Inclusion body myositis is an insidious, progressive inflammatory and degenerative muscle disorder causing weakness of skeletal muscles. It is histologically characterized by the non-necrotic inflammatory cell invasion of muscle fibers, the presence of abnormal rimmed vacuoles, intra- or extravacuolar congophilic inclusion protein deposits, and an increased number of mitochondrial cytochrome c oxidase negative fibers.

It is the most common acquired inflammatory muscle disorder in adults older than 50 years. Asymmetric proximal and/or distal muscle atrophy and weakness are typically seen. Classically, this first is noted in the quadriceps muscles, with weakness in knee extension more than flexion, resulting in difficulty with walking, climbing, and rising from a seated position. This may eventually predispose the patient to falls. Distally, asymmetric finger flexor and wrist weakness interfere with gripping. The biceps, triceps, facial muscles, and muscles of deglutition are commonly involved. Rarely, respiratory insufficiency may occur if the diaphragm is involved.

Men are more commonly affected than women. While most cases occur sporadically, rare familial occurrences have been reported. There is a genetic association with HLA-DRB1. A subgroup of individuals will be positive for cytosolic 5'-nucleotidase 1A (cN1A) autoantibodies. While this autoantibody is fairly specific for inclusion body myositis, it may occur in other autoimmune diseases.

There is no effective treatment. Management consists largely in easing symptoms, exercise therapy, use of mobility devices, and medications. Immunosuppressive medications such as corticosteroids, methotrexate, azathioprine, and intravenous immunoglobulin (IVIG) may be tried. These medications do not alter the long-term natural course of the disease.

Typically, life expectancy is not directly affected. However, those with cN1A antibodies may have a higher mortality risk. Significant morbidity does occur. As muscle weakness progresses, most individuals lose the ability to ambulate and require a wheelchair, especially after extended periods of disease duration. Dysphagia may become severe enough to interfere with nutritional needs, requiring alternative feeding methods. As respiratory muscle function declines, the risk for pneumonia may increase.