Background/Purpose:

The idiopathic inflammatory myopathies (IIM) constitute a potentially steroid-responsive group of conditions that must be differentiated from other causes of muscle weakness. Histopathology from muscle biopsy remains pivotal to diagnosis; however, muscle involvement in IIM is often patchy with associated risk of non-diagnostic biopsies. There has been mounting evidence for the use of magnetic resonance imaging (MRI) prior to biopsy to target biopsy site and improve diagnostic yield [1], though this is not routinely used. This single-centre 10-year clinical audit aimed to examine whether the use of MRI prior to muscle biopsy in patients with undifferentiated myopathy resulted in high rates of diagnostic results compared to selection of biopsy site based on clinical assessment of muscle involvement alone.

Methods:

Patient admissions over a 10-year period with an ICD code of “myopathy” or “myositis” were extracted from the hospital iSOFT database and screened for eligibility. Each eligible case was assessed to determine (1) clinical localisation of muscle involvement (2) whether a musculoskeletal MRI was performed pre-biopsy (3) site of muscle biopsy (4) histopathology result (+/- electron microscopy/muscle markers) (5) congruence of muscle involvement by clinical assessment and selected biopsy site (6) congruence of muscle involvement on MRI and selected biopsy site.

Results:

Forty-eight eligible cases were identified for inclusion in the study. Of these, 15 had musculoskeletal MRI performed prior to muscle biopsy and 33 did not. Of the patients who did not have an MRI, the biopsy was positive in 30 cases (91%) and negative in 3 (9%). In the patients who did undergo MRI, the result was positive in 12 (80%) and negative in 3 (20%). The selected biopsy site correlated with clinical sites of muscle involvement in 40 cases, 3 did not, and 5 lacked sufficient clinical documentation to determine this. Two of the three negative biopsies in the MRI group had been targeted appropriately; biopsy correlated to sites of involvement on MRI. One of these cases was ultimately given a clinical diagnosis of dermatomyositis.

Conclusion:

Inflammatory myopathies are uncommon clinical conditions. Despite the limitations of a small sample size and retrospective analysis, our results suggest that clinical targeting for muscle biopsy site alone yields high rates of diagnostic results and doesn’t support the use of MRI to target biopsy site. However the results are perhaps surprising, and therefore a prospective study would be of interest.

1. Day J, Patel S, Limaye V. The role of magnetic resonance imaging techniques in evaluation and management of the idiopathic inflammatory myopathies. Semin Arthritis Rheum. 2017;46(5):642-9.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/in-patients-with-suspected-idiopathic-inflammatory-myopathy-does-pre-biopsy-musculoskeletal-mri-result-in-greater-yield-of-diagnostic-biopsy-results-summary-of-data-from-a-10-year-single-hospital-au/