These are common diseases that affect primarily the striated muscle and skin and sometimes connective tissues. The term used to describe the disease reflects the tissues involved. If the inflammatory changes are restricted clinically to the striated muscles, the disease is called polymyositis (PM); if, in addition, the skin is involved, it is called dermatomyositis (DM), although the two diseases are now understood to be immunopathologically distinct and some authorities question the existence, or at least the frequency, of an independent idiopathic polymyositis. Either may be associated with a rheumatologic disorder, in which case the designation is PM or DM with rheumatoid arthritis, rheumatic fever, lupus erythematosus, scleroderma, Sjögren syndrome, or mixed connective tissue disease, as the case may be. There is also an important but inconsistent relationship of these myositides and systemic carcinoma, as discussed further on. Dalakas (2015) has provided a comprehensive review of the subject.
Both diseases have been known since the nineteenth century. Polymyositis was first described by Wagner in 1863 and 1887, and DM was established as an entity by Unverricht in a series of articles written from 1887 to 1891. A modern classification introduced in the monograph of Walton and Adams included categories associated with neoplasia and with connective tissue diseases. References to the original articles and a survey of the literature since that time can be found in the monograph of Kakulas and Adams and in the chapters on the PM and DM syndromes by Engel and colleagues.
It is mentioned above and emphasized further on that there is disagreement regarding the frequency of PM as an independent entity. Amato and Griggs have expressed the opinion that many cases so classified are a result of DM, an immune necrotizing myopathy commented on below, or IBM, or are related to an underlying connective tissue disease. Even other cases are examples of muscular dystrophy with secondary inflammatory changes. The main point of controversy has been the proposal they favor, that isolated PM is rare and overdiagnosed (see van der Muelen et al).
Inflammatory myopathy coexists with numerous systemic diseases as discussed, and some authors consider it to be a syndrome rather than a disease. The current authors continue to see a few well-studied and convincingly documented cases of “classic” PM that are unassociated with other disease.
Recently added to the traditional group of inflammatory myopathies is an increasingly recognized immune-mediated necrotizing myopathy (IMNM); these are instances of myopathy that were previously classified as either dermatoor polymyositis but are now recognized as being the result of antibodies to anti-signal recognition particle (SRP), and some cases of necrotizing myopathy that are due to statins are similarly caused by antibodies directed at HMGCoA reductase, and not a direct toxic effect of the medication. This emphasizes that clinicians should conduct a careful evaluation before concluding that a patient has idiopathic polymyositis.
This is the representative example of inflammatopry myopathy. The onset is usually insidious and the course progressive over a period of several weeks or months. It may develop at almost any age and in either sex; however, the majority of patients are 30 to 60 years of age, and a smaller group shows a peak incidence at 15 years of age; women predominate in all age groups. A febrile illness or benign infection may precede the weakness, but in most patients the first symptoms develop in the absence of these or other apparent initiating events.