Overlap Myositis

Overlap myositis (OM) is a form of idiopathic inflammatory myopathy (IIM) characterized by myositis with at least one clinical and/or autoantibody overlap feature.

Epidemiology

Prevalence and annual incidence of OM are not known. Estimates are difficult to determine because of low recognition levels of this form of IIM.

Clinical description

Overlap myositis is a clinically heterogeneous, poorly recognized subtype of inflammatory myopathy. Patients with myositis and one clinical and/or autoantibody overlap feature are considered to have OM, excluding pure polymyositis and pure dermatomyositis (see these terms). Possible clinical overlap features include polyarthritis, Raynaud phenomenon, sclerodactyly, scleroderma (proximal to metacarpalphalangeal joints), lung interstitial pneumonia, and/or clinical signs of systemic lupus erythematosus (SLE). Overlap autoantibodies include those observed with scleroderma, SLE, and/or some inflammatory myopathies such as anti-synthetase syndrome (see these terms).

Etiology

OM covers a range of inflammatory myopathies and other connective tissue diseases for which the etiology is generally poorly understood.