Cystinuria is an inherited defect of the renal tubules in which reabsorption of cystine (the homodimer of the amino acid cysteine) is impaired, urinary excretion is increased, and cystine stones form in the urinary tract. Symptoms are colic caused by stones and perhaps urinary infection or the sequela of chronic kidney disease. Diagnosis is by measurement of cystine excretion in the urine. Treatment is with increased fluid intake and alkalinization of the urine.
Cystinuria was originally classified according to urinary excretion of cystine and dibasic amino acids in obligate carriers. In this classification, parents of affected children were assessed as having either normal (type I), moderate (type III), or significant (type II) increases in cystine excretion.
A newer classification is based on genotype: Type A patients have homozygous mutations in the gene SLC3A1 and type B patients have homozygous mutations in SLC7A9. These genes encode proteins that together form a heterodimer responsible for cystine and dibasic amino acid transport in the proximal tubule.
Cystinuria should not be confused with cystinosis ( see Hereditary Fanconi syndrome).
Pathophysiology of Cystinuria
The primary defect results in diminished renal proximal tubular reabsorption of cystine and increased urinary cystine concentration. Cystine is poorly soluble in acidic urine, so when its urinary concentration exceeds its solubility, crystals precipitate and cystine kidney stones form. Males are generally affected more than females.
Symptoms and Signs of Cystinuria
Symptoms of cystinuria, most commonly renal colic, may occur in infants but usually appear between ages 10 and 30.
Urinary tract infection and chronic kidney disease due to urinary tract obstruction may develop.
Diagnosis of Cystinuria
Analysis of collected kidney stones
Microscopic examination of urinary sediment
Measurement of urinary cystine excretion
Radiopaque cystine stones form in the renal pelvis or bladder. Staghorn stones are common.
Cystine may appear in the urine as yellow-brown hexagonal crystals, which are diagnostic.
> 400 mg/day in cystinuria (normal is < 30 mg/day).
Treatment of Cystinuria
High fluid intake
Alkalinization of the urine
Dietary sodium restriction
Dietary animal protein restriction (when possible)
End-stage renal disease may develop. Decreasing urinary cystine concentration below about 250 to 300 mg/L (1 to 1.25 mmol/L) decreases renal toxicity and can allow clearance of cystine in solution. This decrease is accomplished by increasing urine volume with fluid intake sufficient to provide a urine flow rate of 1.5 to 2 L/m2/day, which could require fluid intake as high as 2 to 4 L/day (1). Hydration is particularly important at night when urinary volume and pH drop.
Alkalinization of the urine to pH >
Mild restrictions of dietary sodium (100 mEq/day) and animal protein (0.8 to 1.0 g/kg/day) may help reduce cystine excretion.
nephrotic syndromepenicillaminepenicillaminepenicillamine but is less toxic. Close monitoring of response to therapy is very important.
Animal models (eg, gene knockout mice) are currently being used toward the development of novel therapies for cystinosis (2
Treatment references
1. Claes DJ, Jackson E: Cystinuria: Mechanisms and management. Pediatr Nephrol 27(11):2031–2038, 2012. doi: 10.1007/s00467-011-2092-6
2. Sahota A, Tischfield JA, Goldfarb DS, et al: Cystinuria: Genetic aspects, mouse models, and a new approach to therapy. Urolithiasis 47(1):57–66, 2019. doi: 10.1007/s00240-018-1101-7
Key Points
Defective urinary resorption of cystine increases urinary cystine levels, leading to cystine kidney stones and sometimes chronic kidney disease.
Yellow-brown hexagonal crystals in the urine are pathognomonic; quantitative cystine excretion is typically > 400 mg/day.
Restrict dietary sodium and animal protein.
