Definition of Stones, cystine kidney

Stones, cystine kidney: Cystine kidney stones are due to cystinuria, an inherited (genetic) disorder of the transport of an amino acid (a building block of protein) called cystine. The result is an excess of cystine in the urine (cystinuria) and the formation of cystine stones.

Cystinuria is the most common defect in the transport of an amino acid. Although cystine is not the only overly excreted amino acid in cystinuria, it is the least soluble of all naturally occurring amino acids. Cystine tends to precipitate out of urine and form stones (calculi) in the urinary tract.

Small stones are passed in the urine. However, big stones remain in the kidney (nephrolithiasis) impairing the outflow of urine while medium-size stones make their way from the kidney into the ureter and lodge there further blocking the flow of urine (urinary obstruction).

Obstruction of the urinary tract puts pressure back up on the ureter and kidney. causing the ureter to widen (dilate) and the kidney to be compressed. Obstruction also causes the urine to be stagnant (not moving), an open invitation to repeated urinary tract infection. The pressure on the kidneys and the urinary infections results in damage to the kidneys. The damage can progress to renal insufficiency and end-stage kidney disease, requiring renal dialysis or a transplant.

The stone are responsible for all the signs and symptoms of cystinuria, including:

• Hematuria -- blood in the urine;
• Flank pain -- pain in the side, due to kidney pain;
• Renal colic - intense, cramping pain due to stones in the urinary tract;
• Obstructive uropathy -- urinary tract disease due to obstruction; and
• Urinary tract infections.

Prevention of stone formation is directed at decreasing the cystine concentration in the urine. This is done by increasing the amount of fluid intake and keeping the urine volume at up to four liters per day. A low-sodium diet has also been reported to reduce urine cystine excretion. If the urine pH is increased, the solubility of the cystine is increased. It is difficult to maintain the elevated pH on a long-term basis. Some medications have been found to bind cystine and help reduce the risk of stone formation; side effects may limit their use, however.

For a fuller understanding of cystinuria, including the diagnosis, the different modes of treatment, the genetics of cystinuria and the prognosis for persons with this disease, please see the Cystinuria article.