Which nonautoimmune myopathies are most frequently misdiagnosed as myositis?

To discuss the spectrum of nonautoimmune myopathies that may be misdiagnosed as autoimmune myopathy.

Inherited myopathies, such as dysferlinopathy, calpainopathy, and facioscapulohumeral dystrophy may be misdiagnosed as autoimmune myopathy, especially when they have inflammatory muscle biopsies. Inclusion body myositis is frequently misdiagnosed as polymyositis when rimmed vacuoles are absent on muscle biopsy, and a careful neuromuscular evaluation is not performed. Hypothyroid myopathy can be misdiagnosed as immune-mediated necrotizing myopathy if thyroid function tests, including a T4 level, are not obtained. Self-limited statin myopathy can be distinguished from statin-associated autoimmune myopathy because patients with the former do not have autoantibodies recognizing 3-hydroxy-3-methylglutaryl-coenzyme A reductase.

Autoimmune myopathies can usually be distinguished from nonautoimmune myopathies based on a combination of the patient history, neuromuscular exam, laboratory findings, and/or muscle biopsy features.