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(Vitamin D–Resistant Rickets)
Hypophosphatemic rickets is a disorder in which the bones become painfully soft and bend easily because the blood contains low levels of the substance phosphate.
This very rare disorder is nearly always hereditary and is most commonly inherited as a dominant gene that is carried on the X chromosome, one of the two sex chromosomes. The genetic defect causes a kidney tubule abnormality (see Introduction to Congenital Kidney Tubular Disorders) that allows an inappropriately high amount of phosphate to be excreted into the urine, resulting in low levels of phosphate in the blood. Because bones need phosphate for growth and strength, this deficiency causes defective bones. Females with hypophosphatemic rickets have less severe bone disease than do males. In rare cases, the disorder develops as a result of certain cancers, such as giant cell tumors of bone, sarcomas, prostate cancer, and breast cancer. Hypophosphatemic rickets is not the same as rickets caused by vitamin D deficiency (see Vitamin D Deficiency).
Hypophosphatemic rickets usually begins to cause abnormalities in the first year of life. Abnormalities may be so mild that they cause no noticeable symptoms or so severe that they cause bowing of the legs and other bone deformities, bone pain, joint pain, and poor bone growth with short stature. Bony outgrowths where muscles attach to bones may limit movement at those joints. The space between a baby’s skull bones may close too soon (called craniosynostosis). If the skull bones close too soon, the child's growing brain has no room to expand and pressure increases within the brain. The increased pressure on the brain can cause developmental abnormalities.
Laboratory tests show that calcium levels in the blood are normal but phosphate levels are low. Urine is also tested to detect the levels of phosphate that have been excreted. The phosphate levels in the urine are high. Doctors may also take x-rays of bones.
The aim of treatment is to raise phosphate levels in the blood, which promotes normal bone formation. Phosphate can be taken by mouth and should be combined with calcitriol, the activated form of vitamin D. Taking vitamin D alone is not enough. The amounts of phosphate and calcitriol must be adjusted carefully because this treatment often leads to high levels of calcium in the blood and urine, a build up of calcium in the kidneys, or kidney stones. These side effects can harm the kidneys and other tissues. In some adults, hypophosphatemic rickets resulting from various types of tumors lessens dramatically after the tumors are removed. If hypophosphatemic rickets does not lessen, people who have rickets caused by a tumor are given calcitriol and phosphorus.
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