Introduction

Necrotising fasciitis is caused by beta-haemolytic Streptococcus spp. Group A and affects the subcutaneous layers, with undermining of surface tissues and dermal gangrene, but minimal muscle involvement, leading to widespread tissue destruction, systemic illness and shock.

Fournier’s gangrene is a necrotising infection of the scrotum and perianal area: because the skin in this area is thin, the condition tends to progress rapidly.

Synergistic gangrene is a polymicrobial infection.  There is evidence that the alpha-lysin produced by Staphylococcus aureus is necessary to potentiate the infection caused by beta-haemolytic Streptococcus spp. Group A.

Meleyne’s post-operative synergistic gangrene is a form of synergistic gangrene in which anaerobes and aerobes may act synergistically.  Painful black skin ulcers form after surgery.

Gas gangrene is an infection of the muscle layers by clostridia.  Extensive gas production is accompanied by tissue death and severe systemic illness.

These infections are rare but serious.  They lead to widespread tissue death, resulting in high morbidity with disfigurement and loss of function.  They occur in poorly oxygenated tissue, often after a wound or other skin break (which may be very minor). Mortality rate can be very high, often greater than 25%.

Urgent surgical referral for consideration of debridement is mandatory.

Risk factors for necrotising fasciitis, synergistic and gas gangrene include:

Aetiology of necrotising infections

Necrotising fasciitis is caused by beta-haemolytic Streptococcus spp. Group A.

Synergistic gangrene is a polymicrobial infection and includes beta-haemolytic streptococci Group A plus Staphylococcus aureus, Gram-negative aerobes or anaerobes.   

Gas gangrene is caused by Clostridia and the most common causative organism is Clostridium perfringens, which produces exotoxins that cause local tissue damage.  If systemic spread occurs, widespread haemolysis and renal failure may occur and death may rapidly follow. 

Gram-negative organisms, including Escherichia coli, may lead to a gas gangrene-like syndrome in diabetic patients.  This presents with cellulitis and crepitus, and may be mistaken for gas gangrene.

Fournier’s gangrene is often polymicrobial: typical organisms include Bacteroides fragilis, Peptostreptococcus spp., Streptococcus spp., Clostridia, Escherichia coli, Enterobacter, Pseudomonas, Klebsiella, Staphylococcus spp. and Proteus.

Meleyne’s post operative synergistic gangrene results from synergistic infection with anaerobes and aerobes.

Pyoderma gangrenosum is an ulcerative cutaneous disease; tissue destruction can be extensive and scarring can occur.  Patients often have some condition associated with reduced immune function, including inflammatory bowel disease, polyarthritis or haematological disorders, particularly leukaemia.  It is not clear whether pyoderma gangrenosum is primarily an infectious disorder, but systemic and topical antibiotics have been used, together with corticosteroids and immunosuppressives.

Heroin injection into muscle has been associated with deaths in the UK from infection by Clostridium novyi type A, which was a frequent cause of gas gangrene in the two world wars. 

Disease progression can be very rapid, with the infection visibly spreading over about one hour.

Complications include:

• Sepsis
• Renal failure
• Extensive tissue destruction (possibly requiring amputation).

If untreated, systemic toxicity progresses to shock and eventually death.

Diagnosis of necrotising infections

Early intervention is mandatory.  Do not wait for microbiology results before commencing empirical treatment.

• Diagnosis is by clinical signs and symptoms. 
• Microbiological investigations include aspirates, swabs or tissue samples for Gram-stain, culture and sensitivity.  

Management of necrotising infections

Antibiotic rationale

Empirical therapy must be comprehensive and cover all likely pathogens, including aerobes and anaerobes and must always be an adjunct to and not a substitute for surgery.  A broad-spectrum beta-lactam antibiotic should be used, together with an antibiotic that is active against anaerobes. 

The disease progresses rapidly: the earlier the diagnosis and treatment and in particular, the earlier the surgical debridement, the better the survival rates and the lower the resulting morbidity.

These infections are serious and life-threatening; it is therefore important to administer IV antibiotics in high doses to quickly achieve bactericidal concentrations in the affected tissue.

Empirical therapy

Examples of empirical antibiotic regimens include:

• A broad-spectrum penicillin such as amoxicillin plus metronidazole plus an aminoglycoside such as gentamicin.
  or
• A second-generation cephalosporin such as cefuroxime, plus metronidazole.
Penicillin-allergic:  clindamycin plus a quinolone eg. ciprofloxacin.
• If pyoderma gangrenosum is suspected, eg. in patients with cystic fibrosis, anti-pseudomonal agents such as ciprofloxacin or a ureidopenicillin such as piperacillin plus an aminoglycoside should be used.
If the patient is immunocompromised then pseudomonal infection is also common and a quinolone should be used.

Aminoglycoside blood levels must be monitored, particularly in the elderly, if high doses are used, if the patient has renal impairment, or if treatment is prolonged beyond seven days.

Once microbiology results are known, consult your hospital microbiologist and tailor the antibiotic regimen accordingly.

Additional interventions may include:

• Hyperbaric oxygen treatment
• Tetanus toxoid and immunoglobulin
• Skin grafts. 

Fournier’s gangrene

This is a form of necrotising infection affecting the scrotum and perianal area, with very high mortality (up to 80%).  It often follows local tissue injury, including surgery and abscess drainage, and diabetes is a common underlying disease.

It presents with scrotal skin pain and itching, tenderness, oedema and redness, with severe swelling of the scrotum, crepitation, and necrosis of the skin.  The thin scrotal skin means that the clinical picture is altered, compared with other forms of necrotising fasciitis.  Fournier’s gangrene may spread to a more typical picture of necrotising fasciitis on the surrounding skin.

A similar perineal disease occurs in women but has a more typical necrotising fasciitis course, as the skin of the affected area is thicker than that of the scrotum.

Early aggressive surgery is vital in Fournier’s gangrene.

Treatment is similar to that for necrotising infections, but the extensive debridement necessary almost always results in loss of the scrotum and testes, and often of the penis.  Less extensive surgical intervention has been tried with some success, provided treatment is commenced early enough.