Inflammatory muscle diseases

The three major immune-mediated inflammatory myopathies, dermatomyositis (DM), polymyositis (PM) and inclusion body myositis (IBM), each have their own distinctive clinical features, underlying pathogenetic mechanisms and patterns of muscle gene expression. In DM a complement-dependent humoral process thought to be initiated by antibodies to endothelial cells results in a microangiopathy with secondary ischemic changes in muscles. On the other hand, in PM and IBM there is a T-cell response with invasion of muscle fibers by CD8+ lymphocytes and perforin-mediated cytotoxic necrosis. In IBM degenerative changes are also a feature and comprise autophagia with rimmed vacuole formation and inclusions containing beta-amyloid and other proteins whose accumulation may be linked to impaired proteasomal function. The relationship between the inflammatory and degenerative component remains unclear, as does the basis for the selective vulnerability of certain muscles and the resistance to conventional forms of immunotherapy in most cases of IBM. Patients with DM or PM usually respond to treatment with glucocorticoids and immunosuppressive agents but their use remains largely empirical. Intravenous immunoglobulin therapy can be used to achieve disease control in patients with severe weakness or dysphagia, or in patients with immunodeficiency, but its use is limited by expense. Emerging therapies for resistant cases include TNFalpha inhibitors (etanercept, infliximab) and monoclonal antibodies (rituximab, alemtuzumab). However, experience with these therapies is still limited and there is a need for randomized trials to test their efficacy and establish guidelines for their use in clinical practice.