Cystinuria is a genetic cause of kidney stones with an average prevalence of 1 in 7000 births . Cystine stones are found in 1 to 2 percent of stone formers, although they represent a higher percentage of stones in children (about 5 percent) .
Cystinuria is a different disorder from cystinosis, which is characterized by intracellular cystine accumulation leading to the Fanconi syndrome and progressive renal failure. (See "Cystinosis".)
PATHOGENESIS, GENETICS, AND CLASSIFICATION
Cystine is a homodimer of the amino acid cysteine. Patients with cystinuria have impairment of renal cystine transport, with decreased proximal tubular reabsorption of filtered cystine resulting in increased urinary cystine excretion and cystine nephrolithiasis [3-5]. The cystine transporter also promotes the reabsorption of the other dibasic amino acids, including ornithine, arginine, and lysine, but these compounds are relatively soluble and an increase in their excretion does not lead to stones. Intestinal cystine transport is also diminished, but the result is of uncertain clinical significance.
Classification — Previously, cystinuria was classified according to the amount of cystine excreted by the parents of the affected child (phenotypic classification). Patients were classified as type I if their parents excreted normal amounts of cystine, and types II or III if parental cysteine excretion was either greatly (II) or moderately (III) increased. However, with the identification of the genes responsible for this disorder (specifically, SLC3A1 and SLC7A9), a genotypic classification of cystinuria has been introduced [5-8]. Neither the genotypic nor the phenotypic classification appears to be relevant with respect to the clinical course in patients with cystinuria, as is discussed below. (See 'Genotype-phenotype correlation with stone risk' below.)
The genotypic classification scheme is summarized here: