Necrotising fasciitis is defined as a necrotising infection of the deep soft tissue layer. The deep fascial layer can act as a potential space, allowing rapid spread of infection through this plane. The relatively avascular nature of this area (and the necrotising effects of the infection) mean that immune system response on antibiotic delivery is poor.
The condition is usually classified as type 1-4 depending on the type of pathogen:
Group A streptococcus
Gram negative monomicrobial
Type 1 is as a result of infection with a mixture of anaerobic and aerobic pathogens, usually in patients with some degree of immunosuppression e.g. DM, chronic alcohol use, chronic infection. Pathogens include E. Coli, strep, staph, Type 2 is caused by group A strep infection and can occur in anyone. Type 3 is usually caused by marine water pathogen e.g. vibrio, and can occur after a relevant injury in these conditions. Type 4 may occur in immunocompromised patients or following significant traumatic injury e.g. burns.
Can be considered as:
Chronic liver disease
The diagnosis can be challenging to detect early on. It may initially look like cellulitis. Physical changes are often delayed. The initial presentation will generally be of severe skin pain:
Limbs (most common)
Perineum (Fournier’s gangrene)
Key features of the early disease are:
POOP - pain out of proportion of the skin changes
POEM - pain outside erythematous margin (If there is one)
Patients will often be significantly systemically unwell.
There can be notable skin changes that develop:
Skin discolouration (grey due to necrosis)
Wooden hard feel
Anaesthesia may develop
The spread of the cutaneous changes can be rapid.
The key to diagnosis is exploratory surgery - this is needed in cases of high index of suspicion. The macroscopic findings at surgery will include:
Grey necrotic tissue
Lack of bleeding
Easy finger dissection
Additional investigations that may be helpful include.
FBC - WBC usually elevated
CRP - usually high
CK - may be elevated
Xray - may show soft tissue gas (poor reliability/late)
CT - soft tissue gas and fluid
Beside finger test
Examination under local anaesthesia for worrying signs
Aggressive debridement of affected tissue - cut back to margin of healthy (bleeding) tissue. No infective tissue must be left. Repeat explorations will often be needed.
Antibiotics have no real ability to affect the local condition (due to inability to penetrate to the site) but may help with systemic infection. They should still be started early and have a broad spectrum (staph, strep, gram negative and anaerobes)
Supportive care may be needed in cases of a septic response or organ failure.
Epidemiology & Prognosis
This is an uncommon condition, with about 500 cases per year. Mortality is high, even with early detection and intervention (20-40%) Delayed diagnosis, additional pathology (e.g. comorbidity) and inadequate resection can increase mortality.