(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 and 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 .)
Cystinuria is caused by an inherited defect of the kidney tubules. The defect causes people to excrete excessive amounts of the amino acid cystine into the urine (amino acids are the building blocks of proteins). The excess cystine causes cystine 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 to form in the kidneys, 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).
There are two gene abnormalities that cause most cystinuria. The genes are recessive Recessive 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 , so people with the disorder must have inherited two abnormal genes, one from each parent ( see Figure: Non–X-Linked (Autosomal) Recessive Disorders Non–X-Linked (Autosomal) Recessive Disorders ). Because two genes are needed when a recessive gene is involved, the parents are carriers of the gene but do not have the syndrome. However, siblings of children with the disorder might have it.
Sometimes, a person who has only one abnormal gene (a carrier), such as a parent of a person who has cystinuria, may excrete larger than normal amounts of cystine into the urine but seldom enough to form cystine stones.
Boys are generally affected more than girls.
Viewing the Urinary Tract
Symptoms of Cystinuria
Although symptoms of cystinuria may occur in infants, they usually start between the ages of 10 years and 30 years.
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 Chronic Kidney Disease Chronic kidney disease is a slowly progressive (months to years) decline in the kidneys’ ability to filter metabolic waste products from the blood. Major causes are diabetes and high blood pressure... read more or, occasionally, kidney failure Overview of Kidney Failure Kidney failure is the inability of the kidneys to adequately filter metabolic waste products from the blood. Kidney failure has many possible causes. Some lead to a rapid decline in kidney function... read more .
Diagnosis of Cystinuria
Analysis of kidney stones
A doctor tests for cystinuria when a person has recurring kidney stones. Stones that have been collected are analyzed.
Cystine crystals may be seen during a microscopic examination of the urine (urinalysis Urinalysis and Urine Culture Urinalysis, the testing of urine, may be necessary in the evaluation of kidney and urinary tract disorders and can also help evaluate bodywide disorders such as diabetes or liver problems. A... read more ), and high cystine levels are found in the urine.
Treatment of Cystinuria
Increasing fluid intake
Medications to make the urine more alkaline
Medications to dissolve cystine
Decreasing salt and animal protein in the diet
Treatment of cystinuria 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 (that is, less acidic) 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 animal protein may help reduce the concentration of cystine in the urine.
If stones continue to form despite these measures, medications 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 can be toxic, so doctors give vitamin B6 Vitamin B6 Deficiency Vitamin B6 is in most foods, but people can have vitamin B6 deficiency if they do not absorb it properly. Many foods contain vitamin B6, but extensive processing can remove the vitamin. People... read more (pyridoxine) supplements to limit the adverse effects of 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 penicillamine but is less toxic.
Although the medications are usually effective, there is a fairly high risk that stones will continue to form.