Abstract
Introduction
Autosomal dominant hypophosphatemic rickets (ADHR) is a condition with variable phenotype in terms of age of presentation, severity, and possible resolution. ADHR is caused by mutations of FGF23, preventing its cleavage, producing high levels of FGF23, which leads to renal phosphate wasting. Studies in mice and adult humans, have shown a correlation between low iron levels and increased FGF23 levels. To our knowledge, three pediatric patients with ADHR resolved with iron supplementation have been reported in the literature.
Clinical case
We report on identical twins born at 28 weeks and 5 days by cesarean section due to premature rupture of membranes with complicated pregnancy due to twin-to-twin transfusion syndrome. Birth weights were 780 grams (2nd percentile) for twin A, 1,200 grams (50th percentile) for twin B. Hypophosphatemia was documented starting at 2 weeks of life and during the first 6 of months of life, with phosphorus levels between 2.9-3.9 mg/dL for twin A and between 2.4-5.1 mg/dL for twin B.
During their NICU admission phosphorus had a positive relationship with the hemoglobin level, which was more severe on twin A. Both were treated with calcitriol and a low dose of phosphorus starting on their 2nd month of life.
At 6 months of age, both had persistent hypophosphatemia, more prominently in twin A (2.7mg/dL) with high alkaline phosphatase (1,209 IU/L) and high FGF23 (343 RU/dL). At that time his hemoglobin was 9.8 g/dL and his hematocrit was 29.5%. Both were started on Polyvisol with iron.
At 14 months of life phosphorus and calcium were within normal limits, therefore calcitriol and phosphate were discontinued. At 15 months of age their hemoglobin, hematocrit, iron level, and TIBC levels were normal for both twins. Phosphorus was 4.6 and 4.3 mg/dL, alkaline phosphatase reduced significantly to 819 and 413IU/L, and FGF23 normalized to 100 RU/dL and 32 RU/dL on twin A and B respectively.
Upon physical examination at 15 months of age, twin A was at the 0.02% for length and weight/length at the 31%; twin B was at the 5% for length and weight/length at the 50%. Both twins had high arched palates. Twin A had craniosynostosis, left renal agenesis, bilateral epicanthal folds, overlapping 2nd toes, and clinodactyly of the fifth digits. Note the donor kid (twin A) had a more severe presentation.
Genetic testing showed heterozygous mutation c536G>a (p.Arg179GLN) in the FGF23 gene. This is the same mutation previously reported to be related with ADHR resolved with iron supplementation
Conclusion
The study of patients with hypophosphatemia and hypophosphaturia should include evaluation of iron status (ferritin, TIBC). Treating iron deficiency on these patients might normalize phosphate levels. This would avoid cumbersome treatment with phosphate and calcitriol.
Laboratory values
Phosphorus:4-8 mg/dL
Alkaline Phosphatase: 130-317 IU/L
FGF23: 44-215 RU/dL
FGF23 Concentrations Vary With Disease Status in Autosomal Dominant Hypophosphatemic Rickets
