Background/Purpose: Idiopathic inflammatory myopathies (IIM) are a rare and heterogeneous group of autoimmune diseases marked by inflammation of muscle tissue and extramuscular manifestations. Their atypical clinical presentations often lead to diagnostic delays, which hinder timely treatment and contribute to worse outcomes. Early diagnosis is essential to reduce disease activity, prevent long-term damage, and improve quality of life. In juvenile dermatomyositis (JDM), delayed diagnosis has been linked to greater morbidity and poorer prognosis, with emerging evidence also pointing to disparities based on race. This underscores the need for further research. We aim to examine the correlation between diagnostic delay, disease activity, and cumulative damage in patients with IIM.

Methods: A descriptive, cross-sectional, and comparative study was performed. Patients over 18 years of age diagnosed with IIM according to the 2017 ACR/EULAR classification criteria or the 1975 Bohan and Peter criteria were recruited. A clinical history was obtained, including the duration of symptoms before diagnosis and the disease duration since diagnosis. The cumulative glucocorticoid dose up to the time of recruitment was calculated.The following scales were assessed:Myositis Damage Index (MDI)Myositis Disease Activity Assessment Tool (MDAAT)Health Assessment Questionnaire (HAQ)Fitzpatrick ScalePatients were divided into two groups based on diagnostic delay: ≤9 months and >9 months. The Kolmogorov-Smirnov test, used to assess normality. Group comparisons were made using the Chi-square test, independent samples t-test, or Mann-Whitney U test. Correlations were analyzed using Pearson or Spearman coefficients, depending on data distribution. A p-value of < 0.05 was considered statistically significant.

Results: A total of 40 patients were included, with a mean age of 46.6 years (SD: 16.2). Most patients were female (n=34, 85%). The most frequent IIM was dermatomyositis, observed in 29 patients (72.5%). The median time from symptom onset to diagnosis was 5 months (IQR: 2–11.5). The most prevalent antibodies were anti-Ro52 (37.5%) and anti-TIFy (27.5%). Demographic and clinical characteristics and disease activity scales are shown in Table 1.Patients with a diagnostic delay greater than 9 months had significantly higher scores on the HAQ, MYOACT, and MITAX scales (p=0.01, p=0.003, and p=0.02, respectively) (Table 2), as well as a younger age (p=0.01). A weak positive correlation was observed between diagnostic delay and MYOACT score (Spearman’s ρ = 0.357, p = 0.02). (Figure 3)Skin phototype showed differences by diagnostic delay. Among patients with a diagnostic delay ≤ 9 months, 44.4% had Fitzpatrick phototype IV or V, while 51.8% had phototype II or III. In contrast, 76.9% of patients with a delay > 9 months had phototype IV or V, compared to only 15.3% with phototype II or III.

Conclusion: According to our results, IIM is not commonly considered an initial diagnosis in younger patients with phototype IV or V. A diagnostic delay of more than 9 months was associated with higher disease activity, greater cumulative damage, and reduced quality of life, highlighting the need for timely evaluation in patients with these characteristics.

Table 1. Clinicodemographic characteristics and disease activity scales

Table 2. Comparison of clinical and laboratory characteristics according to diagnostic delay time

Figure 1. Correlated variables diagnostic delay time and MYOACT scale.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/impact-of-diagnostic-delay-on-disease-activity-and-cumulative-damage-in-patients-with-idiopathic-inflammatory-myopathies-in-a-third-level-hospital-in-mexico/