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

Cystinuria

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. The gene that causes cystinuria is recessive, so people with the disorder must have inherited two abnormal genes, one from each parent (see Section 1, Chapter 2). People who carry the gene but do not have the disorder have one normal and one abnormal gene. These people may excrete larger than normal amounts of cystine into the urine, but seldom enough to form cystine stones.

Symptoms and Diagnosis

Cystine stones form in the 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). Symptoms 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. Blockage of the urinary tract by stones reduces the ability of one or both kidneys to excrete waste products and excess fluid and salts. The stone may also become a site where bacteria can cause 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 cystine levels are measured in the urine.

Treatment

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 8 pints 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 sodium bicarbonate 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.

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. Captopril is slightly less effective than the other drugs, but has fewer serious side effects. Although the treatments are usually effective, there is a fairly high risk that stones will continue to form.