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  • Inclusion Body Myositis: eMedicine Neurology

    http://emedicine.medscape.com/article/1172746-overview[1/21/2010 12:58:29 PM]

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    eMedicine Specialties > Neurology > Neuromuscular Diseases

    Inclusion Body MyositisAuthor: Helen C Lin, MD, Assistant Professor of Neurology, Medical College of Wisconsin, MilwaukeeCoauthor(s): Paul E Barkhaus, MD, Professor, Department of Neurology, Medical College of Wisconsin; Director ofNeuromuscular Diseases, Milwaukee Veterans Administration Medical Center; Michael P Collins, MD, Associate Professor,Department of Neurology, Medical College of Wisconsin; M Isabel Periquet Collins, MD, Assistant Professor, Department ofNeurology, Medical College of WisconsinContributor Information and Disclosures

    Updated: Sep 3, 2009

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    Introduction

    Background

    Sporadic inclusion body myositis (s-IBM) and hereditary inclusionbody myopathies (h-IBM) encompass a group of disorders sharingthe common pathological finding of vacuoles and filamentousinclusions. They collectively demonstrate a wide variation in clinicalexpression, age of onset, associated diseases, and prognosis. Thisarticle focuses on s-IBM. For discussion of h-IBM, the reader isreferred to other sources.1,2

    The term inclusion body myositis was originally used by Yunis andSamaha in 1971 for a case of myopathy that phenotypically suggested chronic polymyositis but showedcytoplasmic vacuoles and inclusions on muscle biopsy. In the subsequent years, s-IBM has been increasinglyrecognized and reported, primarily because of increased awareness of the condition and improved histologictechniques. A relatively common myopathic process, s-IBM is an important diagnostic consideration in theevaluation of progressive weakness in older Caucasian males.

    Expression of s-IBM is variable, but all cases eventually evolve into a syndrome of diffuse, progressive,asymmetric, proximal, and distal weakness that is generally refractory to immunosuppressive treatment.

    Pathophysiology

    s-IBM has been traditionally classified as one of the idiopathic inflammatory myopathies along withdermatomyositis (DM) and polymyositis (PM). However, the pathologic findings of sporadic inclusion bodymyositis (s-IBM) involve both inflammatory and degenerative characteristics, and the true primary pathogenesisof the disease remains a subject of significant debate. Theoretically, the possibilities include (1) a primary T-cellmediated autoimmune response causing muscle damage, (2) a primary degenerative process involving abnormalprotein processing leading to a secondary inflammatory response, and (3) separate and independent immuneand degenerative processes caused by an external trigger.3

    Inflammatory changes

    s-IBM is characterized by the presence of non-necrotic myofibers invaded by mononuclear inflammatory cells,which, as a pathologic phenomenon, is significantly more common than vacuolated, congophilic, and necroticfibers.4 It is found at all stages of the disease in both treated and untreated patients.

    The endomysial infiltrates in patients with s-IBM are composed primarily of CD8+ T cells and macrophages in a2:1 ratio.5 Myeloid dendritic (antigen-presenting) and CD138+ plasma cells are also present in substantialnumbers,6,7 while B cells and natural killer (NK) cells are rare. T cells, macrophages, and myeloid dendritic cellsall have the potential to invade non-necrotic muscle fibers.8 The autoinvasive CD8+ T cells surround major

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  • Inclusion Body Myositis: eMedicine Neurology

    http://emedicine.medscape.com/article/1172746-overview[1/21/2010 12:58:29 PM]

    histocompatibility complex (MHC) class I-immunoreactive myofibers and express perforin and other markers ofactivation.9,10,11

    Identical autoinvasive T-cell clones can persist over time, even in different muscles,12,13 but the amplifiedsubfamilies sometimes change, which suggests of epitope spreading.14 Collectively, these observations implicatean antigen-driven, MHC class I-restricted, cytotoxic T-cellmediated process directed against myofibers. Thespecific antigens responsible for this reaction are unknown.

    Various chemokin

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