Predictors of muscle involvement in portuguese patients with mixed connective tissue disease

BackgroundMixed connective tissue disease (MCTD) is a rare heterogeneous disease, characterized by overlapping features of classic connective tissue diseases. Myositis may be present in up to two-thirds of patients with MCTD and it is included in all diagnostic criteria available. Although some possible associations have been reported, to the best of our knowledge, no independent predictors of MCTD-related myositis have been described.ObjectivesTo identify clinical and laboratorial predictors for muscular involvement in a cohort of Portuguese patients with MCTD.MethodsMulticentre retrospective cohort study including adult-onset patients with a clinical diagnosis of MCTD and fulfilling at least one of the following diagnostic criteria: Sharp, Kasukawa, Alarcón-Segovia or Kahn criteria. Myositis was defined as proximal muscle weakness, creatine kinase elevation, electromyography (EMG) suggestive changes or a positive muscular biopsy. Univariate analysis was performed using Chi-Square, Fischer’s Exact Test and Mann-Whitney Test, as appropriate. Multivariate analysis was performed using binary logistic regression modelling. The linearity of the continuous variables concerning the logit of the dependent variable was assessed via the Box-Tidwell procedure. Cases with missing information and outliers were excluded from the multivariate analysis to fulfil all assumptions necessary to assure the validity of the regression.ResultsA total of 98 patients were included, 43 (44.3%) of whom had muscular involvement at any time of the disease course. Concerning patients with MCTD-related myositis, the mean age at diagnosis was 34.8±12.5 years and the mean disease duration of 4.1±4.9 years. The majority of patients were female (90.7%) and of European ancestry (66.7%).EMG was performed in 24 patients, of whom 10 (41.7%) had a myopathic pattern. Seventeen patients were submitted to a muscular biopsy, of whom 8 (47.1%) had histological myositis features. Capillaroscopy was performed in 24 patients and 12 (50%) had a scleroderma pattern.African ancestry and leukopenia were positively associated with myositis at disease onset. Furthermore, fever at the onset of disease, younger age at diagnosis and shorter disease duration were positively associated with the occurrence of myositis at any phase of the disease.The multivariate analyses predicting myositis at diagnosis included 54 patients and at any time of the disease included 90 patients. These models explained 37.8% and 26.9% (Nagelkerke R2) of the variance in myositis and correctly classified 79.6% and 73.3% of all cases, respectively.African ancestry (OR 8.39, 95%CI: 1.43-49.37, p=0.019), leukopenia (OR 6.24, 95%CI: 1.32-29.48, p=0.021) and younger age at diagnosis (OR 1.07/year, 95%CI: 1.01-1.14, p=0.035) were identified as independent predictors of myositis at diagnosis. Fever (OR 6.51, 95%CI: 1.23-34.37, p=0.027) was an independent predictor of muscular involvement at any time of the disease in MCDT patients.ConclusionAfrican ancestry, leukopenia and younger age at diagnosis are independent predictors of myositis at presentation in MCTD patients, while fever is an independent predictor of myositis at any time of the disease. While evaluating patients with MCTD, these predictive factors should be considered.References[1]Ciang NCO, Pereira N, Isenberg DA. Mixed connective tissue disease-enigma variations? Rheumatol. 2017 Mar 1;56(3):326–33.[2]Hall S, Hanrahan P. Muscle involvement in mixed connective tissue disease. Rheum Dis Clin North Am. 2005 Aug;31(3):509–17, vii.Disclosure of InterestsNone declared