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Objective Among specific autoantibodies in DM, the anti–small ubiquitin-like modifier activating enzyme (SAE) antibody is rare. We aim to describe the clinical characteristics, cancer prevalence, and muscle pathology of anti-SAE–positive DM. Methods Patients with a diagnosis of DM and sera positive for the anti-SAE antibody were recruited from 19 centres in this retrospective observational study. The available muscular biopsies were reviewed. We conducted a comparison with anti-SAE–negative DM and a review of the literature. Results Of the patients in the study (n = 49), 84% were women. Skin involvement was typical in 96% of patients, with 10% having calcinosis, 18% ulceration and 12% necrosis; 35% presented with a widespread skin rash. Muscular disease affected 84% of patients, with mild weakness [Medical Research Council (MRC) scale 4 (3, 5)], although 39% of patients had dysphagia. Muscular biopsies showed typical DM lesions. Interstitial lung disease was found in 21% of patients, mainly with organizing pneumonia pattern, and 26% of patients showed dyspnoea. Cancer-associated myositis was diagnosed in 16% of patients and was responsible for the majority of deaths, its prevalence being five times that of the general population. IVIG therapy was administered to 51% of the patients during the course of the disease. Comparison with anti-SAE–negative DM (n = 85) showed less and milder muscle weakness (P = 0.02 and P = 0.006, respectively), lower creatinine kinase levels (P Conclusion Anti-SAE positive DM is a rare subgroup associated with typical skin features but a potentially diffuse rash, a mild myopathy. Interstitial lung disease defines an organizing pneumonia pattern. Cancer associated DM prevalence is five times that of the general population. Trial registration ClinicalTrials.gov, http://clinicaltrials.gov, NCT04637672.

Objective Idiopathic inflammatory myopathies (IIMs) cause proximal muscle weakness, which affects the ability to carry out the activities of daily living. Wearable physical activity monitors (PAMs) objectively assess continuous activity and potentially have clinical usefulness in the assessment of IIMs. We examined the psychometric characteristics for PAM outcomes in IIMs. Methods Adult IIM patients were prospectively evaluated (at baseline, 3 months and 6 months) in an observational study. A waist-worn PAM (ActiGraph GT3X-BT) assessed average step counts/minute, peak 1-minute cadence, and vector magnitude/minute. Validated myositis core set measures (CSMs) including manual muscle testing (MMT), physician global disease activity (MD global), patient global disease activity (Pt global), extramuscular disease activity (Ex-muscular global), HAQ-DI (HAQ disability index), muscle enzymes, and patient-reported physical function were evaluated. Test–retest reliability, construct validity, and responsiveness were determined for PAM measures and CSMs, using Pearson correlations and other appropriate analyses. Results A total of 50 adult IIM patients enrolled [mean (s.d.) age, 53.6 (14.6); 60% female, 94% Caucasian]. PAM measures showed strong test–retest reliability, moderate-to-strong correlations at baseline with MD global (r = −0.37 to −0.48), Pt global (r=−0.43 to −0.61), HAQ-DI (r = −0.47 to −0.59) and MMT (r = 0.37–0.52), and strong discriminant validity for categorical MMT and HAQ-DI. Longitudinal associations with MD global (r=−0.38 to −0.44), MMT (r = 0.50–0.57), HAQ-DI (r = −0.45 to −0.55) and functional tests (r = 0.30–0.65) were moderate to strong. PAM measures were responsive to MMT improvement ≥10% and moderate-to-major improvement on ACR/EULAR myositis response criteria. Peak 1-minute cadence had the largest effect size and standardized response means. Conclusion PAM measures showed promising construct validity, reliability, and longitudinal responsiveness; especially peak 1-minute cadence. PAMs are able to provide valid outcome measures for future use in IIM clinical trials.