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].
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.