Cracks in the Diagnosis: A Case of Hypermobility Spectrum Disorder as a Mechanical Mimic of Spondyloarthritis With a Brief Review.

Adolescents commonly present with chronic back and knee pain, and distinguishing mechanical from inflammatory causes is a frequent challenge in general and pediatric practice. Juvenile spondyloarthritis (jSpA) is a chronic inflammatory disease affecting joints, entheses, and potentially the axial skeleton that may require prolonged immunosuppression to prevent structural joint damage. Generalized joint hypermobility (GJH) and hypermobility spectrum disorder (HSD) can present with overlapping musculoskeletal symptoms, including chronic joint pain and back stiffness, and may therefore be mistaken for inflammatory disease. Sacroiliac joint bone marrow edema on magnetic resonance imaging (MRI), although an important feature of axial spondyloarthritis, may also be seen in healthy individuals, athletes, postpartum women, and mechanically stressed joints, particularly when interpreted without adequate clinical correlation. This case report and brief review aim to illustrate how GJH can mimic jSpA and how overinterpretation of sacroiliac MRI findings may lead to unnecessary immunosuppression in adolescents. A 17-year-old male from Bangladesh presented for an independent pediatric rheumatology evaluation after a prior diagnosis of jSpA based on reported sacroiliac MRI abnormalities, partial response to nonsteroidal anti-inflammatory drugs (NSAIDs), and intermittent gastrointestinal symptoms. His inflammatory markers were persistently normal, HLA-B27 testing was negative, and his back pain lacked inflammatory features. Examination demonstrated GJH with a Beighton score of 7/9. Repeat independent sacroiliac MRI was normal, and knee MRI showed a partial anterior cruciate ligament injury with bone contusion, supporting a mechanical etiology. Genetic testing for heritable connective tissue disorders was negative. Expert consultation, including review by specialists affiliated with the Ehlers-Danlos Society and an independent pediatric rheumatologist at the National University Health System in Singapore, favored generalized HSD over jSpA. Structured physiotherapy and conservative rehabilitation led to complete symptom resolution without immunosuppressive therapy. This case highlights the risk of overdiagnosing spondyloarthritis when nonspecific sacroiliac MRI findings are prioritized over the clinical pattern, laboratory data, and mechanical risk factors. GJH should be systematically assessed in adolescents with chronic musculoskeletal pain, especially when inflammatory markers are normal and symptoms are predominantly activity-related.