NEJM Makes Myositis as Clear as Possible

Dalakas MC. Review Article: Inflammatory Muscle DiseasesN Engl J Med 2015; 372:1734-1747. April 30, 2015. doi: 10.1056/NEJMra1402225

This review covers dermatomyositis, polymyositis, necrotizing autoimmune myositis, and inclusion-body myositis, including their extramuscular manifestations.

A table conveniently summarizes diagnostic criteria. Diagnosis ultimately depends on muscle biopsy, and three dense figures illustrate the pathology and immune mechanisms. A glossary defines the antibodies anti-cN1A, anti-Jo-1, anti-HMGCR, anti-MDA-5, ANTI-Mi-2, antui-SRP, and anti-TIF-1γ.

In dermatomyositis, activation of complement C3 is an early event. In polymositis and inclusion-body myositis, CD8+ T cells invade muscle fibers that are abnormally expressing MHC class I antigen. In necrotizing autoimmune myositis, macrophages invade necrotic fibers, and antibodies are directed against transport proteins.

Treatment is empirical; controlled trials are few. For dermatomyositis, polymyositis, and necrotizing autoimmune myositis, an initial trial of prednisone is followed by empirical stepped treatment if necessary.

For inclusion-body myositis, all treatments have failed, probably because the degenerative cascade is advanced by the time it is diagnosed. Life expectancy is normal, but most patients require assistive devices.