MON-364 Severe Postoperative Encephalopathy in a Patient with Oncogenic Osteomalacia and Neurofibromatosis Type 1

Background: Acute hypophosphatemia superimposed on chronic phosphate depletion results in decline in intracellular ATP and 2,3-diphosphoglycerate levels, affecting virtually all organ systems. It can lead to metabolic encephalopathy, impaired myocardial and diaphragmatic contractility, skeletal and smooth muscle dysfunction, increase in erythrocyte rigidity leading to hemolysis, and increased risk for postoperative morbidity and mortality. Risk of symptomatic hypophosphatemia may be greater in patients with urinary phosphate wasting syndromes hospitalized for intercurrent illness. Tumor-induced (oncogenic) osteomalacia (TIO) is a rare paraneoplastic disorder associated with mesenchymal tumors caused by tumor production of fibroblast-derived growth factor-23 (FGF-23). FGF-23 impairs transport of phosphorus in renal tubules, inhibits renal 25(OH)-1-α-hydroxylase activity, causing decreased levels of calcitriol, leading to hyperphosphaturia, hypophosphatemia and osteomalacia. We present a case of a patient with underlying TIO who presented with acute hypophosphatemia following hip surgery. Case: 50-year-old woman with Neurofibromatosis type-1(NF-1) complicated by TIO, presented after a fall resulting in left femur fracture requiring surgery. During her initial management in an outside hospital for 4 days and following transfer her home calcitriol and phosphorus replacement regimen was not administered leading to treatment interruption of 7 days. On postoperative day 1 she became hypotensive, tachycardic with non-specific ST-T changes and encephalopathic requiring ICU care. At the time of endocrinology consultation, she was non-verbal, not following simple commands, had severe hypophosphatemia, transaminitis, hyperbilirubinemia, elevated LDH, anemia and elevated troponin. She required aggressive phosphate and calcitriol replacement reaching 4 doses of 20mEq IV Na- Phos daily, Na-KPhos Q6H and Calcitriol IV 0.5mcg Q8H over the course of 72 hours. After 4 days in the ICU, once phosphorous levels normalized, she had dramatic improvement in mental status and gradual resolution of laboratory abnormalities. Discussion: To our knowledge, this is the first report of a patient with TIO associated with NF-1 presenting with severe symptomatic hypophosphatemia causing metabolic encephalopathy and multiple organ dysfunction following surgery. It underscores the fact that abrupt interruption of calcitriol and phosphorus supplementation in such cases can cause acute hypophos