Markedly prolonged compound muscle action potential duration in a patient with coronavirus disease 2019 related critical illness myopathy

A 56‐year‐old man admitted with coronavirus disease 2019 (COVID‐19) became severely ill, required ventilator support and intensive care unit management. After ventilator weaning, he was found to have quadriparesis. Guillain–Barré syndrome (GBS) was suspected and administration of high‐dose intravenous immunoglobulin (IVIg) was considered. However, markedly prolonged compound muscle action potential (CMAP) duration was observed, leading to a diagnosis of critical illness myopathy (CIM). Therefore, IVIg was not given at all, and supportive care was continued. A good functional recovery was obtained. Prolonged CMAP duration is a characteristic finding to CIM. CIM following severe COVID‐19 infection is probably common, although the diagnostic value of prolonged CMAP duration is not widely recognized. This characteristic finding deserves more attention because it contributes to early differentiation between CIM and GBS and the use of IVIg in patients with COVID‐19 may cause thrombotic complications and worsen the prognosis.