Dermatomyositis and polymyositis

Abstract

Dermatomyositis (DM) and polymyositis (PM) are idiopathic inflammatory myopathies characterized by proximal greater than distal muscle weakness, elevated serum creatine kinase levels, electrophysiologic abnormalities, and inflammation on muscle biopsy. Clinically and electrophysiologically, DM and PM appear very similar, and muscle biopsy is the gold standard for diagnosis. Much of the PM literature based the diagnosis on Bohan and Peter's criteria, which is now obsolete given the advances of immunopathology. As diagnostic criteria for the inflammatory myopathies have been refined, it has become apparent that PM is much less common than previously thought, and, in fact, is probably quite rare. More recent literature, using strict histopathologic criteria for diagnosis of PM, has brought into question previously reported associations. Because of this, the clinical entity of PM is poorly defined. The exact incidence of each is unknown because previous epidemiologic studies often grouped them together, but overall the annual incidence of the inflammatory myopathies is approximately one in 100,000. DM and PM respond to immunomodulating therapies. High-dose oral prednisone is generally accepted first-line therapy. In patients who do not respond adequately to prednisone alone, or in whom prednisone cannot be weaned, methotrexate or azathioprine can be added. In the authors' experience, methotrexate works faster and is more effective than azathioprine. However, because of the increased risk of interstitial lung disease with methotrexate, the authors avoid this in patients with anti-Jo-1 antibodies and, obviously, in patients who already have pulmonary disease. If patients do not respond adequately to the combination of prednisone and methotrexate or azathioprine, a trial of intravenous immunoglobulin is administered.

Inflammation induces tau pathology in inclusion body myositis model via glycogen synthase kinase‐3β

“…However, in polymyositis, intracellular amyloid deposits and tau hyperphosphorylation are not present despite the occurrence of increased CD8 ϩ T-cell infiltrates and MHC class I expression. 36,37 Consequently, it is likely that additional factors may be involved in the development of IBM. Our data from chronic LPS administration to PS1 knock-in mice also indicate that increased inflammation by itself does not cause motor dysfunction.…”

Inclusion body myositis: old and new concepts

“…However, these criteria were developed before IBM was widely recognised and cases of IBM would have been misdiagnosed as PM with Bohan and Peter criteria 14 15. Revised criteria for the various idiopathic inflammatory myopathies have been devised to take into account the recent advancements in the field 10 16.…”

Case Report: Elevated CPK, an indicator of idiopathic inflammatory myopathy?

“…However, several epidemiologic features make this a particularly difficult diagnosis in this patient. Foremost, polymyositis is a rare disease with incidence rates occurring in about 1 per 100,000 people annually 4 – 7 . Additionally, polymyositis is seen twice as commonly in women than in men.…”