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Cystinuria is a rare disorder that results in excretion of the amino acid cystine into the urine, often causing cystine stones to form in the urinary tract.
Cystinuria is caused by an inherited defect of the kidney tubules (see Introduction to Congenital Kidney Tubular Disorders). There are several possible gene abnormalities. In type I cystinuria, the gene is recessive, so people with the disorder must have inherited two abnormal genes, one from each parent (see Figure: ). In non-type I cystinuria, the gene is dominant. People who carry one abnormal gene excrete larger than normal amounts of cystine into the urine but seldom enough to form cystine stones.
Cystine stones form in the kidney, bladder, renal pelvis (the area where urine collects and flows out of the kidney), or ureters (the long, narrow tubes that carry urine from the kidneys to the bladder). Occasionally, kidney failure develops.
Although symptoms may occur in infants, they usually start between the ages of 10 and 30. Often, the first symptom is intense pain caused by a spasm of the ureter where a stone becomes lodged. The stone may also become a site where bacteria collect and cause a urinary tract infection.
A doctor tests for cystinuria when a person has recurring kidney stones. Cystine crystals may be seen during a microscopic examination of the urine (urinalysis), and high cystine levels are found in the urine.
Treatment consists of preventing cystine stones from forming by keeping the concentration of cystine in the urine low. To keep the cystine concentration low, a person must drink enough fluids to produce at least 6 to 8½ pints (3 to 4 liters) of urine each day. During the night, however, when the person is not drinking, less urine is produced and stone formation is more likely. This risk is reduced by drinking fluids before going to bed. Another treatment approach involves taking potassium citrate or potassium bicarbonate and sometimes acetazolamide to make the urine more alkaline because cystine dissolves more easily in alkaline urine than in acidic urine. Efforts to increase intake of water and make the urine more alkaline can lead to abdominal bloating, making the treatment difficult for some people to tolerate. Consuming less salt and protein may help reduce the concentration of cystine in the urine.
If stones continue to form despite these measures, drugs such as penicillamine, tiopronin, or captopril may be tried. These drugs react with cystine to keep it dissolved. Penicillamine is effective in keeping the concentration of cystine in the urine low but it is toxic, so doctors give vitamin B 6 (pyridoxine) supplements to people taking penicillamine. Tiopronin can be used instead of penicillamine to treat some children because it has a lower frequency of side effects. Captopril is slightly less effective than the other drugs but is less toxic. Although the treatments are usually effective, there is a fairly high risk that stones will continue to form.
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