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Metabolic disorder involving cystine


Updated November 14, 2004

Cystinuria is an inherited metabolic disorder. In cystinuria, the filtering and reabsorption of cystine and the amino acids ornithine, arginine, and lysine by the kidneys is defective. This causes elevated levels of cystine in the urine, and kidney stones made of cystine form. Cystine accounts for 1% of adult and 6-8% of childhood kidney stones.

Cystinuria is estimated to occur in 1 per 7,000 persons. Research has found that cystinuria is more common in some ethnic groups, such as Jews of Libyan extraction. It is suspected that there may be many more individuals with cystinuria who do not form kidney stones who are therefore undiagnosed.

Generally, the only symptom of cystinuria is the formation of cystine kidney stones, which often recur throughout a person's lifetime. In 10% of individuals, high blood pressure may occur. Others may have kidney insufficiency, recurrent urinary tract infections, or kidney damage.

  • In 25% of individuals, the first cystine kidney stone forms before age 10.
  • In 30-40% of individuals, the first stone forms in adolescence.
  • Stones are often present in both kidneys at the same time, and there may be more than one in each kidney.
  • As the stones pass through the urinary symptom, they cause intense back pain, blood in the urine, and difficulty urinating.

Most often, cystinuria is diagnosed when an individual has a cystine kidney stone. Urinalysis will often show cystine crystals in the urine. There is also a simple test that can detect the level of cystine in the urine.

Abdominal x-ray and intravenous pyelogram can detect the presence of the cystine kidney stones, as can renal ultrasound.

Treatment geared to reducing stone formation
The basic treatment for cystinuria is prevention of the formation of kidney stones. The first step is to dilute the urine by increasing the amount of liquids a person drinks during the day, with the goal of reaching 4 liters per day. This means 8 ounces (240 ml) every hour while awake and 16 ounces (480 ml) at bedtime and once during the night. Beverages which reduce the acidity of the urine, such as mineral water and citrus juice, are preferred.

In addition to drinking juices, the urine acidity can be reduced by taking potassium citrate. Individuals can test the acidity and concentration of their urine at home with a simple dip stick test.

If these conservative measures don't work well, medications that bind to cystine can be used to reduce the amount of cystine in the urine. Penicillamine, alpha-mercaptopropionylglycine, captopril, or bucillamine may be used.

Diet changes
A diet low in methionine will help reduce the production of cystine in the body. A well-balanced mixed diet with low protein content is recommended. Methionine is found in animal proteins such as those in milk, eggs, cheese, and fish. Reducing sodium intake and increasing dietary fiber intake are also helpful.

Surgery may be needed
If the kidney stones are too large to pass through the urinary system, surgery may be needed to reduce or remove them. Shockwave or ultrasonic lithotripsy breaks up large stones into smaller pieces that can pass through the system. If this fails, large stones can be surgically removed.

Information for this article was taken from:
Biyani, S. (2003). Cystinuria. eMedicine, accessed at http://www.emedicine.com/med/topic498.htm

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