Types of Stones

Cystine Calculi

Cystine in the urine, or cystinuria, is a relatively rare autosomal recessive inborn error of metabolism which is characterized by impaired reabsorption of dibasic amino acids (cystine, lysine, ornithine and arginine) from the renal tubules as well as the gastrointestinal tract. Only the basic amino acid, cystine, is associated with stone formation due to its poor solubility. Cystine stones represent 1 to 4% of all urinary calculi. The prevalence in the United States is 1 in 15,000 persons. It accounts for 6 to 8% of pediatric stones. Successful medical therapy in treatment of patients with cystine stones is based on three factors:

1. Decreasing total urinary cystine concentration
2. increasing the solubility of cystine
3. Decreasing urinary cystine excretion.

Successful medical management can be achieved by a combination of these therapeutic measures. This includes adequate fluid hydration and fluid diuresis. In addition, urinary alkalinization to a pH of 7.5 or higher with potassium citrate can decrease the solubility of cystinuria. In addition, D-penicillamine decreases the urinary excretion of cystine by binding cystine to form the more soluble cystine - S - penicillamine complex that is 50 times more soluble than cystine. However, numerous side effects including rash, fever, agranulocytosis, iron depletion, proteinuria and nephrotic syndrome have limited its use. Alpha-mercapto-propionyl-glycine, or Thiola, a newer agent with similar action to penicillamine, has fewer side effects.

Cystinuria, an autosomal recessive disease, results from excessive excretion of the four basic amino acids, cystine, ornithine, lysine, and arginine (COLA) into the urine. Cystine is relatively insoluble in acid urine. Patients with cystinuria (>250mg/24 hr.) produce a supersaturated urine and are at risk for crystallization and stone formation. The chemoprevention of cystine stones includes hydration and urinary alkalinization, cystine-binding agents (D-penicillamine, alphamercaptopropionylglycine, captopril), and irrigation chemolytic therapy (tromethamin E, acetylcysteine).

Suggested readings
Fitomer WL, Pak CYC. Recent advances in the biochemical and molecular biological basis of cystinuria. J Urol 1996, 156:6:1907-1912.

Rutchik SD, Resnick MI. Cystine calculi: Diagnosis and management. Urol Clin North Am 1997, 24:1:163-171.


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