Case Report: Complete hydatidiform mole mimicking spondyloarthritis: a probable paraneoplastic rheumatologic syndrome.

Background: Complete hydatidiform mole (CHM) is a gestational trophoblastic disease characterized by abnormal proliferation of trophoblastic tissue and markedly elevated β-human chorionic gonadotropin (β-hCG) levels. Although CHM typically presents with vaginal bleeding and uterine enlargement, it may occasionally manifest with paraneoplastic systemic symptoms that mimic autoimmune rheumatic diseases. Case presentation: A 33-year-old woman presented with a 2-month history of polyarthritis, inflammatory low back pain, and morning stiffness, initially suspected as spondyloarthritis (SpA). She had a history of missed miscarriage at 7 weeks of gestation with medical abortion two months prior. Physical examination revealed dactylitis of the left third finger and right fifth toe, enthesitis at bilateral heels, and tenderness over the lumbar spine. Laboratory tests showed elevated erythrocyte sedimentation rate (50 mm/h) and C-reactive protein (42.8 mg/L). HLA-B27 was positive, while rheumatoid factor, anti-cyclic citrullinated peptide antibodies, and antinuclear antibodies were negative. Pelvic ultrasonography revealed a heterogeneous echogenic intrauterine mass (4.8 × 4.3 × 3.6 cm) with a "honeycomb" or "snowstorm" appearance, suggestive of gestational trophoblastic disease. Serum β-hCG was markedly elevated at 31,870 IU/L. Bone scintigraphy demonstrated increased uptake in the left wrist, metacarpophalangeal joints, interphalangeal joints, left ankle, and L1 spinous process. Uterine curettage was performed, and histopathology confirmed complete hydatidiform mole. Following the procedure, arthralgia and back pain resolved dramatically without disease-modifying antirheumatic drugs or biologics. Serum β-hCG declined progressively to normal levels, and inflammatory markers normalized. At 2-year follow-up, the patient remained asymptomatic, and repeat bone scintigraphy showed no abnormal uptake. Conclusion: This case highlights that CHM can present with rheumatic manifestations mimicking SpA, including inflammatory back pain, peripheral arthritis, dactylitis, enthesitis, and even HLA-B27 positivity. The parallel course of rheumatic symptoms with β-hCG levels, rapid resolution after uterine evacuation, and sustained remission during follow-up are consistent with a probable paraneoplastic rheumatologic syndrome rather than primary SpA. Clinicians should maintain a high index of suspicion for gestational trophoblastic disease in women of childbearing age presenting with new-onset inflammatory arthritis and elevated β-hCG.