Vitamin D resistant hypophosphatemic rickets
Which are the symptoms?
The characteristic decrease in renal phosphate reabsorption leads to disruption of bone metabolism and the development of rickets and / or osteomalacia. Affected patients at the time of birth have absolutely normal size and bone structure. Growth is normal for the first 6-9 months, then the growth rate decreases and there is low growth or, in any case, final growth not proportional to the genetic target. The most important deformities are the deformities of the lower extremities when the child begins to walk: it is for this reason that most affected children come to the attention of the doctor. More often, patients have a very pronounced varus of the femur-tibia, which is aggravated by excess weight. Other problems encountered in affected patients are dental problems: delayed teething, poor dentin mineralization and tooth weakness, abscesses. Sick adults and adolescents may also experience bone and joint pain, enthesopathy (calcification of tendons, ligaments, and joint capsules), pseudofractures, and decreased joint mobility. Laboratory tests are necessary to confirm clinical suspicion: this pathology is characterized by the detection of hypophosphatemia associated with hyperphosphaturia, as well as inappropriate normal or low levels of calcitriol.
How is it diagnosed?
How is it treated?
The objectives to be set in the therapy of hypophosphorus rickets are to reduce deformities and optimize statural growth, trying to minimize side effects. Forms caused by genetic mutations associated with low or inappropriately normal levels of calcitriol (due to the inhibition of renal 1a-hydroxylase by FGF23) require combined therapy with phosphate and active metabolites of vitamin D orally. Hypophosphatemic conditions caused by mutations in renal Na-Pi cotransporters (i.e. the form associated with hypercalciuria) require, instead, phosphate therapy alone, since the calcitriol values in these patients are normal. Severe lower limb deformities may require surgical correction.