Necrotising fasciitis: recognition and diagnosis

What it is

Necrotizing fasciitis is necrosis of the deep and/or superficial fascia, along with the deeper dermis, subcutaneous tissues and muscle. Put in one line, it is a bacterial infection of the underlying fascia. Microvascular thrombosis in it leads to ischemia of the tissue, and that is why antibiotics alone do not reach the dead fascia.

Type 1 against type 2

Type 1, 2/3 of cases:

Polymicrobial: gram positives, gram negatives, anaerobes
Follows perineal infections, penetrating abdominal injury or bowel surgeries, and decubitus ulcers
Underlying risk factors are common: diabetes, illicit drug use, vascular disease

Type 2, 1/3 of cases:

Monomicrobial: Group A streptococcus, and also Groups B/C/G streptococcus, Staph aureus, Vibrio and Aeromonas species
Often a normal host
Minor contusions or trivial injuries may predispose
Limbs predominate

So most cases are polymicrobial, and where the infection is monomicrobial it is most commonly caused by group A streptococcus, that is streptococcus pyogene.

The typical history

The history set out as typical is a middle-aged man who sustains an open fracture of his leg and develops painful, oedematous, bronze coloured skin around the fracture site, with haemorrhagic blebs and bullae noted along with crepitus and tenderness on palpation of the affected skin.

A second one is the diabetic male who comes to ED with a skin rash that developed a few hours ago and is progressively increasing. He is febrile and sick-looking, and his right lower limb is severely tender with restricted motion and palpable crepitus. The lesion is described as cutaneous necrosis, palpable crepitus, erythema and hemorrhagic bullae.

A third runs on a ward. A patient was admitted and treated with IV cloxacillin; the day after, the redness is more extensive and she is complaining of pain, and she develops hyponatremia and increasing WBC. The two top ddx there are necrotizing fasciitis and a collection.