Pyomyositis / Bacterial Myositis

Risk factors: underlying HIV infection (17%), strenuous activity, rhabdomyolysis, muscle trauma (hematoma as nidus for infection)

Others: skin infection, infected insect bite, injection of illicit drug, underlying diabetes mellitus

Location: usually single muscle; multiple in 11–43%; quadriceps m. >gluteal m. >iliopsoas m. >upper extremity

Iliopsoas abscess: TB of spine (formerly), GI / GU infection (currently)

Stages:

1. invasive: muscle edema → pain
2. suppurative: fever → abscess
3. late stage: toxicity → life-threatening sepsis

CT:

• enlarged hypoattenuating muscle
• effacement of surrounding fat planes
• intramuscular fluid collection
• nonenhancement of necrotic tissue
• rim-enhancement of abscess

Cx:

1. Compartment syndrome
2. Osteomyelitis
3. Septic arthritis
4. Muscle scarring, weakness, dysfunction

DDx: cellulitis (involvement of SQ tissue >muscle group)