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Whole-Body MR Imaging in the Diagnosis of Polymyositis

¹ Department of Radiology, Cappagh National Orthopaedic Hospital, Finglas, Dublin 11, Ireland.
² Department of Radiology, Mater Misericordiae Hospital, Eccles St., Dublin 7, Ireland.
³ Department of Neurology, Mater Misericordiae Hospital, Dublin 7, Ireland.
⁴ Department of Rheumatology, Mater Misericordiae Hospital, Dublin 7, Ireland.

Received October 11, 2001; accepted after revision April 4, 2002.

 
Address correspondence to M. J. O'Connell.

Abstract

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OBJECTIVE. Our purpose was to evaluate the use of whole-body MR imaging in the assessment of the extent and distribution of muscle inflammation in patients with polymyositis.

CONCLUSION. Whole-body turbo short tau inversion recovery imaging is a convenient complete method of evaluating patients with muscle inflammation caused by polymyositis. This imaging technique allows us to evaluate the total inflammatory burden by revealing multiple muscle groups not seen with standard protocols.

Introduction

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Polymyositis is a rare autoimmune and sometimes paraneoplastic inflammatory myositis. The term "polymyositis" is applied when the condition spares the skin, and "dermatomyositis," when polymyositis is associated with a characteristic skin rash. It is a progressive condition causing severe debilitation if untreated [1]. Whole-body MR imaging has an established role in assessing skeletal metastases in a variety of solid tumors, in further evaluation of patients with multiple myeloma, and in whole-body surveys in patients with unknown primary tumors [2]. Turbo short tau inversion recovery (STIR) imaging, used in whole-body scanning, gives excellent detail of muscle inflammation [3,4,5,6]. The purpose of our study was to evaluate the use of whole-body MR imaging in the assessment of patients with suspected polymyositis.

Materials and Methods

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Patients
Patients with clinical evidence of polymyositis based on symmetric proximal limb weakness, raised serum skeletal muscle enzyme levels, and abnormal findings on electromyography were referred for whole-body scanning. The study included three men and four women. The patient age range was 32-56 years (mean, 45.1 years). The creatinine kinase level range was 485-4460 U/L (mean, 1558 U/L). Two patients had a history of underlying connective tissue disease (lupus erythematosus and Raynaud's disease). One patient with a diagnosis of polymyositis made previously on the basis of conventional criteria with no MR imaging presented with evidence of disease recurrence.

Methods
All images were acquired on either a Harmony 1.5-T system (Siemens, Erlangen, Germany) or a Gyroscan 1.5-T system (Phillips Medical Systems, Best, The Netherlands). Imaging was performed in the coronal plane using a turbo STIR technique. Both excitation and signal acquisition were achieved with a body coil. Initial survey scans using sagittal and coronal localizers were obtained. Next, turbo STIR images were acquired using a TR of 4000 msec, a TE of 40 msec, and an echo-train length or turbo factor of 6. Fat suppression was obtained with a 180° inversion pulse, followed by tissue excitation after 160 msec. To achieve whole-body coverage, coronal slabs were acquired at four stations using a field of view of 50 cm. The number of coronal slices at each station varied according to patient body habitus. Slice thickness was 8 mm with interslice spacing of 0.8 mm. Respiratory triggering was used.

A whole-body image was obtained after imaging by manual image realignment and cropping at a workstation before presentation to the referring clinician. Both the individual coronal sections and the generated whole-body scans were reviewed by the interpreting radiologist to facilitate diagnosis. After the whole-body MR study, muscle biopsy guided by scanning findings was performed.

Image Interpretation
Two independent reviewers analyzed images at a workstation by studying individual image sections separately, noting the distribution of muscle inflammation and examining for evidence of underlying neoplastic disease. Differences in observations were resolved by consensus at a second film review.

Results

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Whole-body MR imaging was achieved in less than 20 min in each patient, dictated by respiratory triggering and the number of coronal slices taken at each level based on patient body habitus. A widespread distribution of muscle inflammation was identified, mainly involving lower limb muscles, with myositis of the vastus medialis or lateralis muscles seen in all patients (Fig. 1). Severe muscle edema (Fig. 2A,2B) was seen in four (57%) of seven patients. Moderate inflammation was identified in the remaining three patients. Myositis was predominantly seen in a symmetric distribution in five (71%) of seven patients. Muscle biopsy guided by MR imaging findings confirmed the diagnosis of polymyositis in each patient. Inflammatory change, manifested as signal hyperintensity, was identified in a myofascial distribution in the gluteus medius muscles bilaterally and at the myofascial junctions along the outer borders of the vastus lateralis muscles bilaterally in a patient with lupus erythematosus (Fig. 3). Moderate myositis in an asymmetric distribution was identified in distal lower limb muscles (soleus and gastrocnemius) in one patient with a history of Raynaud's disease. Subcutaneous inflammation was seen in one patient who had no evidence of a skin rash clinically (Fig. 4A,4B,4C). Pharyngeal muscle involvement could not be confirmed in the same patient, who initially presented with myalgia and dysphagia. No evidence of underlying malignancy was identified in any of the patients in this series. Allowing motion artifact due to respiration and despite respiratory gating and relatively poor spatial resolution, we found no gross evidence of pulmonary fibrosis.

