Case Presentation: Autoimmune myopathies are rare diseases, with a prevalence of just 9–14 cases per 100,000 people and only about 10% of these have Immune-mediated necrotizing myopathy (IMNM). Instead of muscle biopsy, recent guidelines describe IMNM based on antibodies. Here we present a case of Anti-SRP myopathy, a rare subtype of IMNM.This is a case of a 52-year-old female with no significant past medical history who presented to the emergency department with myalgia, and generalized weakness beginning four weeks prior. She denied any associated rash, dysphagia, shortness of breath, statin or alcohol use. It was determined that she had significant proximal muscle weakness since she had difficulty standing up, and combing her hair. Her thighs were tender on palpation. Laboratory studies were significant for mildly elevated creatine kinase, ESR, and CRP. EMG/NCS was normal. Initial differential diagnosis included autoimmune myositis for which serology including ANA panel was sent, a muscle biopsy was performed which showed insignificant results and she was empirically started on prednisone. ANA panel was negative but given the high clinical suspicion, a myositis panel was also sent which was significant for elevated Anti- S1 SRP confirming antibody specific IMNM.
Discussion: Previous myositis classification criteria relied exclusively on muscle biopsy features to categorize patients as having IMNM. However, it is now appreciated that predominant necrosis on muscle biopsy may be found in a significant number of patients with other types of myositis. Following this trend, the most recent 2017 European Neuromuscular Centre (ENMC) criteria for IMNM divide this syndrome into three subtypes: anti-SRP myopathy, anti-HMGCR myopathy, and antibody-negative IMNM. The presence of elevated creatine kinase level and proximal muscle weakness is sufficient to diagnose the disease subtype in those patients that are positive for anti-SRP or anti-HMGCR autoantibodies; for those that are autoantibody-negative, it may be necessary to obtain a muscle biopsy to demonstrate the characteristic features of a necrotizing myopathy. Normal muscle biopsy in the presence of overt clinical symptoms can lead to over-testing and mislabeling as a psychiatric condition. Most experts agree that the treatment of IMNM should be initiated early and may need to be intense to avoid long-term disability. Corticosteroids plus methotrexate may be a good initial treatment. Rituximab and/or intravenous immunoglobulin (IVIG) are recommended for refractory disease.
Conclusions: In summary, muscle prognosis in IMNM is worse than in most other types of myositis. Our case highlights the importance of recognition and early initiation of treatment as this can improve prognosis significantly. Further clinical trials are needed to define optimal treatment strategies for IMNM patients.