Clinical Profile and Diagnostic Spectrum of Autoimmune Comorbidities in Juvenile Idiopathic Arthritis: A Descriptive Single-Centre Observational Study.

Background/Objectives: Children with juvenile idiopathic arthritis (JIA) frequently develop additional autoimmune conditions during follow-up, yet the clinical and diagnostic profile of this comorbid subgroup is incompletely characterised in single-centre paediatric series. We aimed to describe the prevalence, clinical pattern, and diagnostic features of autoimmune comorbidities in a seven-year cohort of children with JIA monitored at a single tertiary paediatric centre, and to document the diagnostic protocols applied for each comorbidity. Methods: We conducted a retrospective descriptive observational study of 103 consecutive children with JIA classified according to the ILAR 2001 criteria and monitored at the Paediatric Rheumatology Unit of St. Mary Children's Emergency Hospital, Iași, Romania, between 2017 and 2023, with the year 2020 excluded by design owing to the institutional reorganisation during the early COVID-19 pandemic. Autoimmune comorbidities were ascertained from medical records using ICD-10 coding and confirmed by subspecialty evaluation. The diagnostic approach for each comorbidity is reported in detail. Continuous variables are described using mean ± standard deviation, and categorical variables as frequencies (%). No inferential analysis was performed on predictor variables; the study is exploratory and hypothesis-generating. Results: Autoimmune comorbidity was identified in 21 of 103 children (20.4%; 95% confidence interval [CI] 13.1-29.5%), the JIA-AID subgroup. Patients were predominantly female (17 of 21, 81.0%) and aged over 12 years (10 of 21, 47.6%). Autoimmune thyroiditis was the most frequent comorbidity, present in 10 of 21 cases (47.6%) when the euthyroid, hypothyroid, and vitiligo-associated forms were combined, followed by inflammatory bowel disease (4 of 21, 19.0%), alopecia areata (4 of 21, 19.0%), localised scleroderma (2 of 21, 9.5%), and coeliac disease (1 of 21, 4.8%). Three patients (14.3%) had polyautoimmunity, defined as two or more autoimmune diagnoses in addition to JIA. The HLA-B27-positive enthesitis-related arthritis subtype, although small in absolute numbers, was over-represented within the JIA-AID subgroup: 5 of 7 HLA-B27-positive ERA patients (71.4%; 95% CI 29.0-96.3%) carried a coexisting autoimmune diagnosis, compared with 16 of 96 patients in the remainder of the cohort (16.7%; 95% CI 9.8-25.6%); the predominant comorbidity in this subtype was inflammatory bowel disease. Conclusions: Autoimmune comorbidity affected approximately one in five children with JIA in this single-centre cohort, with autoimmune thyroiditis and inflammatory bowel disease as the most frequent associations and a notable concentration of comorbidity within the HLA-B27-positive enthesitis-related arthritis subtype. These descriptive observations are hypothesis-generating and support the case for proactive multidisciplinary screening in selected subgroups. Prospective registry-based studies with explicit exposure classification and standardised functional outcomes will be needed to confirm the patterns reported here.