What are the causes of acute myositis?

Causes of Acute Myositis

Acute myositis is most commonly caused by viral infections, particularly influenza viruses, followed by bacterial infections, autoimmune conditions, medications (especially immune checkpoint inhibitors), and other systemic disorders. 1

Infectious Causes

Viral Causes

• Influenza viruses (A and B) - most common viral cause, typically developing after the subsidence of respiratory symptoms 1
• Enteroviruses (Coxsackie B) 1
• Parvovirus B19 1
• Human herpes virus 6 1
• HIV 1, 2
• SARS-CoV-2 2
• Cytomegalovirus 1
• Epstein-Barr virus 1
• Varicella zoster virus 1

Bacterial Causes

• Streptococcus pneumoniae 1
• Staphylococcus aureus 1, 3
• Haemophilus influenzae 1
• Group A, C, and G β-hemolytic streptococci 1
• Clostridium species (causing gas gangrene) 3
• Borrelia burgdorferi (Lyme disease) 1
• Mycobacterium tuberculosis 1
• Diphtheria 1
• Syphilis 1

Other Infectious Agents

• Parasites (Trichinosis, cysticercosis) 1, 3
• Fungi (rare, usually in immunocompromised hosts) 3

Non-Infectious Causes

Autoimmune/Inflammatory

• Sarcoidosis 1
• Myocarditis with associated myositis 1
• Overlap syndromes with other rheumatologic diseases 2
• Giant cell myocarditis (with associated myositis) 1

Medication-Induced

• Immune checkpoint inhibitors (ICPIs) - especially anti-PD-1/PD-L1 agents 1
• Statins 1
• Colchicine 1
• Chloroquine 1
• Emetine 1

Toxic Exposures

• Organophosphates 1
• Lead, thallium, arsenic 1
• Diethylene glycol, ethylene glycol 1
• Methanol 1
• N-hexane 1
• Ethanol 1
• Paraquat 1
• Snake venom 1

Metabolic/Endocrine

• Hypokalaemia or hypokalaemic thyrotoxic periodic paralysis 1
• Hypophosphataemia 1
• Hypermagnesaemia 1
• Vitamin deficiencies (B1, B12, E) 1
• Hyperthyroidism and hypothyroidism 1
• Copper deficiency 1

Malignancy-Associated

• Paraneoplastic syndromes 1, 2
• Leptomeningeal metastases 1
• Neurolymphomatosis 1

Clinical Presentation and Diagnosis

• In viral myositis (most common form), symptoms typically develop after the subsidence of acute respiratory symptoms 1, 4
• Gastrocnemius and soleus muscles are typically involved with pain and tenderness to palpation 1
• Laboratory findings include elevated serum creatine phosphokinase (CK), aldolase, transaminases, and lactate dehydrogenase 5, 4
• Inflammatory markers (ESR, CRP) are often elevated 5
• Complete recovery usually occurs within three days for viral causes 1
• Rarely, myositis may be associated with myoglobinuria and renal failure 1
• Electromyography (EMG) can show muscle fibrillations indicative of myopathy 1, 5
• MRI may show increased signal in affected muscles 1, 5

Management Considerations

• Treatment depends on the underlying cause 5, 2
• For viral myositis, supportive care is usually sufficient 4, 6
• For bacterial myositis, appropriate antibiotics are required 3
• For immune-mediated myositis, corticosteroids and immunosuppressive therapy may be needed 1, 5
• Severe cases may require hospitalization, especially if there is risk of rhabdomyolysis 5, 6
• Monitor creatine phosphokinase levels to assess for complications like rhabdomyolysis 5, 6

Prognosis

• Most cases of viral myositis are self-limiting with complete recovery 1, 6
• Bacterial myositis may require more aggressive treatment 3
• Immune-mediated myositis may have a more prolonged course requiring immunosuppression 1
• Early recognition and treatment are crucial to prevent complications 5