Background/Purpose: Immune Mediated Necrotizing Myopathy (IMNM) is a debilitating but understudied entity.  Correlations of demographic and clinical features with pathological findings remain underexplored.  In addition, while intravenous immunoglobulin (IVIG) is thought to be effective based on case series, optimal treatment duration and effectiveness remain uncertain.

Methods: We performed a retrospective study of IMNM patients managed in an academic medical center from 2014 through 2024.  We assessed associations of IVIG duration with presence of flares, time to remission, and glucocorticoid exposure (mg). We also evaluated for associations of demographic and clinical features with pathologic findings and biomarkers (aldolase, creatine phosphokinase [CPK]) using the linear and logistic regression, where appropriate.

Results: We studied 24 patients. For demographics of our population, the mean age was 59.3 years with approximately 33% male and 67% White. The antibodies were divided as follows; HMG-CoA reductase antibody positivity in 62.5%, SRP antibody positivity in 29.2%, and 12.5% were seronegative. IVIG was administered in 18 patients (75%) and rituximab in (45.8%), with a variety in use of disease modifying antirheumatic drugs (DMARDs). Aldolase concentrations trended with statin use (Atorvastatin P=0.086, Simvastatin P=0.097). No similar trend was observed with CPK values.  

Of note, a vacuolar muscle pathology was observed in 3 patients (12%). All were females with HMG-CoA reductase antibody, while two out of the three were African American (p=0.03) with a positive antinuclear antibody. Two had protracted courses despite high-dose glucocorticoids and IVIG use. All three required rituximab and all achieved remission. 

Among the IVIG treated patients (18), the mean IVIG duration was 15.6 months (SD 25.9). Remission was achieved in about 77% (mean time: 11.2 months). Flares occurred in 39% (mean onset approximately 20 months from IVIG initiation). IVIG duration correlated with time to remission and flare likelihood (P=0.03 for each).

Conclusion: Once IVIG therapy is adopted, the duration exceeded one year, on average. There was no specific duration that prevented flares. The positive correlation with flares suggests that flares prompted providers to treat longer with IVIG. Aldolase showed a trend toward correlation with statin use, a trend not observed with CPK.  This needs to be explored further to determine the relative value of aldolase vs. CPK as biomarkers. We identified a distinct variant of HMGCR antibody positive IMNM that we tentatively termed Vacuoles-Associated Immune Mediated Myopathy (VASIMM). It presented with vacuoles on muscle biopsy, associations with female gender and African American race and maybe a favorable clinical response to rituximab compared to IVIG. This variant and its associations should be investigated further in larger panels of patients and ideally in a prospective fashion. 

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/ivig-treatment-duration-in-immune-mediated-necrotizing-myopathy-and-a-possible-association-of-vacuolar-changes-with-black-african-american-female-patients/