Diagnosis: Methicillin-resistant Staphylococcus aureus pyomyositis
Reports of the 3 blood cultures, acquired on the seventh day in the hospital, showed growth of methicillin-resistant Staphylococcus aureus (MRSA) with antibiotic susceptibility patterns common to community-acquired MRSA.1 A purulent aspirate was obtained from the lesion in the FIGURE with CT-guided drainage. Community-acquired MRSA grew in the aspirate culture. The final diagnosis was a primary skeletal muscle abscess without contiguous spread from an adjacent site, or pyomyositis.
Epidemiology of pyomyositis
Pyomyositis is uncommon in immunocompetent individuals living outside of the tropics. It is usually caused by S aureus. In the United States between 1981 and 2002, 330 cases were identified in a literature review,2 which noted an increasing incidence. Of these cases, 70% were caused by S aureus and 61.5% of patients were immunocompromised; 1 involved MRSA.
In 2003, 2 of 3 MRSA cases were reported in patients with hematologic disorders.3 In 2005, 4 community-acquired MRSA cases were reported;1 2 had other illnesses leading to increased risk. MRSA pyomyositis is now being reported in immunocompetent individuals, but most cases arise in patients with cancer, diabetes mellitus, rheumatologic disorders, hematologic disorders, renal failure, liver cirrhosis, intravenous drug use, or HIV infection.2,4,5 With staphylococcal infections increasing,6 including MRSA and community-acquired MRSA, the incidence of pyomyositis may increase correspondingly.
Consider the possibility of pyomyositis in patients with localized muscle pain and tenderness and who have risk factors for acquiring MRSA—living in a community with prevalent MRSA, recent hospitalization, antibiotic use, invasive procedures, or chronic venous catheters.
Most commonly, pyomyositis occurs in a leg or arm.2 In the 330 cases mentioned above, the sites of infection, in descending order of frequency, were lower extremity, upper extremity, buttocks, chest wall, paraspinal, and psoas.