View larger version (35K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —Schematic drawing shows distribution of muscle involvement in patient cohort. Note both muscle group involved and number of times muscle appears to be affected.

View larger version (79K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —46-year-old woman with suspected polymyositis. Whole-body turbo short tau inversion recovery (STIR) MR image (TR/TE, 4000/30; inversion time, 160 msec) shows gross symmetric inflammation or muscle edema in proximal upper (curved arrows) and lower limb girdle muscles. Note florid inflammation or muscle edema (straight arrow) in psoas muscles bilaterally. Edema is worse on right.

View larger version (79K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —46-year-old woman with suspected polymyositis. Whole-body turbo STIR MR image in plane anterior to that seen in A shows inflammation in neck flexor muscles (thin arrow) in addition to gross involvement of glutei that is worse on left (thick arrow).

View larger version (69K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3. —46-year-old man with suspected polymyositis and systemic lupus erythematosus. Whole-body turbo short tau inversion recovery MR image (TR/TE, 4000/30; inversion time, 160 msec) shows focal symmetric edema or inflammation (arrows) in myofascial distribution in gluteus medius muscle group bilaterally. Muscle biopsy helped confirm myofascial inflammation.

View larger version (81K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A. —56-year-old man with suspected polymyositis. Whole-body turbo short tau inversion recovery MR image (TR/TE, 4000/30; inversion time, 160 msec) of posterior aspect of upper torso shows extensive inflammation in deltoid, trapezius, triceps, and latissimus dorsi muscles bilaterally (straight arrows) and further muscle edema and inflammation in vastus medialis and lateralis muscles of left thigh (curved arrow). Note susceptibility artifact at site of right hip prosthesis, limiting assessment of muscles adjacent to this site.

View larger version (109K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B. —56-year-old man with suspected polymyositis. Individual image from coronal slab of thorax allows detailed assessment of shoulder girdle muscle involvement (arrows).

View larger version (128K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4C. —56-year-old man with suspected polymyositis. Individual image from coronal slab of thighs facilitates detailed evaluation of muscle and subcutaneous connective tissue inflammation in left thigh (curved arrows) and of prosthesis-induced artifact on right (straight arrow).

Discussion

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Polymyositis typically has a symmetric distribution, initially involving the proximal lower limb girdle and progressing to involve the proximal upper limb girdle, neck flexors, and pharyngeal muscles [1]. With this distribution, the clinical differential is infectious myositis, including parasitic infestations such as trichinosis, inclusion body myositis, HIV myositis, sarcoidosis, and eosinophilic myositis. The most common form of polymyositis affects patients in their fourth decade and women twice as often as men [7]. It usually has an insidious onset, but it may present with a rapidly progressive course, causing severe disability. As many as 30% of patients develop associated pain or muscle tenderness. Joint manifestations, including arthralgia, arthritis, and periarticular soft-tissue calcification, are seen in 20-50% of patients [8]. The cause is unknown, but a viral cause in a genetically susceptible individual appears the most likely precipitant. An association with an underlying connective tissue disease is seen in one third of patients and underlying malignancy in one tenth of patients [7]. Some clinicians consider screening tests for tumor, including chest radiography, bone scintigraphy, and mammography, particularly in patients older than 60 years, who are at an increased risk.

The diagnosis of polymyositis is based on a typical clinical presentation, serum skeletal muscle enzyme levels, and findings on electromyography and muscle biopsy. Clinical presentation of a symmetric proximal myositis is not always apparent. Patients with atypical presentations of this disease have dysphagia, due to involvement of muscles of the pharynx and the upper esophagus, and myositis of neck flexor muscles [1]. This presentation gives a clinical differential diagnosis of motor neuron disease and myasthenia gravis. Depicting a typical distribution of muscle inflammation in limb muscles on whole-body MR imaging has the potential to help establish the diagnosis of polymyositis.

