TOPLINE

Four distinct pain phenotypes were identified in patients with idiopathic inflammatory myopathies (IIM), ranging from localized, flare-related pain to widespread, constant fibromyalgia-like pain. Approximately 93% of the patients in the most severe pain group met fibromyalgia criteria.

METHODOLOGY

• Researchers performed latent class analysis using survey data from 426 participants with self-reported IIM and myositis-related pain to identify distinct pain phenotypes based on pain distribution, pain pattern, flare-related changes, and the duration of the disease.
• The electronic survey was circulated through patient organizations between September 2025 and January 2026, collecting information on demographic characteristics, disease characteristics, and pain features; the survey also included the Fibromyalgia Survey Questionnaire (FSQ), which comprised measures of symptom severity and widespread pain.
• Respondents were primarily from the US (64.6%), Australia (12.1%), and Canada (9.1%), with a mean age of 57 years; 82.9% were female participants.
• Dermatomyositis represented the predominant IIM subtype (42.5% of participants), followed by inclusion body myositis (24.9% of participants) and polymyositis (11.2% of participants).
• The analysis included four variables: pain distribution by body region derived from the FSQ, pain pattern (persistent vs intermittent), differential pain intensity comparing survey completion and myositis flare, and disease duration categorized as 1 year or less, 2-5 years, 6-10 years, or exceeding 10 years.

TAKEAWAY

• Four distinct pain phenotypes were identified. Class 1 represented a disease-related nociceptive pain phenotype and comprised 20.2% of the study population. Participants in class 1 had pain predominantly in the proximal arm and leg, with the greatest difference in flare-to-current pain (mean, 3.6) and the least percentage of persistent pain (50.0%).
• Class 2 and class 3, comprising 33.8% and 29.1% of participants, respectively, were characterized by intermediate phenotypes with regional pain predominance (upper-arm/axial and lower-leg/upper-leg pain, respectively) and moderate fibromyalgia symptom burden; approximately 70% of participants in both groups met the fibromyalgia criteria.
• Class 4, comprising 16.9% of participants, exhibited the most extensive distribution of pain, affecting all extremities and axial regions, and represented a fibromyalgia-like nociplastic pain phenotype, with the highest proportion of participants with persistent pain (72.2%) and the greatest fibromyalgia symptom burden (93.1% fulfilled fibromyalgia criteria; P < .001 across all classes).
• Each 1-point increase in the measure of symptom severity was independently linked to increased odds of assignment to class 4 vs class 2 (adjusted odds ratio [aOR], 1.35; P < .001) and decreased odds of assignment to class 1 vs class 2 (aOR, 0.82; P = .002).

IN PRACTICE

"[The study] findings highlight the heterogeneity of myositis-related pain and may inform future mechanistic studies and phenotype-directed management strategies," the authors wrote.

SOURCE

The study was led by Mohammad Radaideh, MD, Rush University Medical Center in Chicago. It was published online on September 15, 2026, in Arthritis Care & Research.

LIMITATIONS

The cross-sectional survey design precluded determination of whether pain phenotypes changed over time or subsequent nociplastic features developed. Objective disease activity measures, imaging findings, and treatment response information were not collected, thereby limiting the determination of the cause of pain. Lack of information on current analgesic or immunomodulatory medication usage affected medication-stratified analytical approaches.

DISCLOSURES

The study received no specific funding. One author's work was supported by a National Institutes of Health grant. Another author was supported by the Rheumatology Research Foundation Scientist Development Award. Support was received from the Myositis Support and Understanding patient organization for survey distribution. Two authors reported institutional grant support from NIH, Rheumatology Research Foundation, and Rush Imaging Research Core. Some authors reported having consulting relationships and other ties with various companies. One author reported a leadership role as a Chief Science Officer/Board Member with Myositis Support and Understanding and another author reported serving as a Vice Chair for International Myositis Society. 

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.