Necrotising Fasciitis
Bacteria spreading along fascial planes thrombose the vessels supplying the overlying tissue, so fat and fascia die while the skin above can still look nearly normal.
Definition
Necrotising fasciitis is bacterial infection of the deep fascia and subcutaneous fat, with vascular thrombosis and tissue necrosis. There is no diagnostic test. The diagnosis is clinical and is confirmed at operation: fascia separates without resistance to blunt dissection, does not bleed, and yields thin grey 'dishwater' pus.
First principles
The infection starts too deep for the skin to show it
Cellulitis begins at the junction of dermis and superficial fascia, so it goes red at the surface. Necrotising fasciitis begins in subcutaneous fat and deep fascia, so the surface can look unremarkable while the fascia is already dead. That gap between agonising, rapidly escalating pain and a modest-looking limb is the diagnosis. Blotches and blisters are harder to see on black or brown skin, so weight the pain and the observations more heavily.
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