Polymyositis (PM) and dermatomyositis (DM) are uncommon systemic rheumatic disorders characterised by inflammation and degenerative changes in the muscles (polymyositis) or in the skin and muscles (dermatomyositis).
Diagnosis is by clinical findings and abnormalities on muscle tests, including muscle enzymes, electromyography, magnetic resonance imaging (MRI), and muscle biopsy. Treatment is with corticosteroids, sometimes combined with immunosuppressants or intravenous immune globulin.
Laboratory testing can increase or decrease suspicion for the disorder. They support assessment of severity, help to identify PM/DM overlap syndrome, and help detect complications. Autoantibodies to be tested are antinuclear antibodies (ANA), which are positive in up to 80% of patients. Detailed ANA testing is important in identifying overlap syndrome with another autoimmune disease.
About 30% of PM/DM patients have myositis-specific autoantibodies: aminoacyl-tRNA synthetase antibodies, including Jo-1 antibodies. SRP antibodies and Mi-2 antibodies can also be found. The relationship between these autoantibodies and disease pathogenesis remains unclear, although antibodies against Jo-1 are significant markers for fibrosing alveolitis, Raynaud’s syndrome, pulmonary fibrosis, and arthritis.