(See also Introduction to Congenital Kidney Tubular Disorders Introduction to Congenital Kidney Tubular Disorders The kidneys filter and cleanse the blood. They also maintain the body’s balance of water, dissolved salts ( electrolytes, such as sodium, potassium, and calcium), and nutrients in the blood... read more .)
This very rare disorder is nearly always hereditary. It is most commonly inherited as a dominant gene Dominant x-linked disorders Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are made of a very long strand... read more that is carried on the X chromosome, one of the two sex chromosomes. Because only one gene is needed when a dominant gene is involved, one of the parents is likely to have the syndrome. Siblings of children with the disorder have a 50% chance of having it. There are several other even rarer forms of hypophosphatemic rickets.
The genetic defect causes a kidney tubule abnormality that allows an inappropriately high amount of phosphate to be excreted into the urine, resulting in low levels of phosphate Overview of Phosphate's Role in the Body Phosphorus is an element that plays an important role in the body. In the body, almost all phosphorus is combined with oxygen, forming phosphate. Phosphate is one of the body's electrolytes... read more in the blood. Because bones need phosphate for growth and strength, this deficiency causes defective bones.
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 Vitamin D Deficiency Vitamin D deficiency is most commonly caused by a lack of exposure to sunlight. Some disorders can also cause the deficiency. The most common cause is lack of exposure to sunlight, usually when... read more .
Symptoms of Hypophosphatemic Rickets
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 Bowlegs Bowlegs and knock-knees are the two major types of knee deformities. They are birth defects. Birth defects, also called congenital anomalies, are physical abnormalities that occur before a baby... read more 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 Craniosynostosis Craniosynostosis is a birth defect in which one or more of the skull's sutures close too early. Birth defects, also called congenital anomalies, are physical abnormalities that occur before... read more ). 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.
Diagnosis of Hypophosphatemic Rickets
Blood and urine tests
Sometimes bone x-rays
Sometimes genetic testing
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.
Genetic testing can help confirm the diagnosis.
Siblings of children who are affected should have a medical evaluation, including laboratory testing, imaging tests, and sometimes genetic testing. Genetic testing also may be offered to other family members.
Treatment of Hypophosphatemic Rickets
Phosphate and calcitriol
Burosumab for the most common form of hypophosphatemic rickets
Sometimes surgical removal of tumors
Treatment of hypophosphatemic rickets is aimed at raising 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 Vitamin D Deficiency Vitamin D deficiency is most commonly caused by a lack of exposure to sunlight. Some disorders can also cause the deficiency. The most common cause is lack of exposure to sunlight, usually when... read more . 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 Stones in the Urinary Tract Stones (calculi) are hard masses that form in the urinary tract and may cause pain, bleeding, or an infection or block of the flow of urine. Tiny stones may cause no symptoms, but larger stones... read more . These side effects can harm the kidneys and other tissues.
People who have the most common form of hypophosphatemic rickets are given the drug burosumab and are not given the phosphate and calcitriol therapy described above. Burosumab is a monoclonal antibody Monoclonal Antibodies Immunotherapy is the use of drugs that mimic or modify components of the immune system (such as tumor antigens and immune checkpoints—see also Overview of the Immune System) to fight disease... read more and is given as an injection under the skin. It helps increase the levels of phosphorus in the blood, helps decrease the severity of rickets, and in children may increase height. Burosumab can be given to children and adults.
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.