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Inflammatory bowel diseases (IBD) – Crohn's disease (CD) and ulcerative colitis (UC) – are a group of idiopathic abnormal bowel conditions with unclear etiology and similar clinical courses. In addition to intestinal syndrome, these diseases are often associated with extraintestinal manifestations, the most common of which is the involvement of the peripheral joints and axial skeleton. The increase in the incidence of IBD, especially CD, in childhood warrants further study of this condition's features, including the most common extraintestinal manifestation: joint involvement (JI) in pediatric patients with IBD. 
Aim. To analyze the features of JI in CD and UC and the timing of its occurrence associated with IBD onset.
Materials and methods. We reviewed 146 case histories of patients aged 4 to 18 diagnosed with CD and UC treated in the Gastroenterological Department and Rheumatological Department No.1 of the Mother and Child Center of Sechenov University from 2017 to 2024. The diagnosis of CD and UC was established following the clinical guidelines of the Union of Pediatricians of Russia for diagnosing and treating CD (2021) and UC (2024) in children. A rheumatologist examined the joints and assessed the JI. The classification criteria of the Assessment of SpondyloArthritis International Society (ASAS, 2011) for predominantly peripheral spondyloarthritis and the Garmisch-Partenkirchen JAS Diagnostic Criteria (1987) were used to verify the cause of the joint involvement. Mean values and standard deviation (M±σ) were used to summarize and estimate demographic continuous variables. Absolute values and percentages were calculated for patients in each study group.
Results. Of the 146 patients, 60 (41%) had a combination of JI and IBD. Sixty children aged 4 to 18 with IBD and JI were divided into three groups: 36 (60%) patients with CD, 19 (32%) with UC, and 5 (8%) children with undifferentiated colitis (UndC). The mean age of patients with CD was 14.06±0.5, with UC–13.6±0.3, and UndC–15.2±0.2 years. Among all 60 patients, JI manifested with peripheral arthritis (PA) in 25 (42%) subjects, spondyloarthritis (SpA) in 16 (27%) subjects, or only arthralgias without evident inflammatory changes in the joints in 19 (31%). The analysis of JI variants by groups in children with CD (n=36) showed PA in 15 (42%), SpA in 12 (33%), and isolated arthralgia in 9 (25%) patients. The prevalence of PA and arthralgia among boys and girls with CD was comparable (60 and 40% of PA, 56 and 44% of arthralgias, respectively); the prevalence of SpA in boys with CD was significantly higher: found in 11 (92%) boys. In the group of children with UC (n=19), a high prevalence of PA and isolated arthralgia was reported: 8 (42%) and 9 (47%) patients, respectively. SpA was diagnosed in 2 (11%) children with UC. Of the 60 children with JI and IBD, 30 (50%) developed the disease with joint involvement. CD onset was associated with JI significantly more often than UC onset: of 30 children with JI, 21 (70%) were first diagnosed with rheumatic joint disease, and then CD was detected during follow-up. The disease first manifested with joint involvement in 6 (20%) patients with UC and 3 (10%) with UndC. The time interval from the onset of JI to the onset of the intestinal syndrome (and/or IBD diagnosis) varied widely from 1 month to several years; in 1 patient, it was more than 8 years. In 8 (27%) patients, this time interval was 1 to 3 months; in 6 (20%) patients, the symptoms of the underlying disease appeared within 12 to 24 months from the joint involvement manifestation. The atypical onset and course of the disease make diagnosis challenging and require increased doctor attention to patient management. As an illustration, the article presents a clinical case of an atypical onset and course of CD associated with juvenile SpA.

Keywords: children, Crohn's disease, ulcerative colitis, spondyloarthropathies, inflammatory bowel disease