26th February 2025

Investigations in inflammatory myopathies

Muscle enzymes are classically elevated. Creatine kinase is the most commonly used; however, the transaminases and LDH can be elevated. Although cardiac involvement can occur, elevated troponin T values may be due to cross-reactivity with the creatine kinase assay, and in such situations, troponin I should be used to investigate further.

Inflammatory markers such as ESR and CRP do not correlate well with disease activity. 

Multiple different autoantibodies are associated with inflammatory myopathies and tend to predict the disease course. ANA is the most common antibody. In exams, it is often stated that anti-Mi2 is most associated with dermatomyositis and anti-Jo-1 with polymyositis; however, there are many other antibodies. 

MRI of muscles can show oedema in active myositis and can also be used to identify an appropriate site for biopsy. Muscle biopsy is gold standard; however, is not always required if the patient has classical features, raised muscle enzymes and a suggestive autoantibody.

EMG can be used to identify a myopathic process and will be abnormal in 90% of patients.

 And so, when patients are diagnosed, it is common to screen for malignancy with thorough examination and sometimes further investigations. Certain antibodies are more associated with this, such as TIF1y.