Lessons of the month: Herpetic viral dermatomyositis.

We present the case of a man who presented with severe left lower back pain radiating to the anterior aspect of left thigh. He also had fever and headache. Due to the exquisite tenderness along the inguinal region, the possibility of a psoas abscess was considered. Magnetic resonance imaging of the spine and thigh were performed. These revealed left psoas muscle abnormalities suggestive of an evolving myositis or abscess. However, the next day, he displayed florid rashes in the left L2-L3 dermatomes consistent with herpes zoster. The clinical manifestations of herpes zoster include neuralgic pain and dermatomal skin rashes. It also presents with a prodrome of fever, headache, myalgia, myositis and Guillain-Barré syndrome. In a developing embryo, somites split to form dermatomes, myotomes (skeletal muscles), syndetomes (tendons and cartilage) and sclerotomes (bones). Our case illustrates that herpes zoster can involve the so called 'dermomyotome', a combination of the dermatome and myotome and result in a localised dermatomyositis. © Royal College of Physicians 2020. All rights reserved.