Lower limb necrotising fasciitis descending from malignant colonic perforation: a rare pattern.

Necrotising fasciitis (NF) is a rare surgical emergency characterised by soft tissue necrosis and systemic compromise. Typically it originates following adjacent skin and soft tissue insult; however, our unusual case required a high index of clinical suspicion to avert potential mortality. A man in his 60s with diabetes mellitus presented with 2 weeks of knee pain, swelling and necrotic skin on the posterior calf. X-ray demonstrated subcutaneous emphysema. Initial debridement confirmed extensive necrotising fasciitis of the whole lower limb with tracking through femoral canal into the abdomen. CT confirmed sigmoid colon perforation. Both life-saving Hartmann's and hip disarticulation procedures were performed with good outcomes. Histology confirmed locally invasive sigmoid colon adenocarcinoma. Our case highlights lower limb necrotising fasciitis as a rare complication secondary to sigmoid perforation associated with malignancy. In cases where the infection nidus cannot be identified, an abdominal source should be considered.