Background The severity of muscle involvement and the response to immunosuppressive drugs are highly variable among patients with polymyositis (PM) and dermatomyositis (DM). Although previous studies reported that some clinical and laboratory features were associated with a poorer prognosis, the results were not consistent through those studies. A decade ago, we reported that the height of the baseline serum creatine kinase (CK) elevation predict the course of disease among patients treated with glucocorticoids (GCs) alone. However, other groups reported conflicting results. In addition, we later changed our basic treatment strategy to combination therapy with another immunosuppressive agent to improve response and reduce the need for GCs.

Objectives We aimed to identify predictive factors for insufficient improvement of muscle weakness after treatment among patients with PM/DM, focusing more recent patients than those included in our previous report.

Methods Patients were included in this retrospective study when they met all these criteria: 1) fulfill the Bohan and Peter classification criteria for PM/DM; 2) were administered to our university hospital for the treatment of muscle manifestations of PM/DM from 2008 through 2015. In addition, these patients were excluded from the study: those with clinically amyopathic DM and those complicated with other overlapping systemic autoimmune diseases or cancer. Manual muscle testing (MMT) was assessed using traditional 5 point MMT scales (i.e. the Medical Research Council Scale.) The Total MMT score included 2 axial and 8 proximal muscle groups, with a maximum potential value of 50. The Total MMT score less than 48 at 6 to 8 weeks after initiation of GC treatment was defined as insufficient improvement of muscle weakness. Clinical and laboratory parameters were statistically compared between patients with sufficient and insufficient improvement of muscle weakness. Odds ratios (ORs) for insufficient improvement of muscle weakness were assessed by multiple logistic regression analysis.

Results Ultimately, 45 patients, comprising 30 PM and 15 DM patients were included in the present study. Among these, 22 were positive for anti-aminoacyl-tRNA synthetase (anti-ARS) antibodies, the mean age was 55 years, and the mean CK level before treatment was 2,544 U/l. GCs (≥0.5 mg/kg/day) were administered in all the patients and another immunosuppressant were concomitantly used in 39 patients. By univariate analyses, age and CK levels before treatment were significantly higher in patients with insufficient improvement of muscle weakness than those with sufficient improvement (p<0.05). The cut off values were determined by receiver operating characteristic curve analyses as ≥65 years for age and ≥1,900 U/l for CK level. The patients with ≥65 years old, CK ≥1,900 U/l, or negative anti-ARS antibodies were more frequent in patients with insufficient improvement. By multivariate analyses, ORs (95% confidence interval) for ≥65 years old, CK ≥1,900 U/l, and negative anti-ARS antibodies were 16 (2 to 342), 22 (3 to 526), and 9 (1.3 to 104), respectively.

Conclusions The present study suggested that ≥65 years old, CK ≥1,900 U/l before treatment, and negative anti-ARS antibodies were predictive factors for insufficient improvement of muscle weakness in patients with PM/DM.

Disclosure of Interest None declared