Overlap myositis

Overlap myositis is a form of idiopathic inflammatory myopathy (IIM) characterized by the combination of features of polymyositis or dermatomyositis with clinical manifestations of another systemic autoimmune connective tissue disease, such as systemic lupus erythematosus, systemic sclerosis (scleroderma), rheumatoid arthritis, Sjögren syndrome, or mixed connective tissue disease. It is also referred to as myositis overlap syndrome or overlap inflammatory myopathy. This condition primarily affects the musculoskeletal system, causing proximal muscle weakness and elevated muscle enzymes (such as creatine kinase), but also frequently involves other organ systems including the skin, lungs (interstitial lung disease), joints (arthritis or arthralgia), and the gastrointestinal tract (dysphagia, esophageal dysmotility). Raynaud phenomenon, mechanic's hands, and sclerodactyly may also be present depending on the associated connective tissue disease. Overlap myositis is commonly associated with specific myositis-specific or myositis-associated autoantibodies, including anti-PM-Scl, anti-U1-RNP, anti-Ku, and anti-synthetase antibodies (such as anti-Jo-1). The presence of these antibodies often helps define the clinical phenotype and may guide prognosis. Interstitial lung disease is a particularly important complication that can significantly affect morbidity and mortality, and screening with pulmonary function tests and high-resolution CT imaging is recommended. Treatment of overlap myositis typically involves immunosuppressive therapy. First-line treatment generally includes systemic corticosteroids (such as prednisone), often combined with steroid-sparing immunosuppressive agents such as methotrexate, azathioprine, mycophenolate mofetil, or calcineurin inhibitors (tacrolimus, cyclosporine). In refractory cases, intravenous immunoglobulin (IVIG), rituximab, or other biologic agents may be considered. Physical rehabilitation and monitoring for organ-specific complications, particularly pulmonary and cardiac involvement, are essential components of long-term management. Prognosis varies depending on the severity of organ involvement and response to treatment, but overlap myositis may carry a better muscular prognosis compared to some other IIM subtypes, though extramuscular manifestations can be challenging.

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