Cystinuria is an inherited disease that causes stones made of the amino acid cystine to form in the kidneys, bladder, and urethra. Inherited means that the disease is passed down from parents to children through a defect in their genes. In order to get cystinuria, a person must inherit the defect from both parents.
The defect in the gene causes cystine to accumulate inside the kidneys—organs that help regulate what goes in and out of your bloodstream. The kidneys have many functions, including reabsorbing essential minerals and proteins back into the body, filtering the blood to remove toxic waste and producing urine so that you can excrete the waste out of the body.
Instead of going back into the bloodstream, the amino acid cystine builds up and forms stones. The stones can then get stuck in the kidneys, bladder, and urethra (the tube that carries urine out of the body). This can be very painful until the stones pass through urination. Very large stones may need to be surgically removed.
The stones can recur many times. However, treatments are available to manage pain and to prevent more stones from forming.
Defects (mutations) in the genes called SLC3A1 and SLC7A9 cause cystinuria. These genes provide the instructions for your body to make a certain transporter protein found in the kidneys. This protein normally controls the reabsorption of certain amino acids.
Amino acids are formed when the body digests and breaks down proteins. The amino acids are used to perform a wide variety of bodily functions. They are important to your body and are not considered waste. Therefore, when they enter the kidneys, the amino acids are normally absorbed back into the bloodstream. In people with cystinuria, their genetic defect interferes with the transporter protein’s ability to reabsorb the amino acids.
One of the amino acids—cystine—is not very soluble in urine. If it isn’t reabsorbed, it will accumulate inside the kidney and form crystals, or cystine stones. The rock-hard stones then get stuck in the kidneys, bladder, and urethra. This can be very painful.
You are only at risk of getting cystinuria if your parents have the specific defect in their gene that causes the disease. You can only get the disease if you inherit the defect from both of your parents. Cystinuria occurs in about one in every 10,000 people around the world (Lin & Zieve, 2011).
Although cystinuria is a lifelong condition, symptoms typically first occur in young adults in their twenties and thirties (Biyani & Cartledge, 2006). However, there have been rare cases in infants and adolescents. The symptoms may include:
- blood in the urine
- severe pain in the side or the back (almost always on one side)
- nausea and vomiting
- pain near the groin, pelvis, or abdomen
Cystinuria is asymptomatic (causing no symptoms) when there are no stones. However, the symptoms will recur each time stones form in the kidneys. The stones commonly return more than once.
Cystinuria is usually diagnosed when someone experiences an episode of cystine stones. A diagnosis is then made by testing the stones to see if they are made out of cystine. Additional diagnostic testing could include:
24-Hour Urine Collection
You will be asked to collect your urine in a container over the course of an entire day. The urine will then be sent to a laboratory for analysis.
This is an X-ray examination of the kidneys, bladder, and urethra—to look for the presence of stones. This method uses a dye in the bloodstream to help see the stones.
Abdominal Computed Tomography (CT) Scan
This is an imaging method that uses X-rays to create images of the structures inside the abdomen to look for stones inside the kidneys
This is an examination of the urine in a laboratory. This may involve looking at the color and physical appearance of the urine, viewing the urine under a microscope, and conducting chemical tests to detect certain substances, like cystine.
If not treated properly, cystinuria can be extremely painful and may lead to serious complications. These complications include:
- kidney or bladder damage from a stone
- urinary tract infections
- kidney infections
- blockage of the ureter, the tube that drains urine from the kidneys to the bladder (ureteral obstruction)
Treatment is aimed at preventing stones from forming. It may include any of the following:
Increasing Water Intake
Drinking large amounts of water is vital to lowering the concentration of cystine in the urine. According to a 2006 study published in European Urology, hydration alone can prevent the reoccurrence of stones in 30 percent of patients (Biyani & Cartledge, 2006).
Reducing salt intake to less than two grams per day has also been shown to be helpful in preventing stone formation (Biyani & Cartledge, 2006).
Adjusting pH balance
Cystine is more soluble in urine at a higher pH. pH is a measure of how acidic or basic a substance is. Alkalinizing agents, such as potassium citrate, will increase the pH of the urine to make cystine more soluble. Some alkalinizing medications can be purchased over the counter; however, it is strongly recommended that you talk to your doctor before taking any type of supplement.
Medications known as chelating agents will help to dissolve the cystine crystals. These drugs work by chemically combining with the cystine to form a complex that can then dissolve in the urine. Examples include D-Penicillamine and alpha-mercaptopropionylglycine. D-Penicillamine is effective, but it has many side effects.
Pain medications may also be prescribed to control pain while the stones pass through the bladder and out of the body.
If the stones are very large and painful, or block one of the tubes leading from the kidney, they might need to be removed by surgery. There are a few different types of surgeries to break up the stones. These include the following procedures:
- extracorporeal shock wave lithotripsy (ESWL): a procedure that uses shock waves to break up large stones into smaller pieces. However, this procedure is not as effective for cystine stones compared to other types of kidney stones.
- percutaneous nephrostolithotomy (or nephrolithotomy): a procedure that involves passing a special instrument through your skin and into your kidney to take out the stones or break them apart.
Cystinuria is a life-long condition. However, it can be managed effectively with treatment. The stones themselves appear more commonly in young adults under the age of 40, so they may lessen with age (Lin & Zieve, 2011).
Cystinuria does not affect any other parts of the body. The condition rarely causes kidney failure. Frequent stone formation causing blockage and surgical procedures can impact kidney function over time.
Cystinuria cannot be prevented if both parents are carrying a copy of the genetic defect. However, drinking large amounts of water and reducing your salt intake can help to prevent stones from forming in the kidneys.