June 2009, VOLUME 192
NUMBER 6

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June 2009, Volume 192, Number 6

Musculoskeletal Imaging

Pictorial Essay

MRI Findings in Inflammatory Muscle Diseases and Their Noninflammatory Mimics

+ Affiliations:
1Department of Diagnostic Radiology, Eberhard-Karls-University, Hoppe-Seyler-Str.3, 72076 Tübingen, Germany.

2Departments of Internal Medicine and Oncology, Eberhard-Karls-University, Tübingen, Germany.

3Departments of Diagnostic Radiology and Neuroradiology, Eberhard-Karls-University, Tübingen, Germany.

4Department of Pediatrics, Eberhard-Karls-University, Tübingen, Germany.

Citation: American Journal of Roentgenology. 2009;192: 1708-1716. 10.2214/AJR.08.1764

ABSTRACT
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OBJECTIVE. The purpose of this article is to provide a practical review of the spectrum of MRI findings in inflammatory muscle diseases and their noninflammatory mimics.

CONCLUSION. MRI is a highly sensitive tool for the diagnosis of muscle diseases. Although it has low specificity, awareness of the potential imaging findings in the various, sometimes rare, muscular disorders is helpful for accurate diagnosis.

Keywords: denervation syndromes, dermatomyositis, granulomatous myositis, polymyositis, pyomyositis, rhabdomyolysis

Introduction
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The myopathies are skeletal muscle diseases that develop either as a result of autoimmune-induced inflammation, inherited or acquired metabolic defects in energy production, administration or use of drugs and toxins, infections, or miscellaneous causes. Currently, MRI is the mainstay in the diagnosis of muscular disorders. This article deals with the MRI findings in inflammatory myopathies and their noninflammatory mimics.

Inflammatory Myopathies
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Inflammatory myopathies can be classified as idiopathic or secondary. Idiopathic inflammatory myopathies encompass a group of heterogeneous muscle diseases that share the clinical features of slowly progressive weakness of skeletal muscles and muscle fatigue. Polymyositis is a rare autoimmune and sometimes paraneoplastic inflammatory myositis. The diagnosis is based on a typical clinical presentation, elevated serum skeletal muscle enzymes, and findings on electromyography and muscle biopsy. MRI accurately documents the extent and intensity of the muscle abnormalities. The inflammation is usually symmetric and classically involves the proximal muscle groups in both polymyositis and dermatomyositis, but muscle involvement can also be patchy and asymmetric (Fig. 1). High signal intensity is seen in the active phase on STIR and fat-saturated gadolinium-enhanced T1-weighted images. Sometimes inflammation may extend only along or around individual muscles and muscle groups (myofascial distribution) [1] (Fig. 2). In the chronic phase, fatty atrophy of the musculature is seen on T1-weighted images (Fig. 3). In dermatomyositis, the subcutaneous connective tissue septa and sometimes even the muscle fasciae are also involved (Fig. 4). Juvenile dermatomyositis generally takes a more severe clinical course, which is reflected by the extent and intensity of cutaneous, subcutaneous, and muscular signal abnormalities on MRI (Fig. 5). Inclusion body myositis presents with specific inclusions of amyloid-β protein and is refractory to treatment. Focally increased signal is seen on STIR and fat-saturated gadolinium-enhanced T1-weighted images, predominantly in the anterior thigh compartment.

Connective Tissue Diseases
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Connective tissue diseases (e.g., systemic lupus erythematosus [SLE], Sjögren's syndrome, systemic sclerosis, overlap syndromes, and mixed connective tissue diseases) can be accompanied by inflammatory myositis.

