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Dermatomyositis at a Glance
  • Dermatomyositis (DM) is thought to evolve through multiple sequential phases: (1) a genetically determined susceptibility phase, (2) an induction phase triggered by an environmental stimulus (e.g., ultraviolet light, infection) involving loss of tolerance to self-antigens in skin and skeletal muscle, (3) an autoimmune expansion phase, and (4) an injury phase involving multiple immunologic effector mechanisms.
  • All clinical types of the idiopathic inflammatory dermatomyositis are rare orphan diseases.
  • Some clinical subphenotypes of DM such as clinically amyopathic DM have been underdiagnosed in the past, often being confused with isolated forms of cutaneous lupus erythematosus.
  • Clinical features are a function of age of onset (e.g., juvenile-onset classic DM: vasculopathy, dystrophic calcification; adult-onset classic DM: associated occult malignancy, interstitial lung disease).
  • The sine qua non of DM is a hallmark constellation of inflammatory skin changes presenting as patchy or confluent, macular, violaceous erythema that assumes a highly characteristic anatomic distribution and is often photoaccentuated. With time, additional diagnostic skin changes appear, including Gottron papules, prominent periungual nail fold microvascular changes, and poikiloderma atrophicans vasculare.
  • Joint disease (arthritis/arthralgia), pulmonary disease (interstitial lung disease, aspiration pneumonia), esophageal disease (dysphagia), vasculopathic injury of multiple organ systems (gastrointestinal system, central nervous system, eye), and cardiac disease (conduction defects, cardiomyopathy) may be related features.
  • Pathologic features include the following: in skin—a cell-poor interface dermatitis with dermal mucin accumulation; in muscles—a characteristic pattern of myositis.

The idiopathic inflammatory myopathies (IIDMs) are a heterogeneous group of genetically determined autoimmune disorders that predominately target the skeletal musculature and/or skin and typically result in symptomatic skeletal muscle weakness and/or cutaneous inflammatory disease. Some physicians continue to refer to this group of clinical disorders by an earlier designation, idiopathic inflammatory myopathies. Among the IIDM, dermatomyositis (DM) is of special interest to the dermatologist due the universal presence of a hallmark pattern of cutaneous inflammation. DM is the only idiopathic inflammatory myopathy (IIM) that expresses a characteristic pattern of primary cutaneous inflammation.

In this chapter, the term classic DM is used to refer to DM as traditionally defined (i.e., the concurrence of myositis resulting in clinically significant proximal muscle weakness and a set of hallmark inflammatory skin lesions occurring in a specific anatomical distribution). Also of interest to the dermatologist is the subgroup of patients who express the hallmark cutaneous manifestations of DM for prolonged periods (6 months or longer) without developing clinically evident muscle weakness. This pattern of DM expression is referred to as clinically amyopathic DM (CADM); this term is synonymous with the historical designation, dermatomyositis sine myositis.

On occasion, other organ systems, such as the lungs (interstitial lung disease), joints (arthritis), and heart (cardiomyopathy, conduction defects), are targeted by inflammatory injury in IIM patients. In addition, differences exist between classic DM presenting in adults and that presenting in children. For example, juvenile-onset classic DM is associated with higher rates of vasculopathy/vasculitis resulting in the complication of dystrophic calcification, whereas adult-onset classic ...

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