Cystine stones develop in several locations in the urinary system, including in the kidneys, ureters and bladder. These are a relatively rare type of stone, comprising only three percent of all stones that form in the urinary tract. Cystine stones develop because of a genetic defect that causes a disease called cystinuria. This disease affects approximately one in every 10,000 people, with those under the age of 40 being more likely to have cystinuria stones.
Cystinuria develops because of a buildup of the amino acid cystine in urine. Normally, most proteins and amino acids in waste products are filtered by the kidneys and reabsorbed into the blood. In someone with cystinuria, the mechanism that allows cystine to be reabsorbed is defective. Cystine accumulates in urine, and cystine deposits form crystals or stones. Small crystals can be excreted in the urine, but larger stones can get stuck in the urinary tract.
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Symptoms of cystine stones develop when large stones become lodged in the kidneys, ureters or bladder. The most common symptom is flank pain, which is felt in the back or the side. In most cases the pain is on only one side of the body; it is rare to have pain from cystine stones develop on both sides of the body at the same time. This pain often is mild at first but becomes more intense over a period of several days. Some people might experience pain in other locations, such as the groin, genitals or pelvis. Pain occasionally might develop in the back or upper abdomen.
Most people are diagnosed with cystinuria stones after experiencing an initial episode of pain caused by the stones. Medical imaging tests might be used to determine whether stones are present. Urine samples also are taken to determine whether cystine levels are high enough to confirm that the stones are an indicator of cystinuria.
Cystine stone treatments focus on relieving pain and other symptoms and on preventing further stones from forming. If symptoms are very severe, a patient might be admitted to hospital, but in most cases, at-home treatment is sufficient. Two types of medication might be used: medications to relieve pain and medications to promote the dissolution of cystine crystals.
When these measures do not help stones pass through the urinary tract, surgery might be needed. Stones that are located in the lower urinary tract can be removed via ureteroscopy. In this procedure, small tools are inserted through the urethra to remove the stones from the urinary tract. Some cystine stones can be removed via a procedure called extracorporeal shockwave lithotripsy, in which stones are bombarded with sound waves to break them into smaller crystals that can be excreted into the urine.
For prevention of cystine stones, drinking six to eight glasses of water per day is important to help flush out smaller cystine crystals. This helps prevent smaller crystals from growing into larger stones. Another useful preventative measure is to reduce salt consumption, which also helps to decrease the rate of stone formation.