- Author: Ramesh Pappu, MD, DPH, MBBS; Chief Editor: Herbert S Diamond, MD more...
The following laboratory findings may be present in polymyositis:
Complete blood count (CBC) - May show leukocytosis or thrombocytosis; leukocytosis is present in more than 50% of patients
Erythrocyte sedimentation rate or C-reactive protein level - Elevated in 50% of patients with polymyositis
Elevated muscle enzyme levels
Positive rheumatoid factor results - Found in more than 50% of patients
Perform age-appropriate evaluation for malignancy. Testing for associated malignancy is based on age and sex and can be performed using imaging techniques such as chest radiography, computed tomography (CT) scanning, mammography, pelvic ultrasonography, and upper and lower gastrointestinal endoscopy.
Pulmonary function tests and diffusion capacity for evaluation of interstitial lung disease may be appropriate.
Muscle enzyme levels
Serum creatine kinase (CK) levels are usually elevated in persons with polymyositis, ranging from 5-50 times the reference range. A level greater than 100 times the reference level is rare and is a signal of other diagnoses.
Serum CK levels, along with careful physical examination, may be used to monitor myositis activity. However, serum CK levels may be within reference ranges despite increased disease activity (eg, in cases of chronic and late-stage polymyositis). CK levels are usually minimally elevated or within the reference range in patients with inclusion body myositis. CK levels are within the reference range in patients with corticosteroid-induced myopathy.
Other muscle enzymes that may be elevated include the following:
Aldolase - In practice, usually only the CK and aldolase levels are determined
The following antibody findings may exist in polymyositis:
Antinuclear antibody assay - Positive in one third of patients with polymyositis and in only 15% of patients with inclusion body myositis
Antisynthetase antibodies (such as anti-Jo-1 antibodies) - Associated with certain clinical features; antisynthetase syndrome may manifest as idiopathic inflammatory myopathy, interstitial lung disease, arthritis, Raynaud phenomenon, fever, and/or mechanic's hands
Signal-recognition particle (SRP) antibodies - Approximately 4% of patients with polymyositis have antibodies to signal recognition particles (SRPs), which are associated with acute onset of severe weakness, increased incidence of cardiac involvement, and higher mortality rates
Muscle-imaging techniques such as magnetic resonance imaging (MRI) and ultrasonography may be useful to document and localize the extent of muscle involvement. MRI scans show signal intensity abnormalities of muscle due to inflammation, edema, or scarring. (See the image below.)
MRI scans may also be used to guide muscle biopsy and to monitor disease activity. However, many clinicians choose the biopsy site on the basis of findings at electromyography and clinical examination and believe that MRI is not required. Barium swallow studies are helpful for evaluation of dysphagia or dysphonia.
Chest radiography and high-resolution CT scanning of the chest are helpful for the evaluation of interstitial lung disease. CT scanning of the chest, abdomen, and pelvis is considered for screening of associated malignancy. Chest radiography may also reveal evidence of associated malignancy.
Also consider mammography, pelvic ultrasonography, and upper and lower GI endoscopy in screening for associated malignancy.
Electromyographic findings are abnormal in almost all patients (90%) with polymyositis. Various abnormalities consistent with polymyositis may be found, depending on the stage of disease. In patients with inclusion body myositis, the following myopathic and neuropathic changes may be present:
Evidence of membrane irritability, increased insertional activity, fibrillation potentials, positive sharp waves at rest
Myopathic changes of motor unit action potential; decreased amplitude and duration; increased polyphasic potentials; bizarre, high-frequency, repetitive discharges
Chronic changes, evidence of denervation-reinnervation
Muscle biopsy is crucial in helping to diagnose polymyositis and in excluding other rare muscle diseases. MRI can be used to guide the site of biopsy. However, many clinicians choose the biopsy site on the basis of findings at electromyography and clinical examination and believe that MRI is not required.
Avoid biopsy of sites recently studied by EMG by using the contralateral side. Inflammatory changes are seen at muscle biopsy (eg, deltoid or quadriceps femoris). Findings occasionally may be normal because of patchy involvement. (See the images below.)
Muscle biopsy shows muscle fibers in varying stages of inflammation, necrosis, and regeneration. Findings include focal endomysial infiltration by mononuclear cells (consisting of mostly CD8+ T lymphocytes and macrophages), capillary obliteration, endothelial cell damage, and increased amounts of connective tissue. Later in the course of polymyositis, muscle-cell degeneration, fibrosis, and regeneration may be observed. (See the images below.)
Because the inflammatory infiltrates can be small and multifocal, they can be missed in a small muscle-biopsy specimen. Perifascicular atrophy or prominent perivascular infiltrates are not present, and the blood vessels are normal. When the disease becomes chronic, the connective tissue increases. The diagnosis of polymyositis is definite when a patient has subacute, elevated levels of serum CK and findings on muscle biopsy consistent with the histologic features of polymyositis.
Inclusion body myositis is histologically similar to polymyositis, with the additional presence of intracytoplasmic inclusion bodies observed on electron microscopy. Dermatomyositis shows inflammatory changes, predominantly in the perimysial and perivascular regions with CD4+ T and B lymphocytes (see the image below). Corticosteroid-induced myopathy causes no inflammatory changes. Type II fiber atrophy is the characteristic feature.
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