Pyrexia of unknown origin in a patient with ankylosing spondylitis: A rare case of concurrent adult-onset Still‘s disease

Background. Ankylosing spondylitis (AS) is a chronic inflammatory disorder primarily affecting the axial skeleton. While systemic manifestations such as fever are rare, their presence often necessitates consideration of alternative or concurrent diagnoses. Adult-onset Still’s disease (AOSD), by contrast, is an autoinflammatory condition characterized by high-grade fever, an evanescent rash, and polyarthritis. The simultaneous occurrence of AS and AOSD is exceedingly rare and poses significant diagnostic challenges. This case report delineates the complexities associated with diagnosing and managing this uncommon overlap, emphasizing the importance of advanced imaging techniques and tailored therapeutic strategies. Case presentation. A 37-year-old male with a known history of AS presented with pyrexia of unknown origin (PUO), bilateral asymmetric polyarthritis, and a transient, evanescent skin rash. Laboratory investigations revealed markedly elevated inflammatory markers (C-reactive protein [CRP]: 322 mg/L; erythrocyte sedimentation rate [ESR]: 82 mm/hr), hyperferritinemia (>10,000 ng/mL), hyperleukocytosis (16,000/mm³) with neutrophilic predominance, and normocytic normochromic anemia (hemoglobin: 9 g/dL). Extensive infectious and immunological evaluations yielded negative results. Positron emission tomography-computed tomography (PET-CT) demonstrated metabolically active lymphadenopathy, and biopsies ruled out malignancy and macrophage activation syndrome. The diagnosis of AOSD was established using the Fautrel and Yamaguchi criteria. The patient’s treatment was transitioned from nonsteroidal anti-inflammatory drugs (NSAIDs) to corticosteroids (prednisone 1 mg/kg/day), resulting in substantial clinical improvement. Maintenance therapy with disease-modifying antirheumatic drugs (DMARDs), including adalimumab (40 mg subcutaneously twice monthly) and methotrexate (15 mg weekly with folic acid supplementation), was subsequently initiated to address both axial and systemic symptoms and sustain remission. Conclusion. This case underscores the necessity of considering AOSD in patients with AS who present with PUO and systemic inflammation. A timely and accurate diagnosis, coupled with appropriate therapeutic interventions, can significantly mitigate complications. The integration of biologic DMARDs, such as TNF inhibitors, into the treatment regimen highlights the importance of targeting shared inflammatory pathways in managing these overlapping conditions. Further research is essential to elucidate the underlying pathophysiological connections between AS and AOSD and optimize their management strategies.