Patients with SLE may have true muscular inflammation. Patients with myositis are diagnosed at an earlier age and have a worse prognosis. Myositis, muscle atrophy, microtubular inclusion, and mononuclear cell infiltrates are found at histology [2]. Additionally, SLE patients may develop drug-induced myopathy. On MRI, increased muscle signal is found, mostly in a focal pattern of distribution. Although rare, lesions similar to those of inclusion body myositis have been described in primary Sjögren's syndrome [3]. Musculoskeletal symptoms are common in patients with systemic sclerosis. Myalgia can be the result of deconditioning and joint flexion contractures, but bland myopathy due to muscle fibrosis also is often encountered in systemic sclerosis, generally not accompanied by true inflammation. MRI shows hyperintense signal and enhancement on STIR and fat-saturated gadolinium-enhanced T1-weighted images caused by both diffuse inflammatory infiltrates and collagenization of intramuscular paraseptal fasciae (Fig. 6). Children with systemic sclerosis, mixed connective tissue diseases, SLE, and occasionally Still's disease have muscle abnormalities at onset that mimic those of juvenile dermatomyositis (Fig. 7).

The differentiation of these diseases is usually not difficult, however, because clinical features unique to each are always present. An overlap syndrome of systemic sclerosis and polymyositis–dermatomyositis with true inflammatory myositis can occur (Fig. 8). Myositis is present in 83% of overlap syndromes [4]. Mixed connective tissue disease is an overlap syndrome combining features of SLE, systemic sclerosis, and polymyositis (Fig. 9). The pattern of muscle involvement does not differ from that of typical polymyositis. In systemic juvenile idiopathic arthritis, myositis is not a common manifestation, but it can rarely occur. Most of these cases show focal, myofascial abnormalities on MRI (Fig. 10).

Other Autoimmune, Granulomatous, Vasculitic, and Systemic Illnesses
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Crohn's disease can rarely be associated with muscular involvement. Focal granulomas cause local tenderness and induration and respond to therapy directed against Crohn's disease [5]. MRI shows an abnormal focal hyperintense signal on STIR and fat-saturated gadolinium-enhanced T1-weighted images in the affected muscles with perifocal edema (Fig. 11). Granulomatous myositis must be differentiated from pyomyositis and muscular edema due to deep vein thrombosis because both of these latter complications also accompany the course of the disease. Sarcoidosis of the skeletal muscles manifests as either a nodular or a myopathic form. The nodular form presents a characteristic appearance on MRI with a central area of fibrosis that shows low signal intensity on all sequences, which contrasts with the peripheral bright signal on STIR and fat-saturated gadolinium-enhanced T1-weighted images (the “dark star” sign); the myopathic type shows diffuse nonspecific signal alteration [6] (Fig. 12). Behçet's syndrome is a rare form of vasculitic disease involving venous vessels. In rare instances, a focal necrotizing myositis due to inflammatory granulation associated with central necrosis of the muscle and perivasculitis can be encountered. On MRI, focal nonspecific, masslike muscular lesions with a strong peripheral signal, gadolinium enhancement, and central necrosis are found [7] (Fig. 13A, 13B). Familial Mediterranean fever (FMF) is typically characterized by fever and either serositis, synovitis, or a skin rash. Attacks of FMF can also involve the muscles and present with short-lasting myalgia and normal serum levels of creatine kinase. On MRI, hyperintense focal lesions on both STIR- and fat-saturated gadolinium-enhanced T1-weighted images are seen (Fig. 14A, 14B).

Muscle Infection
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Pyomyositis is an unusual infection of skeletal muscles that is seen primarily in immunocompromised and HIV-infected patients. In immunocompetent patients, it results either secondary to hematogenous spread or from direct spread after intramuscular injection or surgery. Staphylococcus aureus is the most common pathogen. On MRI, hyperintense focal lesions with massive perifocal edema, progressing usually to abscess, are seen (Figs. 15 and 16). Muscle involvement can occur with a viral infection (e.g., influenza, dengue, Coxsackie B virus) or parasitic invasion (e.g., trichinellosis, cysti cercosis, or toxoplasmosis). In immunocompromised patients, musculoskeletal fungal infections (e.g., candidiasis, aspergillosis, or mucormycosis) can also occur (Fig. 17). Acute rhabdomyolysis can be encountered with clostridial and streptococcal myositis.

