Cystinuria

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This disorder is caused by a problem in the SLC3A1 gene and/or SLC7A9 gene. These genes control the transport of renal amino acids. Cystinuria literally means ‘cystine in the urine’. Cystine is an amino acid, which is a ‘building block’ of protein in the body. If you suffer from cystinuria, you have a problem with the transport of cystine in the kidney. This leads to a build up of cystine in the urine, which if the levels are high enough can form crystals – these then enlarge and become stones.

Cystinuria is a rare inherited condition affecting about 1 in every 7,000 people. However, its prevalence has high ethnogeographic variation, ranging from 1:2,500 in the Libyan Jewish population to 1:100,000 in Sweden. The exact incidence is unknown but it is thought that 1 in 2000 people in the UK are affected by cystinuria. Men and women are affected equally. Most people present with cystinuria in their 20’s or 30’s, although it can present at any age.

If you are diagnosed with cystinuria, you have an increased risk of forming kidney stones. These stones are usually first noticed between 10 and 30 years old of age, although 1 in 4 people will have stone disease at an earlier age.

Stones in the kidney can cause:

• A dull ache in the loin
• Blood in the urine either visible or detected on urine testing
• Urine infections (burning when passing urine, passing urine frequently, fever)
• Testicular pain (because of pain radiating from the kidney)
• Back ache/ pain

Stone pain is usually referred to as renal colic. Renal colic typically starts as pain in the back and in the side, and may move down to the groin. It comes in waves, building up to severe pain over a few minutes, then easing a little before building up again. Often it causes you to feel sick and it

is not uncommon to vomit. It can build up rapidly or more slowly. In terms of severity, it can vary from a mild ache to ‘the worst pain ever’ which is comparable to childbirth.

Not all kidney stones cause renal colic. Some stones are tiny and pass in the urine without pain. Larger stones tend to block the drainage of urine from the kidney leading to pain. In Cystinuria symptoms vary a lot. Some patients have very few kidney stones in a lifetime and few symptoms, while others get several new stones a year.

In the long-term the kidneys may become damaged. If damage is extensive kidney failure can occur, but this is rare. Tests are needed to find out if Cystinuria has caused kidney damage or reduced kidney function. Usually, an ultrasound scan and a blood test.

Diagnosis is often made after either a stone has passed spontaneously or after a stone has been treated surgically and it is sent for ‘stone analysis’ in the hospital laboratory which confirms it to be a cystine stone. To confirm that cystinuria is the cause of your kidney stones, you may undergo specialized blood and urine tests.

It is important that the diagnosis is made correctly. Kidney stones of any kind are relatively common in the general population, particularly in middle and older age groups. By contrast, Cystinuria is rare. Only 1% of kidney stones in adults are due to Cystinuria. A kidney stone is more likely to be Cystinuria if there is a family history of Cystinuria or if the first stone forms at a young age. Young people with kidney stones and those with a family history of stones need tests to find the cause.

There are different tests that can be done to find kidney stones. Your urologist (a surgeon who specialises in dealing with problems of the kidneys and bladder) will discuss these with you and arrange the best test for you;

• KUB – X-ray
• Ultrasound
• CT
• Intravenous Urogram
• MRI

Cystinuria cannot be cured but it can be managed with little disruption to normal living. The aim of treatment is to keep cystine dissolved in the urine so that it doesn’t form crystals that build up to form stones. Specialised doctors and dietitians will advise you on the best treatment options. There are multiple ways of doing this:

• Keeping the urine dilute – drink extra amounts of water (4-5 litres) each day and sometimes during the night too.
• Reducing the amount of cystine in your urine – You can reduce the amount of cystine in your urine by making some changes to the food you eat. You should avoid salt (sodium), eating less animal protein, eating more vegetable protein, eating more fruit and vegetables
• Making the cystine more easily dissolved (soluble) – You may be given potassium citrate to make the urine more alkaline, which helps with dissolving cystine.
• Chelating medicines – You may be prescribed Tiopronin, D-penicillamine and Captopril.

There are a number of options available to break up (and remove) kidney stones. There are advantages and disadvantages to these and the specifics of these will be discussed with you by your surgeon.

• Lithotripsy (ESWL)
• Ureteroscopy
• Percutaneous nephrolithotomy (PCNL)
• Open surgery

Cystinuria is inherited in different ways. In most patients, Cystinuria is diagnosed if both parents have a copy of the faulty gene. This is called autosomal recessive inheritance. However, for a few, only one copy of the faulty gene is sufficient. This is called autosomal dominant inheritance. A person who has a copy of the faulty gene is known as a carrier.

For those with autosomal recessive inheritance, if both parents are carriers, their child has a one in four (25%) chance of inheriting the disorder, and a one in two chance (50%) of being a carrier. This is the same for each child the parents have.

If the child only inherits one copy of the faulty gene, they will be a carrier but will not have the disorder. In some rare cases, carriers have had mild symptoms of the disorder for which they carry the faulty gene.

Once you are diagnosed, you can speak to a genetic counsellor. They can explain how you may have inherited Cystinuria. They can also tell you about genetic testing for the rest of your family. They can provide advice and support if you go on to have children of your own.

References are available on request. Please contact us by phoning 0845 241 2173 or emailing contact@metabolicsupportuk.org [Resource Library No: AAP002].