Learn More - Cystinuria
What is Cystinuria?
Cystinuria is an inherited disease of amino acid transport affecting the absorption of four structurally similar amino acids from excretory pathways. In all people, cystine, ornithine, lysine and arginine are filtered from the blood into the urine. Normally, those amino acids are reclaimed from the urine for recirculation in the blood. However, cystinuric patients lack the necessary ability to transport those molecules. As a result, they become concentrated in the urine where cystine, being uniquely insoluble, tends to crystalize and form cystine stones. The result is recurring stone formation. These stones can pass spontaneously, however larger stones may become stuck in the urinary tract and require surgical intervention for removal. Frequent stone formation, kidney blockage, surgical procedures and resulting consequences can impact kidney function over time.
Who gets Cystinuria?
Cystinuria is a genetic condition passed from parents to children. It is generally accepted that 1 in 10,000 people worldwide are cystinuric. However, as with any genetic disease, the prevalence can vary greatly among communities. As a genetic condition, the disease is present from birth and persists for life. However, symptoms including kidney stone formation, may not occur immediately. While some patients form stones at infancy, others may not be aware of their disease until decades later. Some cystinuric people will never form kidney stones, despite having high urinary cystine concentrations. The underlying cause of why some people seem to be more affected than others is currently unknown.
What causes Cystinuria?
When genes responsible for making specific transport proteins are defective, the transport system fails to transport cystine from the urine. These defective gene copies are inherited from parents and cannot be changed. When cystine cannot be absorbed from the urine, it can concentrate to a point where it forms solid crystals that grow into stones in the urinary tract. Ornithine, arginine and lysine also accumulate in the urine, but are generally soluble even at the elevated concentrations and do not impact the health of the patient.
How is Cystinuria diagnosed?
Cystinuria is usually diagnosed when a person goes to the doctor with the symptoms of kidney stones (generally pain called "renal cholic"), and those stones are later found through lab analysis to be composed of cystine. Cystine crystals in the urine are also a diagnostic symptom of the disease. In some areas, genetic screening programs have identified cystinuric individuals based directly on finding the defective copies of the cystine transporter gene. Interestingly, not all people genetically identified as cystinuric go on to be stone-forming patients, indicating that not all people with the disease are equally affected by the symptoms.
What is/is there treatment for Cystinuria?
As cystinuric is a genetic disease, it cannot be fundamentally cured. Treatment is instead focused on reducing the occurrence of resulting kidney stone formation. All current efforts are aimed at preventing stone formation by decreasing the amount of solid cystine in the urinary tract. Cystine solubility can be improved by increasing urine volume (by consistently drinking large quantities of fluids) or decreasing urinary cystine excretion via dietary changes like reducing salt and protein intake. Cystine becomes more soluble as urinary acidity decreases, and therefore diets (high in vegetables) or medication (bicarbonate or citrate compounds) that promote less-acidic urine are considered powerful in reducing the rate of stone formation. Finally, for the most severely affected patients, medications are available that break apart the cystine and form more soluble complexes. These medications may not be well tolerated by the patient, and therefore careful monitoring is necessary to determine the lowest effective dose necessary.