P.03Delayed diagnosis and treatment leads to worse outcome in immune mediated necrotizing myopathy

Abstract

Immune mediated necrotizing myopathy (IMNM) is increasingly being recognized over the past several years as unique subtype of the idiopathic inflammatory myopathies with characteristic proximal weakness, high CK, association with anti SRP or anti HMGCR antibody in the majority of patients, and requires early and aggressive immunosuppression. A small subset of patients with IMNM can present with more insidious weakness over years. Diagnosis and treatment can be delayed significantly in these patients, and response to treatment has not been clearly established. We report a series of 16 patients with IMNM with a wide range of duration from symptom onset to diagnosis and treatment. Patient's age ranges from 21 to 86 years with clinical characteristics similar to previously published findings. The average initial CK was 12256 U/L and average peak CK was 14603 U/L. 15 out of 16 patients tested positive for either anti HMGCR antibody (12 patients) or anti SRP antibody (3 patients). Histologic findings consistent with IMNM were present in 14 patients (87.5%). Time from symptoms onset to diagnosis ranges from 2 months to 21 years, with average time to diagnosis being 39.6 months and median time to diagnosis being 11 months. Time from symptoms onset to treatment initiation ranges from 3 months to 21 years, with average being 34 months and median time to treatment being 8.5 months. All of the patients received immunosuppressive treatment with corticosteroid and IVIG being the most common (81.2%), followed by Methotrexate (31%) Rituximab (19%), Azathioprine (19%) and Mycophenolate (6%). The 5 patients with the longest time to diagnosis range from 44 months to 245 months. 9 patients received treatment within 12 months from symptom onset have average mRS (modified Rankin score) of 1.6. 4 patients received treatment 12-36 months from symptoms onset have average mRS of 1. 3 patients received treatment more than 36 months from onset have average mRS score of 4. Of these 3 patients, two had MRI of hip and thigh muscles showing severe fatty replacement of most pelvic and hip girdle muscles. While the number of patients in our study is small, the findings point towards a worse outcome in patients whose treatment are delayed by 3 years or more, reinforcing the need to diagnose these patients as early as possible.