A magnetic resonance imaging scan of his arms and legs did not reveal any abnormalities. A needle electromyography did not show signs of irritable myopathy.
Despite these unremarkable test results, we suspected immune-mediated necrotizing myopathy based on otherwise unexplainable myalgia and CK elevation, as well as the history of daily consumption of shiitake mushrooms. We performed muscle biopsy of the left biceps brachii and obtained blood for further analysis for autoantibodies associated with idiopathic inflammatory myopathies.
The muscle biopsy (see Figure 1) showed one necrotic and several internally nucleated fibers, in addition to moderate upregulation of MHC class I staining, raising a possibility of immune-mediated necrotizing myopathy—albeit complement C5b-9 was not deposited on the sarcolemma. The autoantibody analyses using enzyme-linked immunosorbent assay (ELISA) revealed both anti-HMGCR and anti-SRP antibodies were positive (antibody titer 2.0 IU/mL and 1.4 IU/mL, respectively; cutoff 1.0 IU/mL).