Acute corticosteroid-responsive post-infection myositis in adults.

Functional disability myalgia is a common condition that frequently leads to significant disability, but its diagnosis is challenging because of its diverse etiologies. While hereditary myalgia has been wells tudied, infection-related myalgia, particularly post-infection forms, remains underrecognized. In this study, six adult patients with severe post-infection myalgia were described. The mean age of onset for these patients was 63.2 ± 16.2 years. All patients experienced myalgia and muscle weakness in the proximal muscles of the lower limbs, which occurred 1-4 weeks after the recovery from symptoms of suspected viral infection, resulting in a significant functional disability. Laboratory tests revealed that creatine kinase levels were not elevated, yet increases in C-reactive protein, erythrocyte sedimentation rate, and interleukin-6 levels were observed. Muscle MRI demonstrated significant edema in the lower limb muscles of 5 patients, while muscle biopsy indicated mild inflammatory myopathy changes in 5 patients. Severe muscle pain were unresponsive to nonsteroidal anti-inflammatory drugs but showed a significant response to steroids, and most patients had favorable prognoses. These clinical features differed from previously documented cases of post-infection myositis, suggesting a distinct subset of infection-related myalgia. Our findings highlighted the importance of recognizing this condition in adults and suggested the need for broader diagnostic criteria to better classify infection-related myopathies.