Myopathy and paraproteinemia with serum igm binding to a high‐ molecular‐weight muscle fiber surface protein

Abstract We evaluated a 69‐year‐old man with Waldenström's macroglobulinemia (IgM‐κ M‐protein) and progressive weakness over 2 to 3 years. The neurological examination showed symmetrical, predominantly proximal weakness. Electrophysiological testing revealed small brief motor unit potentials with fibrillations and positive sharp waves consistent with an irritative myopathy. The muscle biopsy specimen showed myopathic changes including variation in fiber size and increased connective tissue but no inflammation. IgM‐κ but not IgM‐λ was deposited along the surface of muscle fiber membranes. Serum IgM‐κ but not IgM‐λ from the patient stained the surface of normal human muscle fibers but not other regions of muscle fibers or other tissues. The serum IgM‐κ at dilutions up to 1: 512,000 bound to a high‐molecular‐weight muscle protein by Western and dot blot studies. By enzyme‐linked immunosorbent assay and Western blot analysis, serum IgM‐κ bound specifically to the muscle protein and not to other muscle or neural antigens, including GM 1 ganglioside, myelin‐associated glycoprotein, and sulfatide. We conclude that the myopathy in our patient is probably related to the presence of serum IgM‐κ antibodies directed against a muscle surface antigen. Characterization of the target antigen, a high‐molecular‐weight protein located specifically in muscle, should further elucidate the pathogenesis of this presumably humorally mediated immune myopathy.