Myositis Differential Diagnosis

The differential diagnosis for myositis can be organized into the following categories:

• Single Most Likely Diagnosis
  • Polymyositis: This is an inflammatory muscle disease characterized by muscle weakness and inflammation. It is a common cause of myositis and should be considered first due to its relatively high prevalence among myositis cases.
• Other Likely Diagnoses
  • Dermatomyositis: An inflammatory disease characterized by skin rash and muscle weakness. It is another common form of myositis and should be considered, especially if skin manifestations are present.
  • Inclusion Body Myositis (IBM): The most common acquired myopathy in adults over 50, characterized by progressive muscle weakness and wasting. It is a likely diagnosis in older adults presenting with myositis.
  • Infectious Myositis: Caused by bacterial, viral, or parasitic infections. This should be considered, especially if there's a history of recent infection or exposure.
• Do Not Miss Diagnoses
  • Malignancy-associated Myositis: Some myositis cases can be associated with underlying malignancies. It's crucial to investigate for cancer in patients with myositis, especially if they have risk factors or other symptoms suggestive of malignancy.
  • Statin-induced Myopathy: While not typically classified under myositis, statin use can lead to muscle inflammation and damage. Given the widespread use of statins, this is an important consideration.
  • Thyroid-associated Myopathy: Both hypothyroidism and hyperthyroidism can cause muscle weakness and inflammation. Given the prevalence of thyroid disorders, this is a critical differential diagnosis.
• Rare Diagnoses
  • Eosinophilic Myositis: A rare condition characterized by muscle inflammation with eosinophilia. It's essential to consider this in patients with myositis and eosinophilia.
  • Focal Myositis: A rare inflammatory pseudotumor of the soft tissues. It presents as a localized swelling and should be considered in cases of asymmetric or localized muscle involvement.
  • Sarcoid Myositis: Part of systemic sarcoidosis, where granulomatous inflammation affects the muscles. This should be considered in patients with known sarcoidosis or other systemic symptoms.
  • Necrotizing Autoimmune Myopathy (NAM): A rare and severe form of myositis characterized by rapid progression of muscle weakness. It's often associated with statin use or connective tissue diseases.