Staff Physician Medical College of Wisconsin West Ben, Wisconsin, United States
Case Diagnosis: Oncogenic Osteomalacia
Case Description: The patient is a previously healthy 32Y male who presented to his local sports medicine clinic with complaints of four months of diffuse joint pain including bilateral foot pain. His foot pain was located in the ankle and dorsal midfoot areas. He had associated weakness. His pain was worse with standing and walking and prevented him from doing his usual exercise and hobbies. Physical exam was remarkable for tenderness with palpation diffusely throughout his feet and pain with resisted range of motion. Prior X-rays were unremarkable. MRI of right foot was performed showing multiple stress fractures throughout his foot. He was referred to endocrinology for further workup which revealed refractory hypophosphatemia and elevated bone specific alkaline phosphatase, concerning for oncogenic osteomalacia. PET scan was performed and identified a mesenchymal tumor in his right proximal thigh. His tumor was resected and lab abnormalities corrected, resulting in resolution of his symptoms.
Discussions: Oncogenic, or tumor-induced, osteomalacia is a rare metabolic disorder involving inappropriate mesenchymal tumor expression of fibroblast growth factor (FGF) 23, which causes impaired phosphate reabsorption and decreased renal conversion of 25-hydroxyvitamin D to its active form. Characteristic symptoms of this disorder include nonspecific muscle and bone pain, weakness, and fatigue as well as findings of stress fractures on exam. Diagnostic workup includes lab work, specifically to assess phosphate, vitamin D, and alkaline phosphatase, and imaging to identify the tumor source. Treatment includes surgical removal of the tumor and/or medical management with phosphorus replacement therapy if a tumor cannot be identified or resected.
Conclusions: This case presents the workup and management of a previously healthy young adult with insidious onset of pain secondary to multiple stress fractures. This case adds to the growing body of literature describing oncogenic osteomalacia, a rare paraneoplastic syndrome caused by mesenchymal tumors.