Polymyositis is a sub-acute inflammatory myopathy affecting adults and, rarely, children. In most cases the actual onset of polymyositis is not easily determined as patients tend to delay seeking medical evaluation.
Other muscular disorders, as discussed in the case presentation, need to be excluded.
FIGURE 1 Heliotrope rash
Patients with dermatomyositis often have a characteristic rash on their face.
FIGURE 2 Gottron’s papules
Many patients also have papules and plaques on their hands, typically at the joints.
Evidence suggests the inflammatory myopathies are autoimmune disorders.6 They are often associated with connective tissue diseases and other systemic autoimmune conditions. Viral infections such as coxsackie, influenza, paramyxovirus, mumps, cytomegalovirus, and Epstein-Barr have been indirectly associated with chronic and acute myositis and may trigger the autoimmune process.
Specific muscle or capillary target antigens have not been identified, and the agents that initiate self-sensitization are still unknown. Other features of these disorders are their association with auto-antibodies, certain histocompatibility genes, T-cell–mediated myocytotoxicity, and complement-mediated microangiopathy.6
Dermatomyositis appears to be primarily a B-cell mediated microangiopathy. Antibodies directed against the endothelium of the endomysial capillaries lead to the primary histological changes in the blood vessels.9 The disease manifests when the complement system is activated to form the membrane attack complex (MAC).
Polymyositis and inclusion-body myositis appear to result from a cytotoxic T-cell response directed specifically against muscle fibers.9 CD-8+ cells are induced via T-cell activation to invade MHC-I antigen-expressing muscle cells.6 Usually most muscle cells do not express MHC Class I or II antigens. Histology demonstrates infiltration of individual muscle fibers by inflammatory cells. Inclusion-body myositis is differentiated from polymyositis by the presence of nuclear and cytoplasmic vacuoles.
Commonly associated clinical findings
Extramuscular manifestations of inflammatory myopathies. Dermatomyositis is a systemic inflammatory disorder that may extend beyond the dermatologic and muscular systems, and patients can exhibit such symptoms as fever, malaise, and weight loss.
Arthralgia and Raynaud’s phenomenon may occur with associated connective tissue disease.
Dysphagia indicates involvement of the oropharyngeal striated muscles and the upper esophagus.
Cardiac disturbances include atrioventricular conduction defects, tachyarrhythmias, myocarditis, heart failure, and possibly hypertension from long-term steroid use. The elevated troponin-I seen in our index case may have been evidence of a mild myocarditis, though the echocardiogram was normal.
Calcinosis (deposition of calcium in the skin or muscles) occurs in up to 40% of children with dermatomyositis but is unusual in adults.10
Pulmonary symptoms may be due to weakness of the thoracic muscles, interstitial lung disease, or aspiration. One retrospective study of 156 consecutive patients with dermatomyositis/polymyositis based on clinical criteria found a 23.1% incidence of interstitial lung disease.11
Malignant disorders. The frequency of cancer is increased in association with these diseases. Studies have placed the highest risk of concomitant malignancy with dermatomyositis and the least risk with polymyositis. (The relative risk for malignancy in dermatomyositis as compared with polymyositis was 2.4.) Malignancy associated with dermatomyositis or polymyositis is twice as likely in women than in men.12
Risk of associated malignancy was highest within the first year of diagnosis. Therefore, consider a diagnostic evaluation for malignancy at the time myopathy is diagnosed. The optimal diagnostic regimen in this setting is unknown. In one retrospective French study of 40 consecutive adult patients with inflammatory myopathy (33 with dermatomyositis and 7 with polymyositis) between the years 1981 and 2000, malignancy was present at the time of myopathy diagnosis in 16 patients (13 with dermatomyositis and 3 with polymyositis).12 An Australian population-based, retrospective cohort study of 537 individuals with biopsy-proven idiopathic inflammatory myopathy from 1981–1995 demonstrated 116 cases of malignancy in 104 patients.13 The risk was highest in dermatomyositis (standardized incidence ratio [SIR] 6.2), next highest in inclusion-body myositis (SIR 2.4), and lowest in polymyositis (SIR 2.0).
Diagnosis: What helps, what doesn’t
Suspect inflammatory myopathy by the constellation of clinical findings; confirm it by looking for elevated muscle enzymes and characteristic findings on EMG and muscle biopsy (see How inflammatory myopathies develop).
The most sensitive muscle enzyme for inflammatory myopathy is CK, levels of which usually parallel disease activity and may be used to assess response to therapy.6,10 Needle EMG demonstrates increased spontaneous activity with fibrillations; complex repetitive discharges; positive sharp waves; and voluntary motor units consisting of low-amplitude polyphasic units of short duration.6 EMG findings alone are not diagnostic.
Serologic tests are commonly done but their clinical usefulness is controversial.10 Antinuclear antibodies are found in about 80% of cases but are nonspecific and not clinically useful.14 Myositis-specific antibodies (MSAs) have been described in about 30% of idiopathic inflammatory myopathies but are also of uncertain diagnostic and pathogenic importance.14 The most prevalent MSA, anti-Jo, is present in only about 20% of cases and correlates with interstitial lung disease, but has uncertain usefulness in differentiating between dermatomyositis, polymyositis, and inclusion-body myositis.6,10,11,14,15