Aschman T, Schneider J, Greuel S, et al. - In this case-control study of patients who died with and without COVID-19, the majority of those with severe COVID-19 had mild to severe myositis. Skeletal muscle inflammation was more pronounced than cardiac inflammation and was related to the duration of illness. Most skeletal and cardiac muscles had low or negative viral load detection, which was most likely due to circulating viral RNA rather than genuine infection of myocytes. This suggests that SARS-CoV-2 is linked to a postinfectious, immune-mediated myopathy.

• There were 43 COVID-19 patients (median [interquartile range] age, 72 [16] years; 31 men [72% ]) and 11 patients with diseases other than COVID-19 (median [interquartile range] age, 71 [5] years; 7 men [64%]).

• Patients who died from COVID-19 had a higher overall pathology score and a higher inflammation score in their skeletal muscle samples.

• Relevant expression of MHC class I antigens on the sarcolemma was identified in 23 of 42 COVID-19 patient specimens (55%) and upregulation of MHC class II antigens in 7 of 42 COVID-19 patient specimens (17%), but neither was found in any of the controls.

• Natural killer cells were discovered to be in higher quantities.

• Skeletal muscles had more inflammatory features than cardiac muscles, and inflammation was more apparent in COVID-19 patients with chronic courses.

• SARS-CoV-2 RNA was discovered in certain muscle specimens by reverse transcription-polymerase chain reaction, although immunohistochemistry and electron microscopy revealed no evidence of direct viral infection of myofibers.