Idiopathic Inflammatory Myopathy

Introduction

Etiology

The idiopathic inflammatory myopathies (IIMs) include polymyositis (PM), dermatomyositis (DM), and sporadic inclusion body myositis (sIBM). The IIMs are thought to be systemic autoimmune disorders, although a specific autoantigen has not been identified. DM is a complement mediated microangiopathy that is also mediated by plasmacytoid dendritic cells, whereas PM and sIBM are T cell–mediated disorders. Autoantibodies are often present in DM and polymyositis (PM) and are of value in the classification and prognosis of subgroups of IIM, but they do not have a well-established pathogenic role. Genetic and environmental factors may contribute to the development of these diseases.

Prevalence and Epidemiology

The IIMs are rare. These illnesses are 3 to 5 times more frequent in adults than children, with an annual incidence of approximately 10 to 20 new cases per million of the population. PM constitutes approximately 10% to 40% of adult IIM and 10% of juvenile IIM, whereas DM constitutes approximately 30% to 60% of adult IIM and 80% of juvenile IIM, depending on the referral population. The sIBM constitutes 15% to 30% of adult IIM. PM and DM are more common in females, whereas sIBM predominates in older adult males.

Clinical Presentation

Disease onset is usually more acute in dermatomyositis (DM) than in polymyositis (PM), whereas sporadic inclusion body myositis (sIBM) is characterized by an indolent disease onset. Symmetric proximal muscle weakness is the primary presenting symptom in PM and DM. In sIBM, weakness is greater in the distal than proximal muscles and more asymmetric than in PM.

In DM, the characteristic cutaneous findings of heliotrope rash or Gottron papules may precede muscle involvement by up to 6 months. Myalgia and tenderness can occur and are more common in DM. Extramuscular involvement, particularly dysphagia, may be present in all three forms of idiopathic inflammatory myopathy (IIM). Arthritis, dysphonia, interstitial lung disease, a number of other photosensitive and nonphotosensitive skin rashes, myocarditis, and intestinal manifestations are often part of the illness.

PM and DM may occur in association with other connective tissue diseases, most commonly scleroderma and systemic lupus erythematosus, and less commonly with rheumatoid arthritis. Adult onset DM and PM have a potential association with malignancy, particularly within 2 years of diagnosis ; the frequency and spectrum of tumor types parallel those found in the general population. All adults with DM and PM should undergo screening and surveillance for occult malignancy.

The Bohan and Peter diagnostic criteria for PM and DM are still widely used: proximal and symmetric muscle weakness, elevated serum levels of muscle enzymes, myopathic pattern on electromyography, and characteristic findings on muscle biopsy. Three of four criteria are necessary for a diagnosis of probable PM, and four criteria are necessary for a definite diagnosis of PM. The diagnosis of DM requires characteristic skin rashes as one of the criteria. The sIBM is reliably distinguished from PM by muscle biopsy only, but it should be suspected when symptoms are refractory to standard immunosuppressive therapy.

Anatomy (Including Gross Anatomy and Normal Variants)

Any skeletal muscle can be involved in IIM. Disease distribution is usually bilateral and fairly symmetric in proximal appendicular or axial muscle groups in DM and PM, whereas in sIBM, distal muscle groups and quadriceps and the finger flexors are most often involved. On occasion, cardiac or smooth muscle may also be involved.

Pathology (with Radiographic/Pathologic Correlations)

The characteristic findings of IIM on muscle biopsy are inflammatory infiltrates, muscle fiber necrosis, and muscle fiber regeneration. In DM, the inflammation tends to be perivascular, whereas in PM, it tends to be endomysial. Muscle hyperintensity on T2 or short tau inversion recovery (STIR) MRI images may reflect endomysial, perimysial, perifascicular, or perivascular inflammation, as well as muscle necrosis and phagocytosis. Less inflammation of involved muscle is present in cases of sIBM than in cases of PM or DM. The characteristic findings of IBM on biopsy are muscle vacuoles rimmed by granular material and the presence of cytoplasmic, tubulofilamentous inclusion bodies on electron microscopy.

