The perils of myositis mimickers with illustrative case reports.

OBJECTIVES: The idiopathic inflammatory myopathies (IIMs) are a heterogeneous group of immune-mediated systemic disorders that commonly target skeletal muscles. The aim of our review is to remind clinicians to be vigilant of common mimickers, and what red flags to look for to avoid misdiagnosis. METHODS: We reviewed the clinical documentation and investigation results of illustrative real-life case examples of significant IIM mimickers with valuable learning points. Following an initial diagnosis of IIM, the patients had been referred to our Adult Neuromuscular Service at Salford Royal Hospital Northern Care Alliance NHS Foundation Trust, UK. RESULTS: Four cases, two males and two females, were analysed. Retrospective review of key case-specific features suggestive of alternative diagnoses were identified and described, prompting a broader discussion of common disease groups that can mimic IIM. CONCLUSIONS: The presentation of IIM is heterogeneous and the differential diagnosis wide. Several non-inflammatory conditions can present as mimickers of IIM, each requiring a different management approach.

Limb girdle muscular dystrophy R12 (LGMD 2L, anoctaminopathy) mimicking idiopathic inflammatory myopathy: key points to prevent misdiagnosis.

OBJECTIVES: Diagnosing the idiopathic inflammatory myopathies (IIMs) can be challenging as several conditions, including genetic myopathies such as limb girdle muscular dystrophy type R12 (LGMD 2 l, anoctaminopathy) mimic the presentation. Here we describe learning points identified from review of four patients with LGMD 2 l who were initially incorrectly diagnosed with IIM. Our aim is to provide clinicians working in adult rheumatology services with a toolkit to help identify non-inflammatory presentations of myopathy. METHODS: We performed retrospective review of medical notes, laboratory results, muscle imaging and histological findings of four patients with LGMD 2 l who were previously misdiagnosed with IIM. We focussed on clinical presentation and progression, therapeutic agents used and events leading to revision of the diagnosis. RESULTS: Three male patients and one female patient with a mean age of 51 years at presentation were reviewed. In each case, treatment with immunosuppressants, in one case for >15 years, was observed without a clear therapeutic response. All patients were negative for anti-nuclear antibodies and available myositis-associated/specific autoantibodies and associated connective tissue disease features were absent. Prominent fatty infiltration and selective muscle involvement on thigh MRI was found in common. CONCLUSIONS: Adult-onset genetic myopathies, particularly LGMD R12, can mimic IIM. Accurate diagnosis is crucial to avoid the use of potentially harmful immunosuppressive therapies, to allow appropriate genetic counselling and to facilitate involvement in research studies.