Four Distinct Pain Phenotypes Identified in Inflammatory Myopathies

Identifying Pain Phenotypes in Inflammatory Myopathies

To uncover these distinct patterns, investigators performed latent class analysis using survey data gathered from 426 participants with self-reported idiopathic inflammatory myopathies and myositis-related pain. The electronic survey ran from September 2025 to January 2026 through patient organizations, capturing demographic characteristics, disease specifics, and detailed pain features. Respondents showed a mean age of 57 years, with 82.9% identifying as female, and hailed primarily from the United States at 64.6%, Australia at 12.1%, and Canada at 9.1%. Dermatomyositis represented the predominant subtype at 42.5%, followed by inclusion body myositis at 24.9% and polymyositis at 11.2%. The analysis incorporated four primary variables: pain distribution by body region derived from the Fibromyalgia Survey Questionnaire, pain pattern distinguishing persistent from intermittent pain, differential pain intensity comparing survey completion to myositis flares, and disease duration categorized from one year or less up to exceeding ten years. Across the cohort, researchers established four clear classes that reflect widely different underlying pain mechanisms.

Breakdown of the Four Pain Classes

Class 1 represented a disease-related nociceptive pain phenotype, comprising 20.2% of the study population. Participants in this group experienced pain predominantly in the proximal arm and leg, exhibited the greatest difference between flare and current pain with a mean of 3.6, and reported the lowest percentage of persistent pain at 50.0%. Classes 2 and 3 accounted for 33.8% and 29.1% of participants respectively, featuring intermediate phenotypes with regional pain predominance in the upper-arm/axial and lower-leg/upper-leg areas alongside a moderate fibromyalgia symptom burden. Approximately 70% of participants in both intermediate groups met fibromyalgia criteria. Class 4 comprised 16.9% of participants and exhibited the most extensive distribution of pain, affecting all extremities and axial regions. This group represented a fibromyalgia-like nociplastic pain phenotype, showing the highest proportion of persistent pain at 72.2% and the greatest fibromyalgia symptom burden, with 93.1% fulfilling fibromyalgia criteria at a statistically significant level of P < .001 across all classes. Each 1-point increase in the measure of symptom severity linked independently to increased odds of assignment to class 4 versus class 2, with an adjusted odds ratio of 1.35, and decreased odds of assignment to class 1 versus class 2, with an adjusted odds ratio of 0.82.

Clinical Implications and Study Limitations

Historically, medical evaluation of idiopathic inflammatory myopathies prioritized muscle weakness over pain, despite scoping reviews showing myalgia frequencies ranging from 64% to 100%. The new findings point to a complex mix of nociceptive mechanisms driven by active muscle inflammation or fasciitis and nociplastic mechanisms involving altered central pain processing.

The study findings highlight the heterogeneity of myositis-related pain and may inform future mechanistic studies and phenotype-directed management strategies.

Despite the detailed classifications, the study faced notable limitations, including a cross-sectional survey design that precluded determining whether pain phenotypes changed over time or if nociplastic features developed subsequently. Objective disease activity measures, imaging findings, treatment response data, and current medication usage were not collected, restricting the ability to determine specific pain causes or apply medication-stratified analytical approaches.