Pain Phenotypes in Patients with Inflammatory Myopathies: A Latent Class Analysis

BACKGROUND: Pain is a common and burdensome symptom in patients with idiopathic inflammatory myopathies (IIM). Our aim was to identify pain phenotypes in IIM that may reflect distinct underlying mechanisms. METHODS: An anonymous electronic survey of patients with self-reported IIM and pain attributable to myositis was circulated. The survey included questions related to demographics, IIM-related information, pain characteristics and the Fibromyalgia Survey Questionnaire. Latent class analysis (LCA) was performed using four variables including distribution, pattern, difference in pain during myositis flare vs now, and disease duration. Class validity was assessed using pain characteristics, fibromyalgia criteria fulfilment and symptom burden. RESULTS: A total of 426 participants were included. LCA identified four classes: i) class 1 with proximal arm- and leg-predominant pain and lowest persistent pain, ii) class 2 with diffuse axial and proximal limb-predominant pain, iii) class 3 with diffuse axial and distal-leg predominant pain and iv) class 4 with widespread axial and extremity pain and the highest persistent pain. In post-hoc analyses, class 1 primarily included participants with earlier IIM and a lower rate of fibromyalgia. Class 2 and 3 included participants with intermediate fibromyalgia symptom burden while class 4 included participants with the highest pain intensity and fibromyalgia. CONCLUSION: In this large international cohort of patients with self-reported IIM and pain, we identified four different pain phenotypes. These phenotypes range from a localized disease-related nociceptive pattern to a widespread fibromyalgia-like nociplastic pattern. Our findings highlight the heterogeneity of myositis-related pain and may inform future mechanistic studies and phenotype-directed management strategies.