Jones, Lyell K.

Abstract Introduction/Aims Intracellular congophilic inclusions within muscle fibers, although nonspecific, are one of the pathological hallmarks of sporadic inclusion body myositis (sIBM). Extracellular amyloid deposits in muscle, on the other hand, are the canonical findings of amyloid myopathies, which occur with or without systemic amyloidosis. Methods We reviewed the muscle biopsy database (1998–2020) to identify sIBM patients with extracellular amyloid deposits. Clinical and laboratory data were reviewed. Results We identified five sIBM patients (three clinicopathologically defined and two clinically defined) with extracellular amyloid deposits in muscle. Mean age at diagnosis was 74.8 y (range, 68–84 y). All patients had a typical sIBM pattern of weakness without associated sensory or autonomic symptoms. None had electrophysiological evidence of peripheral neuropathy. Only one patient had a monoclonal gammopathy (immunoglobulin M‐lambda, IgM‐λ) with normal bone marrow biopsy. This patient with monoclonal gammopathy and three other patients underwent abdominal fat pad aspirate and were negative for amyloid. Cardiac evaluation was unrevealing in the four patients tested. Three patients without monoclonal gammopathy had normal transthyretin gene sequencing and inconclusive mass spectrometry‐based analysis. The patient with monoclonal gammopathy died of pneumosepsis 5 y after diagnosis and autopsy revealed multi‐organ transthyretin amyloidosis. Discussion Detection of extracellular amyloid deposition in muscle should trigger an aggressive search for systemic amyloidosis independently from other associated myopathological abnormalities. Amyloid subtyping is crucial for early therapy and mortality prevention. An isolated monoclonal gammopathy should not halt a search for non‐hematological causes of systemic amyloidosis.

Introduction: Glial fibrillary acidic protein (GFAP) immunoglobulin G is a recently discovered biomarker of an autoimmune central nervous system disorder characterized by a steroid-responsive meningoencephalomyelitis. Case Report: A 63-year-old man with rheumatoid arthritis on etanercept presented with steroid-responsive subacute encephalopathy and foot drop. Brain and sural nerve biopsies demonstrated a T-cell perivascular infiltrate. Cerebrospinal fluid studies 18 months into the course of the illness demonstrated a GFAP antibody on mouse tissue immunofluorescence confirmed by cell-based assay. The patient was treated with steroids and cyclophosphamide leading to resolution of his symptoms. Conclusion: This case expands on the previously reported cases of GFAP immunoglobulin G autoimmunity by describing an associated inflammatory large fiber peripheral neuropathy.