Primary pyomyositis caused by ca-MRSA.

A 22-year-old male with sickle-cell trait presented with a 1-week history of progressive pain and swelling in his left posterior thigh. He denied any trauma, fever, travel outside of the United States, or intravenous drug usage. On examination, he was afebrile at 36.8 °C (98.3 °F) with stable vital signs. His examination was significant for exquisite tenderness, erythema, induration, and swelling of the posterior thigh (Fig. 1). A computed tomography scan of the extremity was obtained that showed a 5.5 cm × 4.4 cm × 20-cm fluid collection within the biceps femoris muscle (Fig. 2). The patient underwent open drainage of the abscess and was placed on linezolid. Wound and blood cultures were consistent with community-associated methicillin-resistant Staphylococcus aureus (ca-MRSA).

Fig. 1

Erythema and swelling of posterior thigh (arrows)

Fig. 2

CT of leg showing a large fluid collection (arrows)

Pyomyositis is an acute bacterial infection of the muscles characterized by subacute abscess formation. While pyomyositis is rare in temperate climates, the incidence has been increasing, with the majority having comorbidities causing an immunocompromised state, such as HIV and diabetes [1]. Sickle-cell disease has also been implicated as a risk factor for developing pyomyositis [2-4].

Staphylococcus aureus has been implicated in the majority of cases [1], and recently, ca-MRSA has been found to be an important causative agent in pyomyositis [5, 6]. Because of the rarity of pyomyositis in temperate climates, the diagnosis can be difficult to make, and the patient’s symptoms and exam can easily be mistaken for other diagnoses, such as cellulites [7]. Physicians should consider pyomyositis in patients with severe muscle tenderness in areas of apparent cellulitis and recognize ca-MRSA as an emerging cause of this potentially devastating disease.