gms | German Medical Science

Background and Purpose: To date, the diagnosis of idiopathic inflammatory myopathies (IIM) presents a clinical challenge and muscle biopsy is widely regarded as gold standard for confirming diagnosis of IIM. However, the significance of muscle biopsy as a diagnostical tool remains inconclusive. The purpose of this study was to determine the clinical relevance of muscle biopsy in patients with suspected myositis.

Methods: In this retrospective cohort study, histopathological findings of all muscle biopsy specimens of adult (≥ 18 years of age) patients with clinically suspected or differential diagnosis of inflammatory muscle disease referred from all over Austria to the Department of Neuropathology at the Medical University of Vienna through the period of 01.01.2007-31.12.2020 were analyzed and eligible for inclusion. Following information were extracted from handwritten assignment sheets: referral department, suspected diagnosis (inflammatory vs non-inflammatory myopathy), sampling location, clinical symptoms, laboratory, serologic and imaging results. Histological findings were extracted and grouped in 1) inflammatory 2) non-inflammatory (neurogenic, metabolic etc.) 3) non-inflammatory (mixed features) 4) inflammatory and non-inflammatory mixed 5) normal. Descriptive analyses were performed.

Results: We identified 743 muscle biopsy specimens of 731 patients with suspected myositis (48.6% female, mean age of 55.7 +/- 16.5 years). Referrals were mainly conducted by neurology (50.2%), rheumatology (17%), internal medicine (12.4%) and dermatologic divisions (7.0%). Sampling locations comprised mainly proximal lower extremities (53.4%) or proximal upper extremities (23.4%). Most frequently reported clinical symptoms included muscle weakness (51.8%), muscle pain (35.4%), atrophy (9.2%) and skin affection (6.7%), respectively. Creatine kinase (CK) levels were reported in 486/743 (65.41%) cases of which 88.5% were elevated. In general, 353/743 muscle biopsies (47.5%) showed histologically inflammatory features solely or in combination with non-inflammatory characteristics. Out of 561 samples with suspected diagnosis of inflammatory myopathy only, diagnosis was confirmed histologically in 260 (46.3%), 47 (8.4%) also showed co-features of non-inflammatory myopathy (such as neurogenic lesions or type-2 fiber atrophy); 235 (41.9%) were non-inflammatory and 17 (3%) had normal features. Out of 182 samples with suspected diagnosis of non-inflammatory myopathies in addition to IIM, 35 (19.2%) were histologically diagnosed as inflammatory myopathy and 11 (6%) showed mixed features, 125 (68.7%) were non-inflammatory and 10 (5.5%) were normal. 306 of 743 (41.2%) muscle biopsies could be further histologically classified into Polymyositis (n=50; 6.73%), Dermatomyositis (n=81; 10.9%), Immune-mediated necrotizing myopathy (n=118; 15.9%), Inclusion body myositis (n=54; 7.3%) and Overlap myositis (n=3; 0.4%); 64/743 (8.61%) were labeled as unspecific myositis that could not be further classified histologically. Normal samples with no pathological features occurred only in 27/743 (3.6%) of the cases, with 0.2% sampling error (no muscle tissue). Interestingly, clinical symptoms were not different in subgroup of different suspected nor final histopathological diagnosis, whereas normal CK-levels were mainly found in patients with suspected and histopathological diagnosis of non-inflammatory myopathies (figure).

Conclusion: With this study we could describe the match of suspected and final histopathological diagnosis in patient with inflammatory myopathies. Further steps involve determination of the diagnostic accuracy and the predictive value of muscle biopsy in relation to clinical diagnosis and treatment initiation in these patients.

Disclosures: None declared

Figure 1 [Fig. 1]