Case Report: When muscle edema is not a myositis flare: Legionella pneumonia complicated by rhabdomyolysis and acute kidney injury in long-standing polymyositis

Legionella pneumophila is an important cause of severe community-acquired pneumonia and can produce extrapulmonary manifestations that obscure the primary diagnosis. Diagnostic error is especially likely in patients with established idiopathic inflammatory myopathy, because fever, myalgia, weakness, creatine kinase elevation and muscle edema may be attributed to autoimmune flare. We report a 70-year-old woman with a 25-year history of polymyositis receiving long-term corticosteroids and azathioprine who developed high fever, severe bilateral thigh pain and progressive proximal weakness after recent travel. On transfer, she had anuria with bladder distension, dark urine, diffuse posterior-thigh muscle edema, pneumonia, hyponatremia, metabolic acidosis, myoglobinemia and acute kidney injury. Legionella pneumonia with rhabdomyolysis and multifactorial acute kidney injury was suspected clinically, and Legionella-active antimicrobial therapy, hydration, urinary drainage and urine alkalinization were started before microbiological confirmation. A targeted sequencing-based respiratory pathogen assay on sputum collected on Day +4 detected L. pneumophila (145 sequences; estimated concentration <10^3 copies/mL) and respiratory syncytial virus A (266,036 sequences; 10^5 copies/mL), raising the possibility of viral-bacterial coinfection. Legionella urinary antigen, targeted PCR and culture were not documented. After infection and renal injury stabilized, methylprednisolone and intravenous immunoglobulin were used because residual inflammatory myopathy activity remained in the differential. However, creatine kinase and myoglobin had already declined, so subsequent improvement cannot be attributed specifically to immunomodulation. Creatinine fell from 538 μmol/L on Day 0 to 110 μmol/L on Day +9 and 78 μmol/L on Day +30; follow-up chest CT on Day +14 showed marked improvement. This case shows that muscle MRI edema is contextual rather than etiologic. In immunosuppressed myositis patients, pneumonia plus hyponatremia, myoglobinuria and renal injury should prompt early Legionella-active treatment while autoimmune activity is reassessed after infection control.