Investigations for idiopathic inflammatory myopathies

Blood investigations that may assist in the diagnosis of an idiopathic inflammatory myopathy include:

• serum creatine kinase (CK) concentration—usually raised; however, in inclusion body myositis (IBM), serum CK concentration may be normal or raised usually no more than 12 times the upper limit of normal
• alanine aminotransferase (ALT), aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) concentrations—raised
• autoantibodies—including myositis-specific antibodies and myositis-associated antibodies; awareness of antibodies in identifying disease subtypes is increasingNeedham, 2016
• thyroid function tests—hyper- and hypothyroidism can be associated with myopathy and are important differential diagnoses.

If additional specialised investigations are needed, they should be at the direction of a specialist, and may include:

• magnetic resonance imaging (MRI) scan
  • can demonstrate the pattern of muscle involvement
  • can show oedema in affected muscles
  • can help guide the site for a muscle biopsy
• electromyography
  • can demonstrate the pattern of muscle involvement and that weakness is primarily muscular in origin
  • can exclude neurogenic causes or neuromuscular junction disorders
  • can help guide the site for a muscle biopsy
• muscle biopsy
  • is often required to confirm the diagnosis
  • is more sensitive if performed before starting immunomodulatory drugs
  • results should be correlated with the clinical picture.