia); MRI is most sensitive but time-consuming. XR can show linear streak of gas ^[3] in gas gangrene. Never delay surgery for imaging.
Surgical exploration is the definitive diagnostic AND therapeutic intervention. Send deep tissue for Gram stain, C/ST, and histopathology.
Gas gangrene clues: crepitus, no neutrophilia on CBC ^[3], linear gas on XR ^[3], foul brownish liquid with gas bubbles ^[5].

High Yield Summary — Management of NF

Two pillars: Aggressive debridement + IV broad-spectrum antibiotics ^[3]

Antibiotic regimens ^[4]:

Seawater/seafood → IV fluoroquinolone + IV amoxicillin-clavulanate
Cuts/abrasion/healthy adults → IV penicillin G + IV linezolid
Post-surgical polymicrobial → IV imipenem or IV meropenem
Gas gangrene → Augmentin + Flagyl ^[5]

Why add linezolid/clindamycin? Suppresses superantigen and exotoxin production — shuts off the toxin factory, not just kills bacteria.

Amputation indications (3D): Dead, Damage, Danger (NF, gangrene, ascending sepsis) ^[6]

Limb salvage vs amputation: Good health + not life-threatening + responsive to inotropes → salvage. Unremitting shock + myonecrosis + PVD + rapidly progressive → amputation ^[1].

Sequelae: 46% of HK patients required amputation; mortality 20–75% ^[1].

BKA is the most common amputation ^[6]; 90% walk again. AKA: 50% walk again ^[6].

Never delay surgery for imaging, HBOT, or any other investigation. Never give NSAIDs ^[1].

High Yield Summary — Complications of NF

Sequelae of delayed treatment ^[1]: Amputation in 46% of HK patients; mortality 20–75% ^[1].

Amputation complications ^[6]:

Early: bleeding, wound infection, phantom limb pain (Mx: amitriptyline, gabapentin), skin necrosis (poor stump perfusion)
Late: stump ulceration, stump neuroma, osteomyelitis, osteophyte formation, fixed flexion deformity, difficult mobilisation

Late / QoL complications: disfiguring scars, contractures, chronic pain, chronic wounds, psychological (PTSD, depression, body image).

Site-specific: Fournier's → urethral fistula, colostomy; Cervical → mediastinitis, airway compromise; Abdominal → hernia.

Key prognostic factors: Time to debridement (most important modifiable factor), truncal involvement, polymicrobial infection, STSS, comorbidities.

Necrotizing Fasciitis

1. Definition

2. Epidemiology

Incidence

Age & Sex

Mortality

Hong Kong Context

3. Risk Factors

4. Anatomy and Function

Layers of Soft Tissue (Superficial to Deep)

The Fascial Planes — Why They Matter

Common Anatomical Sites

5. Etiology (Focus on Hong Kong)

Classification by Microbiology

Specific Antibiotic Regimens by Scenario [4]

6. Pathophysiology

The Cascade — From Portal of Entry to Multiorgan Failure

Gas Gangrene — Specific Pathophysiology [5]

7. Classification

By Microbiology (Most Commonly Used)

By Anatomical Location

Clinical Stages of Necrotizing Fasciitis [1]

8. Clinical Features

Symptoms

Signs

Bedside Clinical Tests [1]

Distinguishing NF from Severe Cellulitis

Fournier's Gangrene — Special Considerations [2]

9. Factors Favouring Limb Salvage vs Amputation

10. Gas Gangrene — Distinct Entity [5][3]

Active Recall - Necrotizing Fasciitis

References

Why is the DDx So Important Here?

The Differential Diagnosis — Organised by Depth of Infection

Detailed Differential Diagnosis Table

Perineal DDx — Fournier's Gangrene Specifically

The Critical Distinguishing Features — What Separates NF from the Rest

DDx of "Pain Out of Proportion" — A Key Clinical Clue

NF vs Gas Gangrene — A Common Exam Comparison

Sequelae of Missed/Delayed Diagnosis [1]

Active Recall - DDx of Necrotizing Fasciitis

The Fundamental Diagnostic Principle

Diagnostic Criteria

A. Clinical Criteria (Most Important — the "Bedside Diagnosis")

B. Bedside Procedure — The Finger Probe Test

C. Laboratory Scoring — The LRINEC Score

Diagnostic Algorithm

Investigation Modalities — Detailed Breakdown

1. Blood Investigations (Routine — Always Obtained)

2. Microbiological Investigations

3. Imaging Investigations

4. Intra-operative Findings (The Definitive "Investigation")

Putting It All Together — Systematic Diagnostic Approach

Active Recall - Diagnosis of Necrotizing Fasciitis

The Overarching Principle

Management Algorithm

Phase 1: Resuscitation

Phase 2: Empirical Intravenous Antibiotics

Management Principles — Antibiotic Regimens [4]

Gas Gangrene — Specific Antibiotic Regimen [5]

Why These Specific Regimens? — Explained From First Principles

When to Adjust Antibiotics

Phase 3: Emergency Surgical Debridement

Principles of Surgical Debridement

Intra-operative Decision: Limb Salvage vs Amputation

Amputation — Principles and Levels [6][7]

Sequelae of Delayed/Inadequate Surgery [1]

Phase 4: ICU Care and Serial Debridements

ICU Support

Serial Debridements ("Relook" Surgery)

Wound Management Between Debridements

Phase 5: Reconstruction and Rehabilitation

Rehabilitation

Complications of Amputation [6][7]

Adjunctive Therapies (Evidence Variable)

Summary — Treatment by Phase

Active Recall - Management of Necrotizing Fasciitis

The Big Picture

Systematic Classification of Complications

A. Immediate / Life-Threatening Complications (Hours to Days)