Definition of Polymyositis
Polymyositis is a rare idiopathic inflammatory myopathy characterized by subacute-to-chronic symmetric proximal muscle weakness, elevated muscle enzymes (particularly creatine kinase), and endomysial inflammatory infiltrates consisting predominantly of CD8+ T cells invading healthy muscle fibers expressing MHC-I antigen. 1, 2
Core Diagnostic Features
Polymyositis represents one of the major subtypes within the inflammatory myositis spectrum and is now recognized as considerably rarer than previously thought, functioning primarily as an exclusion diagnosis. 1, 3
Clinical Presentation
Muscle weakness develops over weeks to months, affecting proximal muscles symmetrically, manifesting as difficulty standing from seated position, climbing stairs, or lifting arms overhead. 1, 4
No skin manifestations are present—this distinguishes polymyositis from dermatomyositis (patients with rash have dermatomyositis, not polymyositis). 1, 2
Extramuscular manifestations can occur but are less prominent than in dermatomyositis, as polymyositis is primarily a myopathic disease. 5, 4
Laboratory and Diagnostic Criteria
Creatine kinase elevation may reach 50-fold above normal in active disease. 2, 4
Muscle biopsy is the gold standard and pivotal for diagnosis, showing endomysial inflammatory infiltrate with predominantly CD8+ T cells invading non-necrotic muscle fibers that express MHC-I antigen. 2, 6
Autoantibodies may be present and support the autoimmune pathogenesis, though specific myositis-specific antibodies help define clinical subsets. 1, 7
EMG and imaging can support diagnosis but muscle histopathology remains the definitive diagnostic tool. 2
Critical Diagnostic Considerations
Polymyositis is an exclusion diagnosis—a broad differential must be systematically ruled out before confirming this diagnosis. 2, 3
Essential Exclusions
Sporadic inclusion-body myositis must be excluded through careful histopathological examination. 2
Immune-mediated necrotizing myopathy (IMNM) presents with more severe weakness, markedly elevated CK (>10 times upper limit), and minimal inflammatory infiltrates on biopsy—this is a distinct entity, not polymyositis. 5
Toxic myopathies (particularly statin-induced), endocrine myopathies, metabolic myopathies, and muscular dystrophies must all be ruled out. 2
Overlap syndromes with connective tissue diseases should be identified, as these represent distinct clinical entities. 7, 4
Evolution of the Diagnosis
The discovery of myositis-specific autoantibodies and improved immunohistochemical techniques have led to reclassification of many cases previously labeled as polymyositis into more specific entities like antisynthetase syndrome and immune-mediated necrotizing myopathies, making true polymyositis relatively rare. 3
Distinguishing from Related Conditions
Dermatomyositis has pathognomonic skin findings (heliotrope rash, Gottron papules) and different pathophysiology involving complement-mediated capillary destruction. 5
Immune-mediated necrotizing myopathy shows muscle fiber necrosis with minimal inflammation, not the endomysial CD8+ T cell infiltration seen in polymyositis. 5
Polymyalgia rheumatica-like syndromes present with pain but not true weakness, normal CK levels, and no evidence of myopathy on EMG or MRI. 8