SAT-076 Hypophosphatemic Rickets Secondary to Elemental Formula Use - Report on Two Cases

Abstract

Background: Hypophosphatemic rickets (HR) is usually an inherited disorder, but it may also occur in several clinical settings as an acquired condition due to phosphate absorption and internal distribution issues. Recently it was described the association between the use of the elemental amino-acid based formula (AAF) and HR. Herein we report two male twins presenting this condition. Clinical case: Two monozygotic preterm (28 weeks + 5 days) brothers, born with extremely low birth weight (895 and 995 grams, -1,4 SDS and -0.9 SDS, respectively), received total parental nutrition from the 1st to the 7th day of life. Afterwards they started oral diet with milk and human milk fortifier FM85 for preventing metabolic bone disease of prematurity. After some weeks they developed abdominal distension, vomiting and hematochezia. Allergy to cow milk protein was suspected and the infants started receiving extensively hydrolyzed milk formula. So, FM85 was suspended and they were put on tricalcium phosphate (12.9%) supplementation from the 4th week of life on. As the gastrointestinal symptoms persisted, the formula was exchanged for elemental formula Neocate at 2 months of age, with improvement of those symptoms.After the 5th week of life the patients developed hypophosphatemia (2.8 mg/dL, reference 4.8-7.4 mg/dL), hyperphosphatasemia (1,619 IU/L and 2,173 IU/L; reference 70-350 IU/L) and low urinary phosphate excretion, keeping normal calcium and PTH ser um levels. Radiographic skeletal inventory showed under mineralized bones with irregular metaphyseal margins, but no signs of fractures. Calcium and phosphate supplementation doses were increased in attempt to correct the metabolic disturbances, and calcitriol was also started. Nevertheless, hypophosphatemia, hyperphosphatasemia and hypophosphaturia persisted.At 4 months of age, Neocate was switched to Alfamino, still an AAF. After that, phosphate serum levels went up until normalization and alkaline phosphatase started to decrease. Calcium and phosphate supplementation were decreased to keep their serum levels at the normal range for age.The boys were then discharged from hospital when they were 4 months old (45 days of life of corrected age), but the family could not afford Alfamino, switching back to Neocate. After that their phosphate serum levels went below normal range again. Conclusion: To our knowledge, this is the first report on monozygotic twins presenting AAF Neocate-related HR. The underlying mechanisms of Neocate induced hypophosphatemia is still elusive, since its content of phosphate and calcium:phosphate ratio are similar to other AAFs. These cases reinforce the importance of evaluating phosphate metabolism in infants receiving AAF while the pathophysiology of this condition is not entirely understood.