Case in Point: Hypophosphatemic Rickets

The child had been born to a 19-year-old gravida 2, para 2 mother via cesarean birth secondary to malpresentation and fetal distress. The child weighed 3.06 kg at birth. She currently weighed 6 kg (5th percentile); she was 62 cm tall (5th to 25th percentile); head circumference, 46 cm (95th percentile or higher). Laboratory findings: calcium, 9.7 mg/dL; phosphorus, 2.8 mg/dL, immunoreactive parathyroid hormone, 95 pg/mL (normal, 7 to 53 pg/mL); alkaline phosphatase, 521 U/L (normal, 50 to 330 U/L); and urinary phosphorus, 870.4 mg/dL. X-ray films revealed florid signs of rickets, including a rachitic rosary and cupping of ribs, and fraying and flaying of the radius, ulna, femur, tibia, and fibula (Figures 1 and 2). A dietary history revealed that the child had been fed a soy-based formula since early infancy because she had been unable to tolerate cow's milk. She drinks approximately 5 oz of soy-based formula every 2 to 3 hours and recently started to eat baby food. A diagnosis of hypophosphatemic rickets was made. Therapy with vitamin D (calcitriol) and phosphorus supplementation was initiated. The child's mother had rickets as a child, which left severe deformities. She was taking vitamin D and phosphorus supplements. The patient's 18-month-old sister had a history of delayed gross motor milestones and also had frontal bossing; however, a workup had never been done, nor had the child been treated.Hypophosphatemic Rickets Familial hypophosphatemic rickets is the most common nonnutritional form of rickets. The term "vitamin D-resistant rickets" was originally used to describe rickets that was refractory to treatment with vitamin D. It was later found that renal phosphate wasting and a low serum phosphate concentration were common factors in many refractory cases. This discovery led to the name "phosphate diabetes." This disorder now is more appropriately named "hereditary hypophosphatemic rickets." The usual mode of inheritance is X-linked dominant. X-linked hypophosphatemic (XLH) rickets is the most common form of rickets in developed countries.1 One in 20,000 persons is affected.2,3 Other forms of hypophosphatemic rickets include autosomal dominant,4 autosomal recessive hereditary hypophosphatemic rickets with