Epidemiology of Juvenile Dermatomyositis—What We Know and What More Is Needed?

Juvenile idiopathic inflammatory myopathies (JIIMs) are a heterogeneous group of immune-mediated disorders—including Juvenile Dermatomyositis (JDM), Juvenile Polymyositis (JPM), immune-mediated necrotizing myositis, myositis associated with other connective tissue diseases, and rare disorders classified under the unified spectrum of idiopathic inflammatory myopathies (IIMs).

The estimated incidence of IIM varies from 1.6 to 19/million/year, and estimated prevalence varyies from 2.4 to 33.8/100000/year [1]. For JDM, the form of JIIM for which most estimates are available, the incidence varies from 1.9 to 3.2/million children/year [2-4]. The exact etiological basis of JIIMs is not yet known, but studies have found associations with several environmental triggers such as ultraviolet (UV) light exposure, air pollution, and a variety of infectious triggers. Genetic associations with HLA haplotypes have also been identified [5, 6]. Historically, JIIMs had a grim prognosis with a chronic and severe course, and as many as a third of all patients succumbing to the disease [7]. With advances in treatment, the prognosis for JIIMs has significantly improved, but mortality rates in JIIM patients remain higher than that in the general population [6], and most patients are seen to have some extent of disease damage [8]. Even among rheumatological disorders, JIIMs are rare, chronic disorders with patients requiring long-term management and rehabilitation at specialist centers. A better understanding of the epidemiology of the JIIMs can help in estimating the burden of disease, which can influence public health policy toward provision of healthcare facilities for these patients. Moreover, studying epidemiological trends over time can contribute toward recognition of risk factors, some of which may be modifiable. Epidemiological studies can also help guide future research and public health interventions [9].

Nossent et al. conducted a 30-year population-based retrospective study of 40 JIIM patients under 18 years in Western Australia, finding an annual incidence of 2.52/million children, with JDM at 2.02/million, female predominance, and a 94.9% 10-year survival rate. Compared with JIA controls, JIIM patients had significantly higher rates of serious infections, with nonsignificant increases in thromboembolic disease, ILD, osteoporosis, and pregnancy complications. Nossent et al.'s study makes a significant contribution by providing Australian population-based JIIM incidence data and enabling outcome comparisons with JIA as a control group. The extended median follow-up of over 10 years offers valuable insights into the long-term complications and functional impacts of JIIM.

As the understanding of JIIMs has evolved over time, it has been seen that different autoantibodies confer different clinical phenotypes and also have a significant bearing on outcomes in patients with JIIMs [6, 8]. Determining the accurate incidence of JIIMs is met with several challenges. Contemporary research reveals significant heterogeneity in classification criteria usage, with many studies using multiple systems or none at all, highlighting the urgent need for a unified, comprehensive classification framework that can accurately capture all IIM subtypes [10]. Limited access to specialized diagnostic tools, particularly myositis-specific antibody testing in many regions, further complicates accurate case identification. The scarcity of pediatric rheumatologists and specialists experienced in recognizing early disease features often results in delayed diagnosis or missed cases, especially in resource-limited settings. Limited research funding, given the rarity of the condition, constrains the scope and scale of epidemiological studies, particularly in developing regions. Additionally, cases managed exclusively in outpatient settings or those who never reach tertiary care centers may go unrecorded, leading to potential underestimation of true disease incidence.

The above study while valuable, the absence of autoantibody profile data limits our understanding of the immunological characteristics of the cohort. Additionally, the baseline disease severity metrics were not reported, making it challenging to evaluate the relationship between initial patient status and outcomes. The study also does not specify the exact causes of mortality. The composition of JIA subtypes in the comparison cohort would have provided useful context for interpretation. The study's reliance on hospitalization records for JIIM case identification, though methodologically sound, may not capture the full spectrum of cases, particularly those managed in outpatient settings or those without access to healthcare facilities. These observations may inform the design of future studies in this area. The study nonetheless represents a significant contribution to the field, offering valuable longitudinal data on JIIM incidence, prevalence and associated morbidities. Its particular strengths lie in its comprehensive temporal scope and the inclusion of multiple healthcare facilities across Western Australia, providing a robust regional perspective.

Future epidemiological studies would benefit from prospective multicentric data collection, including autoantibody profiles and outpatient cases. Collaborative networks such as Myositis International Health and Research Collaborative Alliance (MIHRA) play a crucial role in addressing current research gaps, particularly in developing regions where data remains limited [11]. Such international partnerships, guided by principles of inclusivity and equitable representation, are essential for establishing a comprehensive global understanding of JIIM epidemiology.

Dev Desai: Wrote the first draft of the manuscript. Latika Gupta: Critical review and revising the manuscript. Pandiarajan Vignesh: Critical review, manuscript editing, and finalising the manuscript.

The authors declare no conflicts of interest.