Muscle Infarction
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Diabetic myonecrosis (infarction) is a rare complication of diabetes mellitus manifested by local pain and sometimes low-grade fever. On MRI, marked edema and enhancement around irregular regions of muscle necrosis are seen (Fig. 18).

Muscle Contusion
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Muscle contusion and overuse are the most common causes of muscle pain and functional impairment. Different degrees of muscle strains up to full interruption of myofibrils and hematoma formation are the rule and are encountered more often in athletes (Figs. 19 and 20). Focal muscular signal alteration on MRI depends on the presence of hemoglobin derivatives and their paramagnetic properties. Muscle edema is always present as in delayed-onset muscle soreness, a type of overuse injury that becomes symptomatic hours or days after overuse [8].

Drug-Induced Rhabdomyolysis
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Drug-induced myopathies are relatively uncommon except those caused by cholesterol-lowering agents and glucocorticoids. All classes of lipid-lowering agents have been implicated in muscle toxicity (Fig. 21). Varying degrees of muscle necrosis are seen; in severe reactions, rhabdomyolysis occurs. Several other drugs (e.g., corticosteroids, zidovudine used in the treatment of HIV infection, D-penicillamine, and illicit drugs) are known for their myotoxicity. Misuse of androgenic hormone derivatives by bodybuilders potentiated by exercise can also cause rhabdomyolysis (Fig. 22).

Musculoskeletal Complications After Allogeneic Stem Cell Transplantation
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Polymyositis can occur in association with chronic graft-versus-host disease (GVHD). Focal muscle necrosis with massive lymphocytic infiltration (cytotoxic T cells) is found at histology [9]. The spectrum of MRI findings consists of both patchy and diffuse patterns of distribution showing increased signal on STIR and fat-saturated gado linium-enhanced T1-weighted images (Fig. 23). Increased signal can sometimes be seen on unenhanced T1-weighted images, denoting massive proteinrich exudation by acute rhabdomyolysis. Granulomatous myositis has also been reported in patients with GVHD who present with nonspecific features and myonecrosis [10].

Autoimmune Polyneuropathy
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Myopathies occur in association with polyneuropathies. As with Parsonage-Turner syndrome (brachial plexus neuropathy), chronic inflammatory demyelinating polyneuropathy causes denervation changes that vary with the stage of disease. MRI signal changes occur in the acute to subacute phase of bilateral increased T2-weighted and fat-saturated gadolinium-enhanced T1-weighted image signal that are diffuse in character (Fig. 24). In the chronic stage, fatty involution and atrophy of the involved muscle groups are expected. Similar findings, but confined to the respective enervation territories, are found in entrapment and denervation syndromes (Fig. 25).

Others
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Focal myositis is a benign, rapidly growing painful soft-tissue intramuscular mass that is considered by some authors to be a localized type of polymyositis [11, 12] (Fig. 26A, 26B). Eosinophilic myositis is unusual and is most frequently associated with parasitic infection, eosinophilic fasciitis (Shulman's syndrome), or the eosinophilia–myalgia syndrome, and manifests focally as polymyositis or perimyositis (Fig. 27). Gouty arthritis may rarely be accompanied by inflammation in the adjacent muscles (Fig. 28).

Summary
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Abnormal MRI signal in the musculature is a typical finding in myopathies. Awareness of the broad spectrum of disorders mimicking idiopathic inflammatory myopathies as well as knowledge of the clinical setting, extent, localization, focal or diffuse character, and presence of concurrent skin abnormalities is helpful for reaching the differential diagnosis.

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Fig. 1 64-year-old man with polymyositis. Note diffuse edema and inflammation of obturator externus (arrowhead) and pectineus (arrow) muscles on axial STIR image.

