Immune myopathy with large histiocyte-related myofiber necrosis

Objective To describe the features of a new, pathologically distinctive, acquired myopathy with an unusual pattern of scattered necrotic muscle fibers that are neighbored, surrounded, or invaded, by large, often multinucleated, histiocytic cells.

Methods Retrospective review of records and muscle pathology of 4 patients.

Results Clinical features common to our patients included muscle pain and proximal, symmetric, moderate to severe, weakness in the arms and legs progressing over 1–4 weeks. Patients had other associated systemic disorders, including anemia in all, and hemophagocytic lymphohistiocytosis, hepatic disease, Raynaud phenomenon, metastatic cancer, and cardiomyopathy, in 1 patient each. Serum creatine kinase (CK) levels at presentation were very high, ranging from 10,000 to 102,000 U/L. Three patients improved within 3 months after treatment. Muscle pathology included scattered necrotic muscle fibers with cytoplasm that stained for C_5b-9 complement, especially around fiber peripheries, pale on nicotinamide adenine dinucleotide and often dark on hematoxylin & eosin. Large, often multinucleated, cells with features of histiocytes, including anatomical features on electron microscopy and immunostaining for major histocompatibility complex Class I and histiocyte markers (HAM56, CD68, CD163, and S100), were usually closely apposed to the surface of, or invaded, necrotic myofibers.

Conclusions Patients with large-histiocyte-associated myopathy (LHIM) had a subacute onset of proximal predominant weakness, associated systemic disorders, very high serum CK, and a pathologically distinctive pattern of large histiocyte-associated muscle fiber necrosis. LHIM may be caused by an autoimmune, histiocyte-mediated attack directed against muscle fibers.