Hypermucoviscous presenting as sternocleidomastoid pyomyositis with subsequent lower extremity necrotising fasciitis.

Necrotising fasciitis (NF) is a severe, rapidly progressing infection of the muscular fascia and subcutaneous tissue. Monomicrobial NF from hypermucoviscous (hvKP-NF) is rare but highly virulent, most common in East Asia. This report reviews a unique case of hvKP-NF in a US male in his 40s with undiagnosed diabetes mellitus, presenting with severe sepsis, neck swelling and calf pain. Imaging revealed pyomyositis in the sternocleidomastoid, and debridement cultures confirmed hvKP. Despite unremarkable examination findings, persistent pain led to a diagnosis of necrotising fasciitis after initial imaging misinterpretations. The patient was treated with multiple surgical debridements and tailored antibiotics. This case underscores the importance of maintaining a high index of suspicion for necrotising fasciitis despite atypical risk factors and symptomatology, awareness of hvKP in the Western Hemisphere, prompt consideration of advanced imaging modalities when initial evaluations are inconclusive and the critical role of multidisciplinary care in managing severe infections.