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Fig. 2 40-year-old man with anti-signal recognition particle polymyositis. Note myofascial orientation of edema and inflammation on axial STIR image (arrows).

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Fig. 3 55-year-old woman with chronic polymyositis. Note advanced fatty atrophy, especially of quadriceps muscles (arrows), on unenhanced axial T1-weighted image.

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Fig. 4 42-year-old man with known dermatomyositis. Axial STIR image shows diffuse hyperintensity in some thigh muscles (arrowhead, vastus lateralis muscle). Note also increased signal in subcutaneous tissue septa (arrow) and skin thickening.

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Fig. 5 14-year-old girl with known juvenile dermatomyositis. Whole-body STIR image shows multifocal patchy pattern of muscle edema and inflammation (arrows).

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Fig. 6 60-year-old man with systemic sclerosis. Coronal STIR image of thigh shows muscular increased signal intensity due to accompanying myositis (arrows) as well as lacelike subcutaneous increased signal intensity (arrowhead) due to inflammation and beginning of collagenization.

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Fig. 7 25-year-old man with adult onset of Still's disease and focal muscle tenderness in right calf. Note focal hyperintensity of peroneus muscle (arrow) on coronal STIR image.

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Fig. 8 27-year-old woman with overlap syndrome. Axial STIR image of pelvis shows edema of sartorius (short thin arrow), obturator internus (short thick arrows), and iliopsoas muscles (long thick arrow).

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Fig. 9 51-year-old woman with known mixed connective tissue disease. Note hyperintense signal of flexor digitorum and interossei plantares muscles (arrow) of left foot on axial angulated STIR image.

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Fig. 10 4-year-old girl with newly diagnosed polyarticular juvenile idiopathic arthritis. Note focal hyperintense signal in gluteus maximus muscles on both sides (arrows) on coronal STIR image.

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Fig. 11 29-year-old man with Crohn's disease. Acute muscular pain in calves was first symptom in this patient. Axial fat-saturated gadolinium-enhanced T1-weighted image shows focal masslike enhancing lesion (short arrow) in diffusely homogeneously enhancing muscles (long arrow) of posterior calf compartment. Short arrow indicates muscle granuloma; long arrow, accompanying myositis.

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Fig. 12 15-year-old boy with muscle tenderness in thighs. Axial fat-saturated T2-weighted image shows focal lesion (arrow) with black central core and peripheral slightly hyperintense rim consistent with sarcoidosis.

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Fig. 13A 35-year-old man with Behçet's syndrome. Note focal nodular lesions in posterior calf compartment due to vasculitis involvement. Axial unenhanced T1-weighted image shows slightly hyperintense lesions (arrows).

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Fig. 13B 35-year-old man with Behçet's syndrome. Note focal nodular lesions in posterior calf compartment due to vasculitis involvement. On axial fat-saturated gadolinium-enhanced T1-weighted image, strong enhancement of these lesions (arrows) is seen.

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Fig. 14A 10-year-old boy with known familial Mediterranean fever syndrome who underwent MRI of lower extremity during acute episode of myalgia. Coronal STIR (A) and axial fat-saturated gadolinium-enhanced T1-weighted (B) images show focal hyperintense lesions (arrows) in thigh musculature.

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Fig. 14B 10-year-old boy with known familial Mediterranean fever syndrome who underwent MRI of lower extremity during acute episode of myalgia. Coronal STIR (A) and axial fat-saturated gadolinium-enhanced T1-weighted (B) images show focal hyperintense lesions (arrows) in thigh musculature.

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Fig. 15 10-year-old boy with pyarthrosis in right hip (arrowhead). Coronal STIR image shows focal secondary infectious myositis (arrow) in right quadriceps muscle adjacent to infected hip joint. At this time, there is no abscess formation.

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Fig. 16 70-year-old woman with erysipelas of calves. Axial STIR image shows diffuse reactive muscle edema, especially in posterior muscle compartment (arrow), but no abscess formation.

