Overlap myositis, also known as myositis overlap syndrome, is a loosely defined heterogenous group of conditions characterized by myositis, that fulfills criteria for an idiopathic inflammatory myopathy, in association with extramuscular clinical features overlapping with a systemic connective tissue disease (e.g. systemic sclerosis, mixed connective tissue disease), often in the presence of specific antibodies.
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Epidemiology
The exact incidence of overlap myositis is unknown, likely because it has a very varied, and sometimes subtle, clinical phenotype that is difficult to define and diagnose. It is known, however, that overlap myositis is more common among females 2.
The most commonly associated systemic connective tissue diseases are systemic sclerosis, sometimes known as ‘scleromyositis’, and mixed connective tissue disease 1,2.
Clinical presentation
Overlap myositis can have a varied clinical presentation, encompassing 1-5:
myositis/myopathy
extramuscular clinical features, e.g. arthritis, interstitial lung disease, Raynaud phenomenon, sclerodactyly, hematological dyscrasias, dysphagia, etc.
Pathology
The pathogenesis of overlap myositis varies depending on its underlying associated connective tissue disease, whereby it is thought that the pathogenesis is that of that underlying condition. A number of systemic connective tissue disease associations are possible in overlap myositis, including 1-5:
systemic sclerosis, with presence of overlap myositis associated with anti-PM/SCl, anti-Ku, anti-U3-RNP (anti-fibrillarin), anti-RuvBL1/2
mixed connective tissue disease, with presence of overlap myositis associated with anti-U1-RNP
systemic lupus erythematosus, with presence of overlap myositis not associated with any specific antibody
rheumatoid arthritis, with presence of overlap myositis not associated with any specific antibody
Sjögren syndrome, with presence of overlap myositis associated with anti-Ro52 (anti-TRIM21), anti-PUF-60
Radiographic features
The radiographic features are those generally of an idiopathic inflammatory myopathy and those of the specific other extramuscular clinical features (e.g. interstitial lung disease, Raynaud phenomenon).
Treatment and prognosis
Specific management depends on the underlying associated systemic connective tissue disease. Generally, immunosuppression is utilized 2,3, with corticosteroids being the most commonly used treatment option 3.
Prognosis, in terms of survival, is thought to be similar to idiopathic inflammatory myopathy 3.
History and etymology
The concept of overlap myositis as a distinct form or classification of myositis was first proposed in 2005 5.
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