Imaging Techniques

Techniques and Relevant Aspects

T2-weighted MRI sequences depict increased signal in affected muscles of patients with inflammatory myopathy. These signal changes can be described as edema-like and are typically widespread but patchy in distribution ( Fig. 56-1 ). Edema-like muscle signal is also readily detected on short tau inversion recovery (STIR) sequences, even at relatively short echo times, by virtue of fat signal suppression, and concomitant T1 prolongation in areas of inflammation.

FIGURE 56–1, A , Axial short tau inversion recovery (STIR) image in polymyositis depicts characteristic, diffuse, edema-like signal in the affected thigh muscles bilaterally. There are some areas of muscle sparing, for example, in the right hamstring and adductor muscles. There are also features of fasciitis. B , Axial STIR image in a different patient with polymyositis showing a more localized pattern of muscle inflammation, preferentially involving the hamstring musculature in this case.

Fat suppression enhances the detection of muscle abnormalities in IIM on T2-weighted imaging. Because spectral fat saturation may fail when static magnetic field homogeneity is poor, STIR imaging is preferred for survey studies done with a large field of view.

Nonfat-suppressed, T1-weighted imaging is valuable for identification of fatty involution of muscle, a conspicuous marker of disease damage ( Figs. 56-2 and 56-3 ). Contrast-enhanced MRI does not appear to improve the detection of muscle disease in IIM.

FIGURE 56–2, Axial T1-weighted spin-echo image in a patient with polymyositis depicts diffuse muscle damage manifested by both widespread and advanced fatty replacement of muscle, and a global loss in muscle mass or volume.

$FIGURE 56–3, A , Axial T1-weighted spin-echo image in a patient with polymyositis depicts mild muscle damage manifested by subtle areas of fatty replacement but with preservation of muscle mass or volume. B , Calculated fat-fraction image derived from in-phase and out-of-phase gradient echo images (not shown) corresponding to the image in A . Areas of fatty replacement are much more conspicuous.$

Step table capability and switchable multichannel receiver coil arrays now facilitate high-quality MRI surveys of the whole body. Because the detection of inflammatory myopathy does not require high spatial resolution, a total body survey, typically using a STIR sequence, is clinically feasible in evaluating suspected myopathy. This approach may reveal truncal disease that precedes disease in appendicular sites that are more typically surveyed by MRI.

Pros and Cons

Exercise creates signal changes in work-loaded muscles on STIR or T2-weighted imaging that may simulate inflammatory disease. Muscle strain injuries or delayed onset muscle soreness also exhibit an edema-like signal on MRI but should be distinguishable on clinical grounds.

Signal variations in muscle groups on STIR or T2-weighted imaging may also reflect normal differences in muscle fiber type predominance. Slow twitch muscles are characterized by longer T1 relaxation, and muscle groups with greater type I muscle fiber predominance may exhibit T2 prolongation with aging.

The findings of myopathy on MRI tend to be difficult to quantify. Technical differences in scan acquisition can also alter the apparent intensity or conspicuousness of diseased muscle on MRI. For this reason, MRI scan acquisition parameters should be rigorously standardized in longitudinal evaluations of myopathy. T2 mapping also has potential value as an alternative, quantitative marker of active disease in IIM ( eFig. 56-1 ).

eFIGURE 56–1, A , Axial short tau inversion recovery image in a patient with polymyositis showing marked edema-like signal in the adductor magnus and equivocal signal elevations elsewhere in the thigh. B , Corresponding calculated T2 map (based on multiecho spin-echo acquisition) shows prolonged T2 relaxation in areas of inflammation (units for color bar are in milliseconds) and provides an objective, potentially quantitative marker of active muscle disease.

Controversies

Although MRI arguably has greater diagnostic value in cases of suspected IIM than electromyography or serum enzyme measurements, MRI findings have not been formalized as a diagnostic criterion for IIM. The acceptance of MRI as a diagnostic tool in myositis may be inhibited by high cost and the need for more reliable and validated methods of summarizing the findings of MRI. MRI evaluation before biopsy, however, has become routine at many tertiary care centers. Although changes in MRI signal after therapy show good concordance with changes in clinical indicators of muscle disease, MRI may not accurately reflect changes in inflammatory cellular infiltration of muscle. However, MRI-directed biopsies demonstrate more inflammatory cells than blind muscle biopsies.

Manifestations of the Disease

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