Muscle disease is more prominent among patients with dermatomyositis and anti-Mi2 autoantibodies compared with in those who have anti-Mi2-negative dermatomyositis, or in patients with antisynthetase syndrome, according to study results published in Neurology.

Idiopahthic inflammatory myopathies are a group of diseases that include dermatomyositis and antisynthetase syndrome, as well as immune-mediated necrotizing myopathy (IMNM) and inclusion body myositis. Myositis-specific autoantibodies are frequently found in patients with these myopathies and define unique subtypes of disease. Previous studies have reported that patients with anti-Mi2-positive dermatomyositis have mild muscle disease. The goal of this study was to describe the phenotypes of anti-Mi2-positive and anti-Mi2-negative dermatomyositis, antisynthetase syndrome, and IMNM.

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The study enrolled patients from the Johns Hopkins Myositis Center Longitudinal Cohort Study between 2002 and 2018, including 58 patients with anti-Mi2-positive dermatomyositis, 143 patients with anti-Mi2-negative dermatomyositis, 162 patients with antisynthetase syndrome, and 170 patients with IMNM.

At disease onset, weakness was present in 60% of patients with anti-Mi2-positive dermatomyositis and in 46% of those with anti-Mi2-negative dermatomyositis compared with in 88% of patients with IMNM. During follow-up, weakness was more frequent in those with anti-Mi2-positive dermatomyositis than in those with anti-Mi2-negative dermatomyositis (98% vs 85%, P =.008).

At the initial visit, neck flexors, arm abductors, elbow extensors, hip flexors and extensors, and knee flexors were weaker among patients with anti-Mi2-positive dermatomyositis compared with those with anti-Mi2-negative dermatomyositis or antisynthetase syndrome (P <.05 for all comparisons). Compared to people with IMNM, patients with anti-Mi2-positive dermatomyositis had stronger hip flexors.

During the follow-up, hip flexors and arm abduction strength were significantly weaker in patients with anti-Mi2-positive dermatomyositis compared with in patients with anti-Mi2-negative dermatomyositis or antisynthetase syndrome (P <.01 for all comparisons). Compared with patients with IMNM, those with anti-Mi2-positive dermatomyositis had substantially stronger mean hip flexors but similar arm abduction strength.

Creatine kinase (CK) levels were inversely associated with strength, and compared with patients with anti-Mi2-positive dermatomyositis, levels of CK were lower in those with anti-Mi2-negative dermatomyositis (P <.001), higher in patients with IMNM (P <.001), and similar in those with antisynthetase syndrome (P =.6).

Compared with patients with anti-Mi2-positive dermatomyositis, muscle atrophy (68% vs. 26%, P <.001) and fatty replacement (87% vs 39%, P <.001) were more common in those with IMNM. Review of available muscle biopsies showed no significant differences in the prevalence of perifascicular atrophy or perivascular inflammation between patients with anti-Mi2-positive dermatomyositis and those with anti-Mi2-negative dermatomyositis or antisynthetase syndrome.

There was a weak but significant association between titer of anti-Mi2 autoantibody and increased CK levels (P <.001) and decreased strength (P <.001). During the course of the disease, autoantibody titers declined in all 10 patients with ≥5 available serum samples.

Over the course of the disease, calcinosis, interstitial lung disease, and fever were less common in patients with anti-Mi2-positive dermatomyositis than in those with anti-Mi2-negative dermatomyositis. Similarly, interstitial lung disease, arthritis, and fever were less common in those with anti-Mi2-positive dermatomyositis compared to patients with antisynthetase syndrome.

The researchers noted that many recently developed outcome measures were not collected for a significant number of patients, which was a limitation of this study.

“This finding supports the growing body of evidence that dermatomyositis is a heterogeneous disease and that [myositis-specific autoantibodies] such as anti-Mi2 can be used to classify patients with dermatomyositis into more homogeneous subgroups,” conclude the researchers.

Reference

Pinal-Fernandez I, Mecoli CA, Casal-Dominguez M, et al. More prominent muscle involvement in patients with dermatomyositis with anti-Mi2 autoantibodies. [published online, October 8, 2019]. Neurology. doi:10.1212/WNL.0000000000008443