- Incidence: 1.00 cases per 100,000 person-years
- Peak incidence: 50-60 years
- Sex ratio: 1:1
- Chronic heart failure
- Alcohol abuse
- Systemic lupus erythematosus
- Immunodeficiency e.g. due to HIV, chemotherapy regimes etc.
- Type I - Polymicrobial. Usually caused by multiple anaerobic species, particularly affecting those with multiple comorbidities. This accounts for 70-90% of cases of NF and usually is found on the trunk or perineum
- Type II - Monomicrobial, caused by Streptococcus pyogenes. This type more commonly occurs in younger patients and affects the limbs rather than the trunk.
- Type III - Monomicrobial, caused by the Clostridium species most commonly but also by Vibrios species. Most frequently found in intravenous drug users.
- Type IV - Fungal NF, mainly Candida species. Mainly found in immunocompromised patients, and is particularly aggressive and extensive.
During the early stages of the infection, the superficial layers of the skin are unaffected and the infection spreads via the subcutaneous tissue and fascia. Development of NF occurs due to:
- High virulence of the organism
- Impaired host defence mechanisms
Bacteria produce enzymes that result in necrosis of the fascia and fat, and bacteria also cause thrombosis in the vessels leading to ischaemia which further promotes necrosis. This ischaemia originally produces intense pain for the patient but, as the disease progresses, can result in hypoesthesia/anaesthesia.
- Swelling (80%)
- Pain (79%)
- Erythema (70%)
NF is characterised by pain that is severe and exaggerated compared to what would normally be suspected from a soft tissue infection. As the disease progresses, this pain may subside due to the destruction of nerve endings, and in some cases goes completely. Patients also commonly present with:
- Rapid progression of the lesion
- Blistering and bullae formation
- Bullae contain serous fluid early on but may become haemorrhagic in later disease
- Failure to respond to broad-spectrum antibiotics
- Grey, dusky skin indicating necrosis
- Skin crepitus
- Air under the skin that causes a crackling feel when touched
- Diarrhoea and vomiting
- Systemic features including tachycardia, fever, hypotension and tachypnoea
- Signs of sepsis and organ dysfunction including altered mental status
The Laboratory Risk Indicator for Necrotising Fasciitis (LRINEC) is a scoring system based on six different serum values that is designed to try and increase early identification of cases of NF. A score of ≤5 suggests a low risk (<50%) for NF, a score of 6-7 suggests an intermediate risk and a score of ≥8 suggests a high risk (>75%). The scoring system is as follows:
- Hb (g/dL)
- >13.5 - 0
- 11-13.5 - 1
- <11 - 2
- White cell count (x10^9/L)
- <15 - 0
- 15-25 - 1
- >25 - 2
- Sodium (mmol/L) <135 - 2
- Creatinine (µmol/L) >141 - 2
- Glucose >10 - 1
- C-reactive protein >150 - 4
Other investigations can be performed to aid diagnosis, including:
- Wound culture, to improve antibiotic sensitivity
- Plain x-rays, which can demonstrate subcutaneous gas, a specific finding for NF.
- CT and MRI imaging, which can be used to show asymmetrical fascial thickening with fluid collection and gas tracking along the fascial planes. These findings have a high sensitivity for detecting necrotising fasciitis (80% for CT, 100% for MRI).
- Incisional biopsy with a frozen-section evaluation showing signs of necrosis, microorganisms, polymorphonuclear infiltration.
- Ultrasound/MRI, allowing visualisation of the deeper tissues and subcutaneous fluid levels.
- Blood tests to look for worsening infection including full blood count, erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP).
- Wound culture to assess for causative organism, although this is rarely helpful due to the florid presence of bacteria on the skin.
Pyoderma gangrenosum is another possible differential diagnosis. It is a rare autoimmune ulcerative skin condition that similarly may present with areas of necrosis and bullae formation. In the majority of patients it is associated with other autoimmune disease and it will usually progress within days compared to the hours in which NF progresses. It is important to differentiate between the two as pyoderma gangrenosum is treated non-surgically.
Antibiotic management has little value alone in the treatment of NF but is useful in combination with surgical debridement. Antibiotic therapy is dependent on the organisms suspected to be causing the infection. However, the majority of cases will require broad coverage of anaerobic and gram-negative bacteria. Precise antibiotics used may vary from health board to health board, but a typical regime may be as follows:
- IV Flucloxacillin
- IV Benzylpenicillin
- IV Metronidazole
- IV Clindamycin
- IV Gentamicin