Worst outcomes if missed
Death – between 20 and 40% of people with necrotising fasciitis die, despite surgery.
Disfigurement – early diagnosis and treatment reduces mortality and the disfigurement from surgery. Delays to surgery increase the risk of amputation.
Epidemiology
- There are about 500 cases of necrotising fasciitis in the UK each year.
- Necrotising fasciitis is more common in patients with diabetes, chronic hepatitis and malignancy – particularly leukaemia – people who inject drugs and those who are immunosuppressed.
- Necrotising fasciitis can occur because of infected pressure sores.
- It is rare in childhood, but there is an association with varicella infection.
- A GP should expect to see at least one case in their career.
- Diagnosis is frequently made late, after multiple presentations.
Symptoms and signs
The classic symptoms of necrotising fasciitis are rarely present initially so distinguishing necrotising fasciitis from cellulitis can be difficult in the early phases of the disease. Early symptoms are non-specific:
- fever
- pain out of proportion to the clinical findings
- inability to use the affected limb.
The limbs are most commonly affected and the perineum is also a common site, but any part of the body can develop necrotising fasciitis.
In patients with fever, clinical suspicion may be aroused by something being ‘not quite right’ for a diagnosis of cellulitis. The classic cyanotic and bullous skin changes may only appear late in the process, but the site of infection may appear unusual. The pain may seem too severe for cellulitis, despite relatively mild skin signs, or there may be overlying sensory loss. Pain is caused by tissue necrosis, but the nerves can also be infarcted as perforating vessels to the tissues are thrombosed by the necrotic process. This can cause exquisite pain and tenderness, but also sensory loss to the overlying skin. The patient may seem disproportionately unwell for the degree of skin involvement. The progression of the illness can suggest the diagnosis – the patient may seem relatively well initially, but will deteriorate despite antibiotic therapy. Crepitus and haemorrhagic blisters are a late sign.
In patients presenting with pain alone, the severity of pain and absence of trauma may suggest the diagnosis.
Differential diagnosis
Symptoms of necrotising fasciitis are initially similar to the much more common and benign cellulitis. Patients with severe musculoskeletal pain may suggest that their pain is caused by an assumed or trivial injury.
Investigations
There are no useful investigations that can be done in primary care – necrotising fasciitis is mainly a clinical diagnosis. Where there is doubt, prompt surgical exploration at hospital is probably best, though MRI or CT scans can be used.
If necrotising fasciitis is suspected, the patient should be referred as an emergency. Patients are usually initially cared for by general surgeons or plastic surgeons, depending on local services.
Five key questions to ask
1. Can you walk or use the limb? Inability to use the limb is suggestive of necrotising fasciitis.
2. Are there any patches of numbness? This would indicate whether any sensory nerves have been infarcted.
3. Where exactly is the pain worst? Pain which is greatest slightly distant to an area of cellulitis is suggestive of necrotising fasciitis.
4. Is the pain around a wound? Uncomplicated wound infections are not usually very painful.
5. Are you feverish and unwell? Patients are usually toxic.
Five red herrings
1. Patients may attribute limb pain to a minor or non-existent injury.
2. The skin signs may be relatively mild at first.
3. Patients who inject drugs often present without systemic signs.
4. Patients may look well in the initial stages of the disease, which can last a few days.
5. Lymphangitis is unusual in necrotising fasciitis – this usually suggests a different diagnosis.
Dr Adrian Boyle is a consultant in emergency medicine at Addenbrookes Hospital in Cambridge and an honorary senior research fellow at Cambridge University.
