Late onset axial spondyloarthritis presenting with fever of unknown origin. A case-based review.

Spondyloarthritis (SpA) is a chronic inflammatory disease involving axial (ax-SpA), and peripheral joints (p-SpA). It mainly affects young individuals and symptoms begin before the age of 45 years. Late onset (LO) ax-SpA, presenting after the age of 50, is rare. It is usually underdiagnosed in favor of other disorders, which are very common in this population. A 70-year-old woman presented with fever of unknown origin (FUO), back pain, malaise, weakness and high acute phase reactants, without any other clinical or laboratory findings. After an intensive and extensive investigation, to exclude several diseases and disorders, our patient, despite being old and almost asymptomatic for SpA features was diagnosed as having LO-axSpA. The patient was treated successfully with adalimumab. We discuss the differential diagnosis of FUO in an elderly woman with constitutional symptoms and elevated inflammatory markers. This is the first case of LO-axSpA presenting with FUO, therefore, physicians must be aware of rare manifestations of ax-SpA in the elderly population. Rigorous and extensive investigation is required to reach the correct diagnosis.