Massive Italian Study Maps the Hidden Split Between Rare Autoimmune Muscle Diseases

One of the largest real-world studies of immune-mediated necrotizing myopathy, a rare but potentially devastating autoimmune muscle disease, has revealed striking differences between its main antibody-defined subtypes and identified practical markers that clinicians can use to track treatment response. The research, published in the Journal of Neurology, brings together data from 159 patients treated across 14 Italian neuromuscular centers, offering a level of statistical power that single-center studies of this orphan disease have rarely achieved.

Immune-mediated necrotizing myopathy, or IMNM, belongs to the family of idiopathic inflammatory myopathies, a group of acquired diseases in which the immune system attacks skeletal muscle. What distinguishes IMNM from conditions such as dermatomyositis or inclusion body myositis is its pathological signature: muscle fibers die off in large numbers through a process of necrosis, while the inflammatory cell infiltrate that defines other myositis subtypes remains comparatively sparse. Under the microscope, a biopsy from an IMNM patient shows scattered necrotic and regenerating fibers, evidence of complement deposition on small blood vessels, and remarkably little lymphocytic invasion of otherwise intact tissue.

The disease is defined serologically by two dominant autoantibodies. The first targets the signal recognition particle, or SRP, a ribonucleoprotein complex that ferries newly synthesized proteins into the endoplasmic reticulum. The second targets HMGCR, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, the rate-limiting enzyme of cholesterol biosynthesis and the pharmacological target of statin drugs. Anti-HMGCR myopathy can be triggered by statin exposure, but it also occurs in patients who have never taken the medications. A third group of patients, seronegative IMNM, carries neither antibody and remains biologically enigmatic. Pathogenic studies suggest that both anti-SRP and anti-HMGCR antibodies drive muscle fiber death partly through complement activation, and that they impair muscle regeneration, which helps explain why the disease tends to be severe and relapsing without immunosuppressive treatment.

The new study, coordinated by Silvia Bonanno and Lorenzo Maggi of the Fondazione IRCCS Istituto Neurologico Carlo Besta in Milan, applied the 224th European Neuromuscular Centre diagnostic criteria to adults diagnosed between 2019 and 2022. The cohort comprised 159 subjects, 57 percent of them female, with a median age of 62 years. Of these, 40 carried anti-SRP antibodies, 71 carried anti-HMGCR antibodies, 41 were seronegative, and 7 had not been tested. The investigators compared clinical presentation, laboratory values, functional outcomes, and treatment responses across these subgroups, adjusting their statistical comparisons for multiple testing.

The results sharpen a picture that smaller studies had only sketched. Patients with anti-SRP antibodies more frequently presented with weakness of the proximal muscles of the upper limbs and with involvement of the axial muscles, those of the trunk and neck, compared with both anti-HMGCR and seronegative patients. The odds ratios were substantial: 3.89 for proximal upper limb weakness relative to the HMGCR group, and 15.2 for axial involvement relative to the seronegative group. Lung involvement, a feared extramuscular complication of inflammatory myopathies, appeared in 7.5 percent of SRP-positive patients but was absent in the HMGCR group and rare in seronegative patients, a difference that reached statistical significance. Conversely, extramuscular features overall were more common in the anti-HMGCR group.

Outcomes diverged along the same immunological fault lines. Compared with anti-HMGCR patients, those with anti-SRP antibodies had more than four times the odds of losing motor independence and fourteen times the odds of experiencing a disease relapse, a finding consistent with the widely held clinical impression that anti-SRP myopathy is the more aggressive of the two seropositive forms. Seronegative patients, by contrast, achieved better functional outcomes than the SRP group, suggesting that the absence of a detectable myositis-specific antibody may not signal a worse prognosis, as clinicians sometimes fear, but rather a milder disease course. One prognostic factor cut across subtypes: cancer-associated myositis predicted worse disability, with nearly sevenfold increased odds of poor functional outcome, underscoring the importance of malignancy screening in newly diagnosed patients.

Treatment data from the cohort provide reassurance grounded in numbers rather than anecdote. Overall, 71.7 percent of patients improved on standard therapy combining corticosteroids with immunosuppressants, and the benefit of both drug classes was statistically significant, with odds ratios of 3.83 for steroids and 2.30 for immunosuppressants. Crucially, the treatment response did not differ between serological subgroups, meaning that the conventional first-line strategy remains effective across the spectrum of IMNM. Creatine kinase, the muscle enzyme released into the blood when fibers die, fell in 81.8 percent of treated patients and returned to normal in 40.3 percent.

Perhaps the most practically useful finding concerns how to monitor that response. The change in creatine kinase levels over treatment, which the authors call delta-CK, was statistically associated with clinical improvement, predicted achievement of clinical remission, and correlated with post-treatment functional scores. In other words, a routine and inexpensive blood test tracks the patient’s recovery well enough to serve as a surrogate marker in busy clinics and, potentially, in real-world observational studies where formal muscle testing is impractical. The authors also explored functional outcome measures adapted from other neuromuscular fields, arguing that these capture treatment response in everyday clinical settings better than the trial endpoints designed for dermatomyositis and polymyositis.

The study’s design has limitations worth noting. It was retrospective, relying on medical records compiled before standardized data collection, and the number of patients in some subgroup comparisons was modest, particularly for the seven untested individuals and for rare events such as lung involvement. Response criteria borrowed from other myositis subtypes may imperfectly capture IMNM-specific outcomes. Nevertheless, the multicenter scope, the application of the 224th ENMC criteria to every case, and the sheer size of the cohort relative to prior IMNM studies give the findings unusual weight for a rare disease.

The implications reach beyond the clinic. By demonstrating that antibody status predicts weakness distribution, relapse risk, and extramusular involvement, the Italian cohort strengthens the case that anti-SRP, anti-HMGCR, and seronegative IMNM are biologically distinct entities rather than variations of a single disease. That distinction matters for the emerging pipeline of targeted therapies, including complement inhibitors such as zilucoplan, which showed promise in a phase 2 trial and would logically benefit patients whose fiber death is complement-driven. It also matters for trial design: a future study of a complement blocker might prioritize anti-SRP patients, who carry the highest relapse burden, while reserving other agents for seronegative disease. For now, the study gives neurologists something immediately actionable, a validated laboratory signal to follow and a clearer sense of which patients to watch most closely, in a disease where until recently even the basic epidemiology rested on collections of a few dozen cases.

Subject of Research: Clinico-serological features and treatment outcomes of immune-mediated necrotizing myopathy

Article Title: Immune-mediated necrotizing myopathies: clinico-serological features and treatment outcomes of a large Italian cohort of patients

Article References: Immune-mediated necrotizing myopathies: clinico-serological features and treatment outcomes of a large Italian cohort of patients. (n.d.). https://doi.org/10.1007/s00415-026-14089-1

Image Credits: AI Generated

DOI: 10.1007/s00415-026-14089-1

Keywords: immune-mediated necrotizing myopathy, anti-SRP antibodies, anti-HMGCR antibodies, seronegative myopathy, creatine kinase, autoimmune muscle disease, myositis, corticosteroids, immunosuppressants, ENMC criteria, Italian cohort, disease relapse