Introduction

Photo of a young woman holding her head (PantherMedia / Tristan Ben Mahjoub)

Kidney stones are small, hard deposits that can form in a part of the kidney called the renal pelvis. Kidney stones that enter a ureter are sometimes referred to as ureteral stones. The ureters are the tubes that carry urine from the kidneys to the bladder.

Many of these stones are so small that they are able to travel to the bladder in just a few days or weeks without any treatment, and then exit the body in your urine. So if the stones are smaller, it's often enough to take painkillers, drink plenty of fluids and simply wait for the kidney stones to pass through. Muscle-relaxing medications can be used to help pass medium-sized stones.

Larger stones may get stuck as they exit the renal pelvis or take longer to move through the ureter, causing severe pain and other symptoms. Then they usually need to be broken up by sound waves or surgically removed. The most suitable treatment will depend on the size, the type and the position of the stones in the kidney or the urinary tract.

It's common to have kidney stones multiple times. Finding the cause is a necessary part of preventing that from happening.

Symptoms

Many people have kidney stones that go unnoticed and are discovered only by chance. Some might feel slight discomfort near their kidneys, but not attribute it to a kidney stone.

Kidney stones often go unnoticed until they block the renal pelvis or travel through the ureter. The main symptom is pain, which can range from feeling just a little unwell to waves of severe pain. The pain may affect your lower abdomen, belly or back "flank pain," depending on which part of the ureter the stone is in.

The pain is especially severe when the kidney stone travels through a narrower passage of the ureter, for example when it enters the ureter from the renal pelvis or when it enters the bladder. Sudden episodes of severe pain on one side radiating through your lower abdomen are common. This is sometimes referred to as renal colic. The intensity of pain gets stronger and weaker in waves, sometimes accompanied by nausea and vomiting. Some people also toss and turn to find a position that helps to relieve the pain. A episode of renal colic usually lasts from 20 to 60 minutes.

 

Illustration: Where painful kidney stones commonly occurWhere painful kidney stones commonly occur

Other possible symptoms of a kidney stone passing through the ureter include blood in your urine, painful urination, and a stronger or more frequent urge to urinate. The pain can sometimes spread to your genitals as well.

Small kidney stones don't always cause symptoms. They're sometimes first noticed when they exit the body in urine. Or they may be detected by chance in x-rays or ultrasound scans of the belly region.

Causes

Kidney stones are made up of the dissolved salts found in our urine. The urine becomes supersaturated when there are too many of these salts. If that happens, they start to form crystals, which can develop into stones over time.

These three types of kidney stones are the most common:

  • Calcium stones: About 80% of kidney stones are made up of calcium salts – calcium oxalate or calcium phosphate, specifically.
  • Uric acid stones: Uric acid is the cause of about 5 to 10% of all kidney stones.
  • Struvite stones: About 10% of kidney stones are made up of the mineral struvite, which contains mostly magnesium and phosphate.

Calcium stones usually result from high levels of calcium in your urine. That may happen if your body takes in too much calcium from the foods you eat or if excess calcium from your blood is released through the kidneys into your urine. High levels of calcium in the blood may result from overactive parathyroid glands.

Calcium stones might also be caused by high concentrations of oxalic acid in the urine (hyperoxaluria). Oxalic acid is present in many plant-based foods. Hyperoxaluria may be caused by certain medical conditions or be related to your diet. Often it's not possible to find an exact cause.

Some conditions that affect the levels of uric acid in your blood may increase your risk of developing uric acid stones, including the following:

  • Gout: Gout is a metabolic disorder in which the level of uric acid in the blood is too high. Some of the uric acid is then released into the urine.
  • Diabetes: The exact cause isn't known, but people with diabetes possibly have more acidic urine.
  • Chronic diarrhea: Diarrhea causes dehydration, which makes your urine more acidic.

pH is used to measure acidity. The lower the pH, the more acidic your urine is. A lower pH level increases your risk of developing most of the different types of kidney stones.

This isn't the case with struvite stones, though, which arise when the pH level of urine is too high – that is, when it's less acidic. The cause might be a urinary tract infection such as cystitis or a renal pelvis infection.

Other substances are only rarely the cause of kidney stones. Genetically-related excess levels of the amino acid cystine may cause the formation of cystine stones, for example. Some stones contain a mixture of different crystal types.

