Causes of Myositis and Muscular Abscess
Primary Etiologic Categories
Myositis and muscular abscesses arise from four major categories: infectious agents (viral, bacterial, fungal, parasitic), drug-induced mechanisms (particularly immune checkpoint inhibitors and statins), autoimmune/idiopathic inflammatory processes, and malignancy-associated disease. 1, 2, 3
Infectious Causes
Viral Myositis
- Viral pathogens cause acute, self-limited myositis with diffuse myalgias, typically resolving within 3-7 days 4, 5
- Common viral agents include influenza, HIV, and SARS-CoV-2 1
- Viruses may also cause acute rhabdomyolysis and have been isolated in inclusion body myositis cases 5
- Characterized by normal to mildly elevated CK levels that normalize rapidly, distinguishing it from autoimmune myositis 4
Bacterial Myositis and Pyomyositis
- Bacterial infections typically cause localized myositis or muscular abscesses (pyomyositis) rather than diffuse disease 2, 3
- More common in immunocompromised patients and at sites compromised by trauma or surgery 2
- Specific bacterial syndromes include:
- These conditions are life-threatening and require urgent recognition and treatment 2
Fungal and Parasitic Myositis
- Fungal pathogens cause myositis, particularly in immunocompromised hosts 2, 3
- Parasitic organisms including Toxoplasma have been implicated in myositis 5, 3
Drug-Induced Myositis
Immune Checkpoint Inhibitor-Associated Myositis
- PD-1/PD-L1 inhibitors cause myositis more commonly than ipilimumab, with median exposure of 4 weeks 6
- Presents as de novo myositis or reactivation of preexisting paraneoplastic polymyositis/dermatomyositis 6
- Can be severe and fatal, with fulminant necrotizing course, rhabdomyolysis, and myocardial involvement requiring urgent treatment 6
- Characterized by markedly elevated CK levels, proximal weakness (difficulty standing, lifting arms, climbing stairs), and myalgia in severe cases 6
Statin-Induced Myopathy
- Statins are much more likely than antipsychotics to cause drug-induced muscle disorders 7
- Persistent and marked CK elevation (>10x normal) suggests necrotizing myopathy rather than simple drug effect 7
Autoimmune/Idiopathic Inflammatory Myopathies
Polymyositis
- Characterized by symmetric proximal muscle weakness with CD8+ cytotoxic T cells invading nonnecrotic muscle fibers 6
- Develops subacutely over weeks to months with persistently elevated CK 4
- Distinguished by true proximal weakness (difficulty rising from seated position, climbing stairs, lifting arms overhead) rather than just pain 7, 4
Dermatomyositis
- Presents with characteristic rash: erythematous photosensitive rash, Gottron papules, heliotrope rash, periorbital edema, periungual telangiectasias 6
- Juvenile dermatomyositis affects children <18 years with calcinosis cutis, cutaneous vasculitis, and gastrointestinal vasculopathy 6
- Symmetric proximal muscle weakness develops over weeks to months with elevated muscle enzymes 6
Immune-Mediated Necrotizing Myopathy (IMNM)
- Severe myopathy with minimal inflammatory infiltrate on biopsy 6
- Triggered by viral infections, statins, and malignant neoplasms 6
Inclusion Body Myositis
- Pathologic features suggest degenerative neuromuscular disease with muscle fiber vacuolization 6
- Abnormal accumulation of amyloid-β and phosphorylated tau proteins 6
Malignancy-Associated Myositis
- Myositis may be paraneoplastic, associated with underlying malignancy 6, 1
- Requires specific evaluation and treatment beyond standard immunomodulatory therapy 1
Overlap Syndromes and Other Causes
Myositis with Connective Tissue Diseases
- Myositis occurs with associated rheumatologic diseases, suggesting different pathogenetic mechanisms 5
- Immunological and pathological differences exist between isolated myositis and overlap syndromes 5
Distinctive Myositis Subtypes
- Eosinophilic myositis and granulomatous myositis have distinctive clinical and pathological features reflecting different injury mechanisms 5
- Orbital myositis and localized nodular myositis are confined to single muscles or muscle groups 5
Critical Diagnostic Distinctions
Differentiating Myositis from Myalgia Syndromes
- Polymyalgia rheumatica-like syndrome presents with severe myalgia and fatigue but no true weakness 6
- CK levels remain within normal limits in polymyalgia-like syndromes, differentiating from true myositis 6
- MRI and EMG show no evidence of myopathy or muscle inflammation in pain-only syndromes 6
Key Laboratory Markers
- Persistently elevated CK (often markedly so) indicates true myositis 6, 4
- Myositis-specific autoantibodies (anti-Jo-1, anti-Mi-2, anti-MDA5, anti-TIF1-gamma, anti-NXP2) define autoimmune myositis phenotypes and predict outcomes 6, 7, 4
- Inflammatory markers (ESR, CRP) are highly elevated in immune-mediated myositis 6