Section 11. Kidney and Urinary Tract Disorders

Chapter 146. Tubular and Cystic Kidney Disorders

Topics: Introduction | Renal Tubular Acidosis | Renal Glucosuria | Nephrogenic Diabetes Insipidus | Cystinuria | Fanconi's Syndrome | Hypophosphatemic Rickets | Hartnup Disease | Bartter's Syndrome | Liddle's Syndrome | Polycystic Kidney Disease | Medullary Cystic Disease | Medullary Sponge Kidney | Alport's Syndrome | Nail-Patella Syndrome

Hypophosphatemic Rickets

Hypophosphatemic rickets (previously called vitamin D-resistant rickets) is a disorder in which the bones become painfully soft and bend easily because the blood contains low levels of phosphate and has inadequate amounts of the active form of vitamin D.

This very rare disorder is nearly always hereditary, passed as a dominant gene that is carried on the X chromosome. The genetic defect causes a kidney abnormality 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 to grow, this deficiency causes defective bones. Females with hypophosphatemic rickets have less severe bone disease than 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 Section 12, Chapter 154).

Symptoms and Treatment

Hypophosphatemic rickets usually begins in the first year of life. It ranges from so mild that it produces no noticeable symptoms to so severe that it produces bowing of the legs and other bone deformities, bone pain, and a short stature. Bony outgrowth where muscles attach to bones may limit movement at those joints. A baby's skull bones may close too soon, leading to seizures. Laboratory tests show that calcium levels in the blood are normal, but phosphate levels are low.

The aim of treatment is to raise phosphate levels in the blood, which will promote 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 helpful. However, treatment often leads to high levels of calcium in the blood, the accumulation of calcium in kidney tissue, or kidney stones. These effects can harm the kidneys and other tissues. In some adults, hypophosphatemic rickets resulting from cancer improves dramatically after the cancer is removed.