Risk factors

Because kidney stones have such a variety of causes, there are also many different risk factors:

  • Dehydration: Kidney stones are more likely to form if there is less water in the urine. Then the water isn't able to dissolve as many mineral salts. So not drinking enough fluids or sweating a lot can increase your risk of developing kidney stones.
  • Unusual anatomical structures in the kidney: Renal cysts or a horseshoe kidney can make kidney stones more likely. It is called a "horseshoe kidney" when the lower ends of both kidneys are attached.
  • Family history: Some people have a family history of kidney stones. So genes are likely to play a role in those cases, but the link may also come from having a similar lifestyle or other things in common.
  • Citrate deficiency: Some substances in our urine act to keep crystals from forming, the main one being citrate (a citric acid salt). So it's easier for kidney stones to form if you have a citrate deficiency. A citrate deficiency might result from chronic diarrhea or a lack of potassium. But often, no particular cause is found.
  • Diet: A high-salt diet appears to increase your risk of developing calcium stones: The presence of salt causes more calcium and less citrate to dissolve in your urine. Other aspects related to diet also play a role in kidney stone formation. But many people who have calcium stones don't find a specific cause.

Some medications may increase the likelihood of kidney stones developing. For example, the HIV drug indinavir can cause the formation of crystals in your urine, which may develop into stones. Other drugs can alter the composition of the urine, and create the right conditions for kidney stones to develop. These include the drug topiramate, which is used to treat epilepsy and migraine. But generally speaking, the use of medication is among the less common causes of kidney stones.

Prevalence

Kidney stones are very common: In Germany, 1 to 2% of the population has them each year. Estimates for the U.S. show that 7% of women and 13% of men are diagnosed with kidney stones at least once during their life. Kidney stones affect people of all ages, including children, but are most common in people between the ages of 20 and 40.

Outlook

Kidney stones usually only cause symptoms once they've entered the ureter. The symptoms depend mostly on how large the kidney stones are: Kidney stones less than 5 millimeters in diameter travel to the bladder without any problems and are flushed out of the body in the urine 70% of the time. The same is true for about 50% of stones between 5 and 10 millimeters in diameter. Kidney stones that are larger than 10 millimeters usually need to be treated.

The time it takes for a kidney stone to pass varies quite a bit as well. Small kidney stones are usually passed with the urine after one or two weeks. Kidney stones that are not flushed out on their own within four weeks are usually treated.

About 30 to 50% of people treated for a kidney stone have one again within five years, and some get them even more often, so prevention plays an important role.

Effects

Left untreated, kidney stones can narrow or block the ureters, increasing the risk of infection and temporarily putting added strain on the kidneys. These problems are rare because most kidney stones are treated before they can cause complications. Signs of an infection of the upper urinary tract include fever, chills, side and low back pain, and nausea or vomiting.

Diagnosis

The usual symptoms of kidney stones can provide a basis for a diagnosis, but they aren't always enough to tell what the exact cause is. Ultrasound scans can be used to detect most kidney stones. If doctors still aren't sure, they may also use computed tomography (CT) scans.

A simple x-ray of the kidneys, ureters and bladder is less exact because some kidney stones don't show up. Calcium stones are easy to detect, but struvite stones are harder to see and uric acid stones don't appear at all. But x-ray images can help to determine how successful treatment for calcium stones was.

Blood and urine tests are also important. These tests can help provide evidence of the underlying cause by showing whether you have an infection or whether your levels of calcium or uric acid are too high.

Prevention

When kidney stones are treated without doing anything about the cause, they often come back. The best thing to do will depend on what type of kidney stones you have. It can help to keep track of the kidney stones that you pass in your urine. One way to do this is to use a filter or sieve to catch them when you urinate. Kidney stones that have been surgically removed are analyzed at a laboratory.

Experts generally recommend drinking enough fluids to produce two liters (about 8.5 cups) of urine per day. Depending on the cause, you might also be advised to eat a diet low in oxalate, salt, meat or protein. Medication that can affect the pH level of your urine or lower the levels of calcium or uric acid may also be an option. The best ways to prevent kidney stones will also depend on your individual risk of getting them again.

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Treatment

If you have smaller kidney stones that aren't painful, all you need to do is wait to pass them when you urinate. It's possible to tell by their size and location whether they can pass on their own. You can use painkillers such as diclofenac, ibuprofen or metamizole to relieve any pain. They can be combined with stronger drugs (opioids) for more severe pain.

Certain muscle-relaxing medications (alpha blockers) can make it easier to pass kidney stones that are 5 to 10 millimeters in diameter. Larger kidney stones usually need to be removed. Depending on how large the kidney stones are and where they're located, sound waves can be used to destroy them or they can be removed using endoscopy or surgery.

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