Cohort study of 23 patients with seropositive immune-mediated necrotizing myopathy

Immune-mediated necrotizing myopathy (IMNM) are rare muscular inflammatory muscle diseases that bear poor prognosis. Description of their clinical evolution remains limited in the literature. This study aimed to describe the clinical, biological, and serological features of patients with anti-HMGCR or anti-SRP IMNM presenting with muscle involvement at diagnosis and to identify factors associated with relapse during follow-up.

Between January 1st, 2015 and December 31st, 2023, we retrospectively identified 66 patients with anti-3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR) or anti-signal recognition particle (SRP) autoantibodies in three French immunology laboratories (Rouen, Le Havre, Dieppe). We included 23 patients with a follow-up of at least 3 months: 14 with anti-HMGCR and 9 with anti-SRP autoantibodies. Statistical analysis was performed on clinical data and focused on a score derived from the manual muscle testing score at 6 months after diagnosis and during follow-up, as well as biological data and treatment strategy.

IMNM patients with anti-SRP were significantly younger and had more severe disease at diagnosis compared to patients with anti-HMGCR. We observed an inverse correlation between creatine kinase (CK) levels and muscular strength during follow-up. At diagnosis, women, anti-SRP patients, and patients who experienced a relapse during follow-up had more severe muscle weakness compared to male, anti-HMCGR positive patients and patients who did not experience a relapse, respectively. Relapse occurred in 65.2% of patients. Relapsing patients had higher CK level and higher anti-SRP levels at diagnosis compared to non-relapsing patients. Treatment with intravenous immunoglobulins was associated with a shorter time to response.

Our study provides valuable clinical insights into the management of seropositive IMNM in a real-world setting and highlights the need for further research in this area.