Late-Onset Ankylosing Spondylitis Presenting With Severe Flank and Abdominal-to-Back Pain in a 64-Year-Old Man: A Case Report

Background: Ankylosing spondylitis (AS) is a chronic inflammatory disease that primarily affects the spine and sacroiliac joints and is typically diagnosed in younger adults. However, AS may be overlooked in older patients when symptoms are atypically localized, such as flank or abdominal-to-back pain, rather than classic inflammatory low back pain.

Case Description: We report the case of a 64-year-old man who presented with persistent left-sided abdominal-to-back pain accompanied by markedly elevated inflammatory markers. Initial evaluation focused on abdominal, urinary, infectious, malignant, and degenerative spinal disorders. However, imaging revealed bilateral grade 3 sacroiliitis according to the modified New York criteria, marked narrowing and partial anterior fusion of both sacroiliac joints on pelvic CT, and lumbar vertebral corner ossification. Human leucocyte antigen (HLA)-B27 was negative. Based on the overall clinical and radiographic findings, late-onset AS was diagnosed.

After diagnosis, adalimumab was initiated and rapidly improved disease activity. Because its efficacy later waned, treatment was switched to infliximab (IFX), which restored disease control. Although disease activity worsened after switching to an infliximab biosimilar, reintroduction of originator IFX restored clinical remission, which was maintained for 2.5 years without serious adverse events.

Conclusions: This case highlights that late-onset AS may present with atypical flank and abdominal-to-back pain rather than classic inflammatory low back pain. In older patients with unexplained persistent flank or abdominal-to-back pain and markedly elevated inflammatory markers, AS should be considered in the differential diagnosis after excluding abdominal, urinary, infectious, malignant, and degenerative spinal disorders. Careful differentiation from diffuse idiopathic skeletal hyperostosis is particularly important when extensive spinal ossification is present.