Familial Hypophosphatemia is an inherited disorder of bone metabolism that affects 1 in 20 000 new-borns. The X-linked dominant form of the disease results from a genetic mutation of the PHEX gene. This leads to disordered regulation of fibroblast growth factor 23 resulting in reduced phosphate renal reabsorption and vitamin 1α Hydroxylation. Clinical features include short stature, rickets, craniosynostosis and dental abnormalities.
Case: A 29 year old woman was referred to the joint Antenatal endocrine clinic at 28 weeks gestation. She was diagnosed with familial Hypophosphatemic rickets in childhood of which a mutation in the PHEX(Phosphate regulating gene with homology to endopeptide on X chromosome) gene was identified . This was an x-linked dominant pattern of inheritance. Her mother was also diagnosed with the condition however her brothers were not affected. At 20 weeks gestation, the growth scan showed no significant abnormalities apart from the foetus measuring slightly small for dates. Progressive growth of femur length and head circumference were subsequently noted on serial growth scans. She was managed with Phosphate Sandoz and alfacalcidol 50 mcg daily. Her average calcium was 2.39 mmol/l (2.202.60 mmol/l), Phosphate 0.81 mmol/l (0.801.50 mmol/l). Vitamin D was normal at 66 nmol/l. She delivered at a healthy female infant at 39 weeks gestation.
Discussion: Familial X-linked dominant hypophosphatemic rickets confers a 50% chance of transmission to offspring. The implication of this means that if the gene defect is inherited, potential disorders in bone growth and development could occur. Genetic counselling was sought prior to conception. However following delivery, genetic testing of the infant might be required especially if there are physical manifestations of the disease. The aim of treatment in pregnancy is to ensure adequate phosphate and vitamin D levels to facilitate normal growth and development of foetal bones by treatment with oral phosphate supplements and calcitriol.