Diagnostic evaluation: Know the stages
Making the correct diagnosis at the initial presentation is difficult because nonspecific symptoms resemble a muscle strain. But this infection progresses to bacteremia, making early identification important. To recognize pyomyositis before bacteremia develops, rely on clinical suspicion and familiarity with the 3 stages of the disease.
Stage 1 (invasive) is characterized by muscle pain and tenderness without systemic evidence of illness. Stage 2 (suppurative) is the formation of an abscess. At this time systemic symptoms such as fever may occur; otherwise physical exam findings remain vague with tenderness and induration without overlying skin redness or palpable fluctuance common to skin abscesses. Stage 3 is bacteremia or sepsis. Awareness of the risks and the stages of pyomyositis will aid in quicker diagnosis.
Diagnostic delay occurs because the first stage presents with nonspecific symptoms and examination findings, and also because initial laboratory and radiographic testing are inconclusive. Elevation of the white blood cell count and the erythrocyte sedimentation rate occur early, while serum muscle enzymes remain normal.2,4,5
Imaging studies performed during the first stage may not be productive; however, in the second stage imaging identifies the abscess. Although limited data exist regarding the best imaging modality, MRI and CT appear most useful.1,3,4,7 In 1 retrospective analysis, ultrasound identified 5 of 8 cases correctly, while MRI and CT were diagnostic in 5 of 6 and 9 of 9, respectively.7