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Cystinuria is a genetic disorder in which your kidneys, organs of the urinary system, excrete an excessive amount of cysteine, an amino acid, meaning one of the building blocks of proteins. Cysteine stands out among the amino acids in that each cysteine unit can connect with another cysteine unit that is not next to it in the chain that amino acids form in a protein. The connections are made by way of sulfur atoms, so they are called disulfide bridges, which enable proteins to hold certain three-dimensional shapes that allow them to function. Some types of cystinuria also involve excessive excretion of other amino acids in addition to cysteine. In cystinuria, cysteine accumulates in the urinary system and crystalizes into cystine stones (also called calculi) in the kidneys, ureters, and bladder. The word ‘cystine’ refers to the connection of two cysteine units linked together in a disulfide bridge. Cystinuria is present in approximately 1 out of 10,000 people and is passed down with what geneticists call autosomal recessive inheritance. This means that it occurs roughly equally in males and females and that one needs to receive a cystinuria gene from each parent. It also means that, if you are born with cystinuria, you still have the condition when you are pregnant. In other words, cystinuria can coexist with pregnancy.
Typically, diagnosis is made after you experience an episode of urinary stones in which you pass a stone, or in which a stone is found in urine that is collected over time. In either case, diagnosis is made when the stone is found to be a cystine stone. Diagnosis also can involve genetic testing for mutations of the genes SLC31A and SLC7A9. Additionally, your kidneys, ureters, and bladder may be imaged with ultrasonography, or with forms of magnetic resonance imaging (MRI) that image the urinary system. In some cases, computerized tomography (CT) scanning may be needed for a closer look, even though this exposes you and the fetus to some amount of ionizing radiation.
When cystinuria leads to calculi, stones, getting stuck within the urinary tract, this results in pain in the flank, upper abdomen, or the back. Typically, the pain advances to the lower abdomen or the groin and you also may suffer from urinary urgency/frequency, nausea and vomiting, and blood in the urine. Often, kidney stones will eventually pass through into the urine and the situation will resolve on its own. Otherwise, you will need treatment to break up the stone. If a stone does not pass and the condition is not treated, there is a possibility of severe complications, including an upper urinary tract infection that can damage the kidneys, an abscess (a concentrated big infection), sepsis (infection throughout the body), and various types of rupture and leaking of parts of the urinary system. Serious complications that arise from kidney stones may put you at increased risk for certain pregnancy complications. Such complications include preeclampsia and gestational diabetes mellitus, and may increase the chances that you’ll need a caesarean delivery.
As for the baby, some studies have suggested associations between urinary stones (most of which are not cystine stones, but stones made of something else) and pregnancy complications that can have a negative impact on the baby. These complications include preeclampsia, gestational diabetes, premature rupture of membranes (water breaks too early), low birth weight, and spontaneous abortion (miscarriage), but the findings are not consistent between different studies. However, if you develop a serious complication, such as sepsis, the baby is in danger, because your life is in danger.
Since cystinuria inherited, you may be wondering about your baby’s chances of having this condition. As noted above, this condition is inherited with an autosomal recessive pattern. This means that, if you have cystinuria, but the baby’s father does not have cystinuria and is not a carrier for it, then the baby will not have the condition, but will be a carrier. If both you and the father have cystinuria, then the child will have cystinuria. If you have cystinuria and the father is a carrier but doesn’t have the condition, then the baby has 50 percent chance of having the condition and a 50 percent chance of being a carrier without having the condition.
As for treatment, cystinuria can be treated with medications that make your urine more alkaline (less acidic), such as acetazolamide and potassium citrate. With this strategy, you must also limit your dietary intake of salt. Although there has been some concern that acetazolamide might present a risk for the fetus, studies have not found much evidence supporting this concern, while potassium citrate is thought to be fairly pregnancy-safe. Outside of pregnancy, cystinuria also can be treated with a drug called D-penicillamine, which chemically alters cysteine so that it can dissolve better in urine, but this drug is thought to pose a risk of birth defects, so it’s not an option while you’re pregnant. As for breastfeeding, acetazolamide is generally considered safe in nursing mothers, but it is recommended that you avoid breastfeeding, if you are taking D-penicillamine.
It is important for you to drink plenty of fluids when pregnant, and especially if you suffer from cystinuria. If you do form a cystine stone and it does not pass, there are procedures available, such as breakup and removal of a stone through an instrument called a ureteroscope into the bladder and kidney to remove or break up a stone. The standard procedure for treatment of stones in non-pregnant women, extracorporeal shockwave lithotripsy (ESWL), is contraindicated in pregnancy. However, another procedure, called laser lithotripsy, appears to be safe an effective in pregnancy, particularly for stones located in the ureters.