Page 786 - Clinical Immunology_ Principles and Practice ( PDFDrive )

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Inflammatory Muscle Diseases

Arash H. Lahouti, Lisa Christopher-Stine

The idiopathic inflammatory myopathies (IIMs) are a group of can help to better estimate the burden of disease and differentiate
rare systemic diseases. They consist of polymyositis (PM), acute changes (edema) from chronic changes (atrophy). The role
dermatomyositis (DM), inclusion body myositis (IBM), and more of new imaging modalities such as whole-body MRI needs to
recently described immune-mediated necrotizing myopathy be further investigated.
(IMNM). Traditionally, they are believed to be autoimmune Interstitial lung disease, gastrointestinal involvement, and
diseases, although recent studies suggest that there is a close arthritis are among the most common extramuscular manifesta-
resemblance between IBM and other neurodegenerative diseases. tions of the IIM. Interstitial lung disease commonly occurs as
Similar to patients with other autoimmune diseases, those with part of the antisynthetase syndrome in a subset of patients who
IIM often have serum autoantibodies. Some of these antibodies have antisynthetase autoantibodies. Gastrointestinal manifesta-
are specific for myositis and are not seen in other rheumatic tions include dysphagia and aspiration pneumonia. Dysphagia
disorders. Our knowledge of the myositis-specific antibodies is particularly common and can be seen in all forms of the IIMs.
(MSAs) is incomplete. There has been considerable effort to Certain forms of IIM, particularly DM, can be a paraneoplastic
better characterize antibodies associated with myositis and to phenomenon. The most common cancers associated with myositis
discover new antibodies. Many MSAs are closely linked to a include gynecological (ovarian), pulmonary, gastrointestinal
unique clinical phenotype. Thus they are useful in classifying (pancreatic, stomach, and colorectal), and non-Hodgkin lym-
the IIM. They can forewarn of particular extramuscular manifesta- phoma. Also, the IIMs may be associated with other autoimmune
tions and guide appropriate treatment strategies. diseases such as systemic lupus erythematosus (SLE), Sjögren
IIM patients present with muscle weakness and elevated muscle syndrome, and systemic sclerosis.
enzymes. In patients with DM, skin manifestations may be the Muscle biopsy is critical for the diagnosis of the IIMs. The
initial presentation. The differential diagnosis of muscle disease presence of perifascicular atrophy is strongly suggestive of DM,
is broad and includes drug-induced myopathies, neuromuscular whereas the finding of rimmed vacuoles in the appropriate context
disorders, muscular dystrophies, and metabolic and endocrine suggests IBM. Muscle biopsy also differentiates between the IIMs
myopathies. To add to the complexity, some DM patients do not and other forms of myopathy such as drug-induced myopathies
develop muscle weakness, referred to as clinically amyopathic DM. and muscular dystrophies.
The patient’s age and gender, the pattern of the weakness, the Treatment of the IIMs is based largely on experience.
severity of manifestations, and associated symptoms are commonly Corticosteroids remain the mainstay of therapy. Steroid-sparing
helpful in leading to the correct diagnosis. For example, IBM is agents such as azathioprine, methotrexate, mycophenolate mofetil,
distinguished from other IIMs by a characteristic involvement and hydroxychloroquine are frequently initiated at presentation
of the finger flexor and knee extensor muscles, which is often while a steroid taper is attempted. In patients with refractory
asymmetrical. IBM is more common in older men, whereas PM disease, rituximab, intravenous immunoglobulin, and biological
and DM are commonly seen in young to middle-aged women antirheumatic medications may be tried. IBM is the most resistant
and children. DM is often associated with characteristic skin subset of the IIMs.
findings, which are not a feature of PM and IBM. However, in DM, PM, IBM, and IMNM constitute the largest subgroup
many patients differentiation between the IIMs cannot be made of the acquired myopathies. They are a heterogeneous group
on clinical grounds, and further diagnostic testing is required. and are rare among immunological illnesses. However, the IIMs
For example, both PM and IMNM present in the same fashion share many clinical features and laboratory abnormalities,
with a predominantly symmetrical proximal muscle weakness including related autoantibodies, and are closely related to
and elevated muscle enzymes. They can be distinguished only by major autoimmune diseases. Because their presentation and
pathological examination. On muscle biopsy, IMNM is associated major clinical manifestations are weakness and/or rash, the
with necrosis and regeneration of muscle fibers and a characteristic differential diagnosis includes many more common diseases
sparse inflammatory infiltrate. In contrast, PM is associated with familiar to neurologists and dermatologists.
the presence of cytotoxic inflammatory cells surrounding and
invading muscle fibers. Electromyography is a valuable tool for CLINICAL FEATURES
differentiating between weakness originating from muscle rather
than peripheral nerves. Magnetic resonance imaging (MRI) can The clinical hallmark of PM, DM, and IMNM is the gradual
be extremely helpful in identifying inflammatory changes in onset of symmetrical proximal muscle weakness over weeks to
patients with subtle clinical muscle involvement. Moreover, MRI months. In some cases, myalgia may be the presenting or most

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