Cystinuria

Cystinuria is caused by changes (mutations) in the SLC3A1 and SLC7A9 genes. These genes encode a protein complex that helps control the reabsorption of amino acids (such as cystine) in the kidneys. Mutations in these genes disrupt the function of the protein complex, causing cystine to become more concentrated in the urine. As the concentration of cystine increases, cystine crystals and/or stones begin to form in the urinary tract leading to the many signs and symptoms associated with cystinuria.

Screening for cystinuria should be considered in people with recurrent or bilateral (i.e. affecting both kidneys) stones; those who develop stones at an early age (before age 30); and people who have a family history of cystinuria. A diagnosis is typically made after an episode of kidney stones when testing reveals that the stones are made of cystine.

The following tests may be recommended to detect kidney stones and diagnose cystinuria:

• 24-hour urine collection
• Abdominal imaging (CT scan, MRI, or ultrasound)
• Intravenous pyelogram (IVP)
• Urinalysis
• Genetic testing

Cystinuria is inherited in an autosomal recessive manner. This means that to be affected, a person must have a mutation in both copies of the responsible gene in each cell. The parents of an affected person usually each carry one mutated copy of the gene and are referred to as carriers. Carriers typically do not show signs or symptoms of the condition. When two carriers of an autosomal recessive condition have children, each child has a 25% (1 in 4) risk to have the condition, a 50% (1 in 2) risk to be a carrier like each of the parents, and a 25% chance to not have the condition and not be a carrier.

Cystinuria is a chronic condition and many affected people experience recurrent cystine stones in the urinary tract (kidneys, bladder and ureters). In rare cases, frequent kidney stones can lead to tissue damage or even kidney failure.

Treatment of cystinuria is focused on relieving symptoms and preventing the formation of additional stones. A more conservative approach is typically tried first. This may include increasing fluid intake, regular monitoring of urinary pH, dietary restrictions (i.e. eating less salt) and increasing the pH of urine with potassium citrate supplements. If these strategies do not prevent the formation of stones, medications may be added to help dissolve the cystine crystals.

Treatment for cystinuria-related stones varies depending on the size and location of the stone, but may include:

• Extracorporeal shock wave lithotripsy (ESWL)
• Ureteroscopy
• Percutaneous nephrolithotomy
• Open surgery (in rare cases)

Medscape Reference's Web site offers more specific information regarding the treatment and management of cystinuria. Please click on the link to access this resource.