Myositis Definition And Classification

Twenty years ago Bohan et al. [1] proposed five primary criteria for myositis: symmetrical weakness of proximal limb muscles, elevated serum levels of enzymes present in skeletal muscles, specific electromyographic (EMG) myopathic modifications, abnormal muscle biopsy with muscle fiber necrosis and inflammatory cell infiltrates, associated in DM with specific cutaneous modifications such as: eyelids lilac discoloration and erythema on the extension side of both metacarpophalangeal and proximal in-terphalangeal joints. In reality, any of these criteria can be absent, especially those pertaining to muscle histological modifications, due to the heterogeneous distribution of inflammatory sites as documented by recent MRI studies [2]. Purely cutaneous forms have also been described and called "dermatomyositis sine myositis" [3, 4].

Bohan et al. [1] proposed to classify myositis into five subgroups: primary idiopathic polymyositis (PM), primary idiopathic dermatomyositis (DM), dermatomyositis (or polymyositis) associated with malignancy, childhood DM or PM and myositis overlapping with another connective tissue disease [1], This classification is simple and easy to use, but it has some drawbacks, especially the fact that some individual patients may be classified in more than one subgroup. Furthermore, it does not individualize inclusion body myositis (IBM), whose diagnosis is suggested by "atypical" manifestations, such as distal muscle involvement at times asymmetrical, discrete muscle enzyme modifications, neurological EMG alterations, resistance to both steroid and immunosuppressive therapies, and confirmed by characteristic features on optical and more specifically electron microscopy [5, 6], Macrophage myofasciitis has been described too recently to be included in any of the proposed classification for myositis [7].

Although frequently studied together, DM and PM probably result from distinct processes. In DM, the elementary muscular lesion is microvasculitis with B and T-CD4 lymphocytic infiltrates, whereas in PM there are no endothelial abnormalities and the cellular infiltrate is made of T-CD8, macrophages and natural killer cells [6, 8, 9]. Therefore, it is thought that immune dysfunction is mainly humoral in DM, and cellular in PM [6]. In 1991, based on this observation, Dalakas reclassified myositis into three groups: DM, PM and inclusion body myositis [6]. This classification however does not take into account "specific" myositis autoantibodies (ab), mainly those directed against certain aminoacyl-tRNA synthetases (antisyn-thetases), predominantly anti-Jol ab. Patients who carry these antibodies have indeed been identified as having distinctive clinical and genetic features [9, 10],

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