Potential pitfalls occur in the laboratory diagnosis of polymyositis because serum skeletal muscle enzyme levels can be normal and electromyography findings are not specific [7]. Electromyography causes patient discomfort and is time-consuming for the operator, and postprocedural focal myositis interferes with the interpretation of subsequent biopsy findings. Similarly, iatrogenic electromyography-induced inflammation should not be confused with polymyositis on MR imaging (Fig. 5). Muscle biopsy is expensive, causes morbidity in patients who may be severely ill, and can delay mobilization and interfere with physiotherapeutic interventions. Biopsy has a false-negative rate of 10-25% [7, 9]. MR imaging has previously been documented to reduce this rate when imaging is used to identify the biopsy site [9], and whole-body MR imaging has the potential to provide a wider survey of these potential sites.

View larger version (77K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5. —32-year-old woman with creatine kinase level of 1020 U/L and suspected polymyositis. Whole-body turbo short tau inversion recovery MR image (TR/TE, 4000/30; inversion time, 160 msec) shows moderate myositis of vastus lateralis and to lesser extent of vastus medialis muscles bilaterally. Myositis is worse on right, at site of recent electromyographic study (arrow). Other muscle groups are free of disease.

Conventional imaging methods of documenting polymyositis include limited scanning of the proximal lower limb muscle girdle and scanning of the proximal upper limb girdle if the patient is symptomatic at this site. MR imaging techniques include coronal T1-weighted, STIR, and axial T2-weighted sequences [10]. The technique of STIR imaging has been shown to have 97% specificity for identifying sites of inflammatory myopathy proven at biopsy [6]. However, the findings are not specific [11]. Gadolinium-enhanced T1-weighted MR imaging has no advantage over the STIR technique [11]. Myositis seen in patients with polymyositis typically has a high signal on T2-weighted and STIR images in the muscle body, although inflammation may also be seen in a myofascial distribution around muscles [11]. T1-weighted images show isointensity in regions of inflammation that have been described to involve initially the vastus medialis and lateralis muscles [10]. Although the inflammation is clinically symmetric and classically involves the proximal muscle groups, muscle involvement can be patchy and asymmetric on imaging, and distal muscles of the upper and lower limbs may be involved [11]. High signal intensity on T2-weighted and STIR sequences is also seen in the subcutaneous tissues, with thickened connective tissue septa [11]. This involvement is present in both polymyositis and dermatomyositis, but only the latter has the characteristic skin rash. In an effort to explain why findings on imaging may be normal, Huppertz and Kaiser [12] have suggested that MR imaging findings in patients with myositis may lag behind clinical or serologic improvement.

A limitation of conventional MR imaging has been an inability to scan a large volume of muscles without prolonged acquisition times. In general, only symptomatic muscles were imaged. In our study, myositis was frequently identified in asymptomatic muscle groups. Whole-body MR imaging has the advantage of documenting inflammatory myopathy of multiple muscle groups, including the psoas, intercostal, and neck muscles, not imaged using standard protocols. To our knowledge, inflammation in these muscle groups has never been previously documented on MR imaging in polymyositis. In addition, the distal upper and lower limb muscles can be imaged with little extra scanning time. Upper limb position is dictated by the patient body habitus. In most cases, the patient's arms are placed by the side, but they can be placed across the chest or above the head, the latter requiring an extra coronal acquisition.

Whole-body scanning gives a complete assessment of disease burden in an acute presentation, therefore helping the clinician decide what level of treatment is appropriate. It also has the potential to identify associated malignancy in patients with paraneoplastic polymyositis. This advantage may obviate an extensive search for an unknown primary tumor by conventional scanning techniques.

In the follow-up treatment of patients with polymyositis, frequent diagnostic dilemmas occur as to whether the disease is still active after antiinflammatory treatment. Determining serum skeletal muscle enzymes levels is of limited value [1]. Assessment of disease activity and sequelae is complicated because a validated comparative measure has not been developed for grading disease severity and for follow-up [7]. In the past, these problems have sometimes led to repeated biopsy. Whole-body MR imaging has the potential to document a response to treatment. In addition, it may prove useful in differentiating persistent inflammatory myositis and steroid myopathy, which cause fatty infiltration and atrophy and are best diagnosed on additional T1-weighted MR imaging [11].

Our study shows a wide variation in distribution of muscle inflammation in patients with polymyositis. It also identifies relative sparing of certain muscle groups, including the adductor magnus and hip extensor muscles, seen in one patient. Surveillance scanning could help improve our understanding of the natural history of polymyositis.

In summary, we report the novel application of a whole-body MR imaging protocol using fat-suppressed STIR images that can be rapidly acquired to facilitate a global overview of the extent and symmetry of muscle disease. We advocate its use as the primary diagnostic imaging tool in the evaluation of patients referred for suspected polymyositis.

References

Top Abstract Introduction Materials and Methods Results Discussion References

 

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