P.21.4 Combining MRI and muscle biopsy improves diagnostic accuracy in subacute-onset idiopathic inflammatory myopathy

Abstract

In approximately 10–20% of patients suffering from subacute-onset idiopathic inflammatory myopathy (IIM) muscle biopsy is normal or shows non-specific findings. MRI can be used as triage test prior to a muscle biopsy and as add-on test if a muscle biopsy has proven to be non-diagnostic. A prospective study was performed on forty-eight consecutive patients with subacute-onset IIM of whom MRI scans of the skeletal muscles and muscle biopsies were systematically evaluated. The results of MRI, muscle biopsy findings and definite diagnosis including response to treatment which was considered the gold standard were collected and compared. The false negative rate of all muscle biopsies was 0.23. Biopsies taken from a muscle showing hyperintensity on MRI (as triage test) had a false negative rate of 0.19. Using the result of MRI as add-on test in patients with a non-diagnostic muscle biopsy decreased the false negative rate to 0.06. MRI has a high diagnostic accuracy as add-on test to the muscle biopsy in patients with a negative or non-specific muscle biopsy but otherwise a clinical presentation consistent with a diagnosis of subacute-onset IIM. Our results also suggest that MRI may be useful as triage test in reducing false negative muscle biopsy results. Therefore, in patients presumed to have a subacute-onset IIM it is recommended to perform both MRI and a muscle biopsy.