Rationale: Rheumatoid arthritis (RA) and axial spondyloarthritis (axSpA) are distinct inflammatory rheumatic diseases with different clinical, serological, and genetic profiles. RA is typically characterized by symmetric peripheral polyarthritis and autoantibody positivity, particularly anti-cyclic citrullinated peptide (anti-CCP), whereas axSpA primarily involves the axial skeleton and sacroiliac joints and is often associated with human leukocyte antigen-B27 (HLA-B27). The coexistence of seropositive RA and axSpA is uncommon and presents diagnostic and therapeutic challenges, particularly when axial symptoms are under-recognized in young patients. This case highlights a rare overlap phenotype and emphasizes the role of advanced imaging and individualized management. Patient concerns: An 18-year-old Saudi woman presented with inflammatory peripheral joint pain, associated with chronic axial symptoms suggestive of sacroiliac involvement. Diagnoses: Laboratory investigations revealed markedly elevated anti-cyclic citrullinated peptide (anti-CCP) antibodies (373 U/mL), supporting the diagnosis of seropositive RA based on the 2010 American College of Rheumatology/European Alliance of Associations for Rheumatology criteria. Magnetic resonance imaging of the sacroiliac joints demonstrated bilateral sacroiliitis with active inflammatory changes, fulfilling the Assessment of SpondyloArthritis International Society (ASAS) criteria for axial spondyloarthritis, despite negative HLA-B27 and absence of other typical spondyloarthritis features. Interventions: The patient was initially managed with methotrexate (15 mg/wk) and low-dose prednisone (5 mg/d). Due to persistent axial symptoms, treatment was escalated to adalimumab following appropriate screening and counseling. Outcomes: At 3-month follow-up, the patient demonstrated significant clinical improvement (>70%) in both peripheral and axial symptoms, with normalization of inflammatory markers C-reactive protein (CRP) and erythrocyte sedimentation rate. Lessons: This case underscores the importance of considering overlapping rheumatologic conditions in patients with atypical presentations. Magnetic resonance imaging plays a critical role in detecting axial involvement, particularly in HLA-B27–negative patients. Early recognition of such overlap syndromes is essential to guide appropriate targeted therapy and improve long-term outcomes, especially in young populations.
Alenzi et al. (2026) studied this question.