Diagnostic Criteria Definite inclusion body myositis

Patients must exhibit all of the muscle biopsy features including invasion of non-necrotic fibers by mononuclear cells, vacuolated muscle fibers, and intracellular (within muscle fibers) amyloid deposits or 15-18 nm tubulofilaments

None of the other clinical or laboratory features are mandatory if the muscle biopsy features are diagnostic Possible inclusion body myositis

If the muscle shows only inflammation (invasion of non-necrotic muscle fibers by mononuclear cells) without other pathologic features of inclusion body myositis, then a diagnosis of possible inclusion body myositis can be given if the patient exhibits the characteristic clinical and laboratory features

Modified from Griggs RC, Askanas V, DiMauro S, et al: Inclusion body myositis with IVIg: a double-blind, placebo-controlled study [see comments]. Neurology 1997; 48:712-716.

Muscle biopsy alone can make the diagnosis of IBM but may not always show the typical pathologic features of the disease, even in patients with clinically typical presentations. Additional sections, additional stains (i.e., Congo red to assess for amyloid deposits), or another biopsy may increase this yield. [124

Management. IBM is generally resistant to all therapies, but a trial of corticosteroids and one aggressive immunosuppressive agent is probably reasonable. Although initial reports suggested IVIg was useful in IBM, subsequent studies, including one with placebo control, have shown response in only about one third of patients. W , [125J IVIg is given in a dose of 2 g/kg divided over 2 to 5 days followed by monthly maintenance therapy. Adverse events such as fever, rash, and pruritus can be relieved by slowing the infusion rate or prevented by pretreatment with acetaminophen and diphenhydramine. Anaphylactic reactions to the small amount of IgA contained in IVIg have been reported in patients with congenital IgA deficiency. There is no way to predict a positive response to therapy, and whether the potential benefits warrant the high cost of IVIg remains unclear.^ , [126J

Prognosis and Future Perspectives. Despite treatment, IBM is typically a slowly progressive disease. Further studies with currently available agents may require larger numbers of patients and a longer duration of treatment to demonstrate a clear pattern of benefit or predict which subset of patients may respond. A better understanding of the etiologic mechanisms of the unusual constellation of pathologic findings will likely tailor new approaches to treatment.

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