gas gangrene

definition

· infection of skeletal muscle

· rapidly progressive

· life-threatening

· characterised by

· massive muscle and soft tissue necrosis

· gas production

· due to invasive anaerobic infection with clostridia

· esp. Clostridium perfringens

epidemiology

· seen in

· open fractures

· penetrating wounds, esp war wounds

· surgical wounds, esp. after bowel or biliary surgery

· arterial insufficiency in an extremity

· incidence 1000 cases/year in USA

· incidence in open fractures 0.05%

· incidence in war wounds

· 2% in WWI to 0.2% in Vietnam

· occurs when there is

· muscle injury with necrotic tissue

· ischaemia with low pO2

· contamination with spores of Cl perfringens or other histotoxic clostridia

· incidence greatly increased by

· inadequate debridement

· inadequate antibiotics

· primary wound closure

aetiology

· most commonly due to Clostridium perfringens (welchii) - 80%

· sometimes due to Clostridium novyi - 15%

· occasionally due to Clostridium septicum - 5%

Clostridium perfringens

· large gram-positive rod

· produces spores

· non-motile

· grows only under anaerobic conditions

· encapsulated

· distributed in soil and faeces

· may be found on human skin

· is saprophytic commensal of alimentary tract

· is also ubiquitous organism found in operating rooms and emergency departments

· isolated from skin in 20% of patients

Histotoxins

· produce large variety of toxins and enzymes that result in spreading infection

· substances are necrotising and haemolysing

· most important is alpha-toxin (lecithinase)

· others include haemolysin, collagenase, hyaluronidase, leukocidin, deoxyribonuclease, protease and lipase

Infection

· toxigenic clostridia introduced into deep muscle wound

· organism converted from saprophytic state to fulminating gangrenous state

· by ischaemia and necrosis of muscle with reduction in oxidation reduction potential

· occurs in dirty wounds closed primarily without adequate debridement

· can also be introduced into wound during surgery

· histotoxins produced which cause cell wall destruction and cell death

· this devitalisation allows further colonisation by clostridia

· production of gas from fermentation aids spread

pathology

· involved muscle rapidly undergoes disintegration

· initially, is pale, swollen and inelastic

· later becomes discoloured and friable

· reddish purple

· then greenish purple and gangrenous

· histologically there is coagulation necrosis

clinical findings

Incubation period

· usually 2-3 days (as short as 6 hours)

Symptoms

· earliest and most important symptom is pain

· rapidly progressive

· out of proportion to injury or procedure

Signs

General

· patient is pale and sweaty

· fever may not be marked

· marked tachycardia

· then shock with hypotension

· patient initially alert and often extremely anxious and fearful of death

· then delerium, stupor and unconsciousness

· may progress to death over hours

Local

· tense oedema and local tenderness seen early

· discharge from wound

· serosanguinous, dirty-appearing

· peculiar foul odour

· may contain gas bubbles

· tissue adjacent to wound

· may be crepitus

· skin initially swollen and white

· rapidly develops bronze discolouration

· blebs containing dark fluid develop

· areas of green-black cutaneous necrosis appear

· may progress over 2-4 hours with advancing crepitus and oedema

investigations

· positive blood culture in 15%

· Gram-stain of exudate shows many organisms but few leukocytes and no spores

· positive Nagler’s test

· lecethinase turns egg yolk opalescent in agar

· x-rays show extensive gasseous distension of muscle and fascial planes

differential diagnosis

Anaerobic clostridial cellulitis

· clostridial infection of necrotic tissue occuring after several days in inadequately debrided wound

· gradual onset with slight toxaemia and no pain

· slight brown, seropurulant exudate with no skin lesions

· abundant foul-smelling gas production

· no actual muscle invasion

Streptococcal myonecrosis

· infection with group A beta haemolytic streptococcus

· flesh-eating bug

· similar to clostridial myonecrosis

· longer incubation period

· characteristic pain of clostridia not present

· little gas formation, and profuse seropurulent discharge

prophylaxis

· awareness of at risk injuries

· open fractures

· deep penetrating injuries, esp.to buttock and thigh

· early surgical debridement

· meticulous removal of necrotic tissue

· leave wound open

· avoid tight packing and primary closure

· appropriate antibiotics

· cephalosporin

· + aminoglycoside if extensive

· + penicillin if farmyard, crush or vascular

treatment

Surgery

· emergency exploration indicated

· to examine muscles directly to differentiate between gas gangrene and crepitant cellulitis

· for appropriate debridement

· involves

· excision of involved muscles

· fasciotomies

· may require amputation

Antibiotics

· in form of penicillin G

· 3 million units q3h IV

· if penicillin-sensitive, use chloramphenicol

· may add gentamicin because other organisms are often present

· most clostridia are sensitive to cephalosporins but may not achieve adequate tissue concentration to be effective

· resistance to penicillin developing

Resuscitation

· fluid loss is marked

· prompt replacement with monitoring of fluid balance required

Hyperbaric oxygen

· role controversial

· appears to be effective

· are distinct hazards

· barotrauma, decompression sickness, convulsions, otitis media, lung damage

· useful where trunk involved when surgical excision would be mutilating

· hyperbaric oxygen may reduce extent of debridement necessary

· use hyperbaric chamber at 3 Atm for 60-90 minutes every 8-12 hours for 4-6 sessions

prognosis

· mortality in WWI was 50% and in WWII was 25%

· now even lower

· approaches 50% if infection reaches trunk