Anti‐Signal Recognition Particle Antibody‐Positive Immune‐Mediated Necrotizing Myopathy Diagnosed by Re‐Evaluation 40 Years After Symptom Onset

Immune‐mediated necrotizing myopathy (IMNM) is a frequent subtype of idiopathic inflammatory myopathies. Distinguishing juvenile‐onset IMNM from muscular dystrophy is often difficult, particularly when the disorder takes a chronic course. We describe a 45‐year‐old Japanese woman with progressive generalized weakness. Her early motor milestones were normal until age 4, but by 5 years she developed Gowers' sign. At that time, she was noted to have proximal weakness, scapular winging, gait impairment, and markedly elevated serum creatine kinase (6069 IU/L). A biopsy of the left biceps brachii suggested muscular dystrophy. Her disease advanced, and she required a wheelchair during elementary school. At age 41 she was referred to the neurology department at Tottori University Hospital, and at 45 years she was re‐evaluated. Examination revealed symmetric proximal weakness, atrophy, severe spinal deformity, and bilateral pes cavus. Electromyography demonstrated prominent myogenic changes, and muscle MRI disclosed profound atrophy with fatty replacement of proximal muscles. A repeat biopsy from the left tibialis anterior showed severe myofiber atrophy but no necrosis, regeneration, or inflammatory infiltration. Immunohistochemistry for dystrophy‐associated proteins, MHC class I, and p62 was unremarkable. In contrast, retrospective review of the original biopsy revealed diffuse MHC class I expression and granular sarcoplasmic p62 staining, supporting a diagnosis of IMNM. Although ELISA results were negative, RNA immunoprecipitation identified anti‐signal recognition particle antibodies. This patient illustrates an uncommon presentation of juvenile‐onset IMNM that remained untreated for decades. The absence of characteristic histological features on biopsy performed 40 years after disease onset emphasizes the diagnostic challenges in long‐standing cases. Careful reassessment of earlier tissue samples, in combination with extended antibody testing, may be essential for establishing the correct diagnosis in similar situations.