Inflammatory Myopathies, Myositis

The idiopathic inflammatory myopathies (IIM), also known as myositis, are a heterogeneous group of rare autoimmune diseases of varying prognosis. The IIM:s are characterised by:

  • Proximal muscle weakness and in case of dermatomyositis also skin abnormalities.
  • Inflammatory cell infiltrates in skeletal muscle biopsies except for the polymyositis subtype Necrotizing autoimmune myopathy (NAM).
  • Presence of circulating myositis-specific/associated autoantibodies.

The most common subclassification of IIM in adults includes:

  •  Polymyositis of which Necrotizing auto-immune myopathy often associated with cancer is a subtype
  • Dermatomyositis, often associated with cancer
  • Sporadic inclusion body myositis (sIBM)
  • Myositis overlapping with another connective tissue disorder (myositis/CTD-overlap)

​Myositis may also present in children, the most common subgroup being juvenile dermatomyositis (JDM).​

Myositis subsets may also be subclassified based on autoantibody profiles which are associated with distinct clinical phenotypes [1. Hengstman, G. J., R. Brouwer, et al., 2002].

​Clinical features

Characteristic clinical features of polymyositis and dermatomyositis are the insidious onset of symmetrical muscle weakness and a decreased muscle endurance in proximal muscle groups. Swallowing difficulties are also frequently encountered. Typically, problems with the neck, shoulder, pelvic, and thigh muscles create difficulties in getting up from a chair, walking stairs, and working with the arms above the head.

​Sporadic inclusion body myositis differs clinically due to its involvement, not only of proximal thigh muscles, but also of distal muscles – mainly finger flexors and foot extensors. Furthermore, muscle atrophy may be pronounced. In contrast to polymyositis and dermatomyositis where muscle fatigue is often a major problem, patients with inclusion body myositis often have a severe weakness of their thigh muscles and a history of falling and fractures. Here too, swallowing difficulties even as presenting feature, are frequently experienced.

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