Cystinuria is a rare disorder that results in excretion of the amino acid cystine into the urine. This often causes cystine stones to form in the urinary tract, known as urinary calculi.
The condition is caused by an inherited defect of the kidney tubules. The gene that causes cystinuria is recessive, which means that people with the disorder need to inherit two abnormal genes, or one from each parent. Carriers of the gene do not have the disorder and have one normal and one abnormal gene. These carriers may excrete larger than normal amounts of cystine into the urine, but seldom enough to form stones.
Cystine stones form in the bladder, renal pelvis (area where urine collects and flows out of the kidney), or the ureters (long narrow tubes that convey urine from the kidneys to the bladder).
- Symptoms usually start between ages 10 and 30
- 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 may result in urinary tract infection and kidney failure.
A doctor tests for cystinuria when a person has recurring kidney stones. Cystine may form yellow-brown hexagonal crystals in the urine that are visible under a microscope. Excessive amounts of cystine in the urine can be detected and measured by several tests.
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 with cystinuria must drink enough fluids to produce at least 8 pints of urine each day. During the night, however, when the person isn't drinking, less urine is produced and stone formation is more likely. Consequently it is important to reduce this risk by drinking fluids before going to bed.
Another treatment approach involves making the urine more alkaline by taking sodium bicarbonate and acetazolamlde. Cystine dissolves more easily in alkaline urine than in acidic urine.
If stones continue to form despite these measures, a drug such as penicillamine may be tried. Penicillamine reacts with cystine to keep it dissolved.
Cystine stones can be treated by the same surgical methods as other more common types of urinary calculi. These include shock wave lithotripsy, percutaneous stone removal and endoscopic stone fragmentation and removal. (See kidney stones)
Reviewed by Dr Pieter J. le Roux