Unusual Association of Partial Fanconi Syndrome and Tumor-Induced Osteomalacia Revealed by Multiple Vertebral...

Tumor-induced osteomalacia (TIO) is a rare acquired paraneoplastic syndrome caused by a mesenchymal tumor secreting a phosphaturic hormone called FGF23. Patients present with bone pain, fragility fractures and muscle weakness. Biochemical results show hypophosphatemia, raised serum alkaline phosphatase and reduced calcitriol. We report the case of a 44-year-old man who presented to the Emergency Departement with acute low back pain revealing extensive subchondral fractures between D2 and L5. Investigations showed partial Fanconi syndrome; nevertheless, he had profound hypophosphatemia, low 1,25-OH vitamin D and raised FGF23 levels suggesting a diagnosis of tumor-induced osteomalacia. A subcutaneous lesion was identified in the left leg on a PET-CT initially performed to rule out malignancy in the context of Fanconi syndrome. Tumorectomy enabled complete resolution of the electrolyte disturbances within days of surgery. This case shows that TIO may present as partial Fanconi syndrome, highlighting the importance of testing other electrolytes in cases of hypophosphatemia and the need to look for TIO in cases of partial Fanconi with severe hypophosphatemia.