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Fig. 17 60-year-old man with systemic mucormycosis after allogeneic stem cell transplantation for osteomyelofibrosis. Note focal abscess in temporal fossa (arrowhead) and transcranial invasion of extracranial muscles (arrow). Coronal fat-saturated gadolinium-enhanced T1-weighted image of skull base shows central hypointensity and peripheral enhancement (arrow) by abscess formation.

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Fig. 18 58-year-old man with diabetes mellitus presenting with acute muscle pain in calf. Axial fat-saturated proton density–weighted image shows hyperintensity of gastrocnemius muscle thought to be caused by muscle infarction (arrow). No signs of infection are visible at this time.

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Fig. 19 29-year-old man with compartment syndrome of left calf. After rapid swelling of calf muscle due to inflammation, focal myonecrosis occurred before surgery was performed. Axial fat-saturated gadolinium-enhanced T1-weighted image of left calf shows focal necrosis of musculature (arrows), interruption of myofibrillar structure, and peripheral reactive enhancement.

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Fig. 20 60-year-old man days after excessive biking who has progressive myalgia in both legs, which is consistent with delayed onset of muscle soreness. Note bilateral muscle edema on coronal STIR image involving especially quadriceps femoris muscles. Arrow in right thigh indicates vastus lateralis muscle; arrowhead shows edema of left vastus medialis muscle.

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Fig. 21 67-year-old man with acute onset of polymyositis after administration of statins. Note atrophy and hyperintensity of thigh muscles on axial STIR image (arrows).

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Fig. 22 20-year-old man with severe rhabdomyolysis after administration of methandrostenolone. Note diffuse edema of chest wall musculature as shown on coronal STIR image along pectoralis muscles.

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Fig. 23 43-year-old man with acute polymyositis caused by biopsy-proven graft-versus-host disease. Patient presented with muscle pain and elevated serum creatinine kinase levels. Note patchy myositis pattern in thighs on axial STIR-image.

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Fig. 24 58-year-old man with progressive autoimmune polyneuropathy and tenderness and weakness in muscles, especially in pelvic and thigh regions. Note almost symmetric diffuse edema of involved muscles on whole-body STIR image, initially believed to be expression of polymyositis (arrows). Histology revealed denervation-associated muscle edema caused by polyneuropathy.

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Fig. 25 63-year-old woman with denervation-related atrophy of left-side quadriceps femoris muscle. Axial STIR image shows both atrophy and residual muscle edema in subacute phase of intraoperative damage of deep branch of femoral nerve (arrow).

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Fig. 26A 37-year-old woman with focal myositis presenting as painful swelling of right thigh. Coronal STIR (A) and fat-saturated gadolinium-enhanced T1-weighted (B) images show increased signal in focal distribution along rectus femoris muscle on right, suggestive of focal nodular myositis (arrows). Serum creatine kinase level was normal.

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Fig. 26B 37-year-old woman with focal myositis presenting as painful swelling of right thigh. Coronal STIR (A) and fat-saturated gadolinium-enhanced T1-weighted (B) images show increased signal in focal distribution along rectus femoris muscle on right, suggestive of focal nodular myositis (arrows). Serum creatine kinase level was normal.

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Fig. 27 68-year-old man with newly diagnosed Shulman's syndrome. Because of muscular fascial thickening and thickening of adjacent intramuscular septa, increased signal is seen on whole-body STIR image (arrows). Note contrast between signal of thigh musculature and that of gluteus muscles (arrowhead), which were only minimally involved.

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Fig. 28 64-year-old man with gouty arthritis and tenosynovitis. Axial fat-saturated T1-weighted gadolinium-enhanced image of hand shows diffuse reactive edema of lumbricales muscles (arrow) secondary to adjacent nodular mass (not shown).

Address correspondence to M. Horger ().

CME

This article is available for CME credit. See www.arrs.org for more information.

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