hereditary hypophosphatemic rickets
Hereditary hypophosphatemic rickets, previously known as vitamin D resistant rickets, refers to a genetically heterogeneous group of causes of rickets related to renal phosphate wasting. The disorder is resistant to treatment with vitamin D supplementation.
Children present with abnormally slowed growth. Laboratory findings include low serum phosphate, normal serum calcium, elevated serum alkaline phosphatase, and normal to modestly elevated parathyroid hormone. Vitamin D levels are often normal.
This group of disorders results from a failure in phosphate reabsorption in the renal tubules. The final common mechanism of most, but not all, of these disorders is elevated levels of the hormone fibroblast growth factor 23 (FGF23), a phosphatonin that inhibits the sodium/phosphate cotransporters in the kidney.
Many genetic causes have been implicated :
- X-linked hypophosphatemic rickets (most common): PHEX mutation
- autosomal dominant hypophosphatemic rickets: FGF23 mutation
- autosomal recessive hypophosphatemic rickets: DMP1 mutation
- McCune-Albright syndrome: GNAS mutation
- epidermal nevus syndrome: RAS mutation
- hereditary hypophosphatemic rickets with hypercalciuria: SLC34A3 mutation
Findings are similar to nutritional rickets .
Other causes of hypophosphatemic/phosphopenic rickets include
- Fanconi syndrome, a generalized proximal tubulopathy that can be acquired or inherited
- tumor-induced osteomalacia/rickets, a paraneoplastic syndrome of phosphaturic mesenchymal tumor