PSAT222 Muscle Weakness, Bone Pain, and Stress Fractures: When a Fracture is NOT JUST a Fracture: A Case Report

CASE: A previously healthy 46-year-old male presented to orthopedic clinic with bilateral hip pain, and progressive weakness without history of trauma requiring cane for ambulation. X-ray and MRI evaluation revealed stress fractures of left subtrochanteric femur and right basicervical femoral neck, both requiring surgery. In hospital blood work revealed elevated total alkaline phosphatase 167 units/L, mild hypocalcemia 8.5 mg/dL, and low 25-hydroxy Vitamin D 18 ng/mL. The patient was started on calcium/vitamin D supplements upon discharge. Initial endocrinology outpatient work-up revealed low phosphorus level of 2.1 mg/dl and low 1, 25 OH vitamin D of 5.9 pg/ml (19.9–79.3). Bone mineral density revealed normal spine bone density, and 1/3 radius osteopenia (T score -1.1). Physical exam revealed profound proximal weakness with inability to rise independently from sitting position. Follow-up of low phosphorous level demonstrated an elevated FGF-23 level of 186 RU/mL (normal < 180) and elevated fractional excretion of phosphorus 16% (expect < 5% with hypophosphatemia), indicating inappropriate renal phosphate wasting. The patient was started on Calcitriol and phosphorus. Before above evaluation was completed the patient had an ED visit with complaint of small mass just below left knee associated with mild pain and warmth. Ultrasound and MRI showed a complex cyst which was aspirated and showed cystic appearing mass suggestive of hematoma. This was treated with antibiotic course. Due to suspicion of tumor-induced osteomalacia, octreotide scan (Indium-111 pentetreotide) performed showed focal increased activity in the medial left knee. MRI showed a 26 mm cystic lesion within the subcutaneous tissue along the inferomedial aspect of the left knee, which correlated with the site of radiotracer uptake on the octreotide scan. The patient underwent left knee soft tissue tumor resection, and pathology demonstrated phosphaturic mesenchymal tumor. Follow-up several weeks postoperatively showed normalization of phosphorus and FGF-23 level. The patient's mobility and weakness improved significantly within a few months. DISCUSSION: Tumor-induced osteomalacia is a rare paraneoplastic condition characterized by chronic hypophosphatemia secondary to excessive tumoral FGF 23 production. Typical presenting symptoms are profound muscle weakness, bone pain, and fractures. Diagnosis is often delayed for several years, as phosphorus levels are not included in a routine metabolic panel. Considering high morbidity of this condition including significant mobility limitation requiring use of assistive devices, healthcare provider vigilance is paramount as this condition is potentially curable if the offending tumor is detectable and amenable to surgical resection. This case illustrates the importance of consideration of an underlying metabolic bone disease in patients with unexplained atraumatic fractures and hypophosphatemia. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.