Kidney Stones Explained: Causes, Symptoms, And Treatments

Kidney stones are a medical condition that may not be on your radar, but they probably should be. These painful stones, also known as renal calculi (via Mayo Clinic), are more common than you might think. According to the National Kidney Foundation, about 11% of men and 9% of women will get at least one kidney stone at some point during their lifetime. Kidney stones can strike at any time, but they usually form for the first time between age 20 and 40 (via Intermountain Healthcare). And the bad news is that once you've had one kidney stone, you're at much greater risk of having additional stones. Kidney stones are also becoming more common. The Children's Hospital of Philadelphia noted that between 1997 and 2012, the risk of kidney stones among children doubled, while the lifetime risk for women increased by 45%. For African Americans, rates rose by approximately 15%.

Passing a kidney stone can be excruciating, and individuals who've gone through the ordeal often say it's the worst pain they've ever experienced. The pain is often compared to childbirth, and on subjective rating scales of pain, passing a stone is ranked on par with, or even more intense than, labor and delivery (via Keck Medicine of USC). Fortunately, there are many things you can do to make passing a stone easier, and there are surgical options for stone removal and strategies to prevent stones from forming in the first place.

How kidney stones form

The first step to dealing with (or, better yet, preventing) kidney stones is understanding how and why they develop. To do this, you need to get acquainted with the urinary system, which consists of the kidneys, ureters, bladder, and urethra. As Johns Hopkins Medicine explains, "the kidney and urinary systems help the body to eliminate liquid waste called urea, and to keep chemicals, such as potassium and sodium, and water in balance." As blood flows into the kidneys, waste products and excess water are filtered out, creating urine. Each kidney is connected to the bladder via a narrow tube called a ureter. Muscle contractions in the ureters move urine down into the bladder and prevent it from traveling back up into the kidneys. Once the bladder is full enough, it contracts to pass urine out of the body through the urethra.

Stones form when there's either too much of a particular mineral or not enough water in the kidneys, or both (via Intermountain Healthcare). The substance can't dissolve properly and instead forms a crystal. These crystals clump together and create a stone. The stone leaves the kidney and travels into the ureter, but because ureters are only about an eighth of an inch in diameter, stones often get stuck. This can cause urine to backflow into the kidneys and trigger muscle spasms in the ureter, leading to the intense pressure and pain that are the hallmarks of a kidney stone.

Calcium stones

A number of substances can form stones in the kidneys, and doctors classify kidney stones based on what they're made of. Calcium stones are the most common. According to a 2006 paper published in the Canadian Medical Association Journal, "about 80% of stones are calcium based, and about 80% of those are calcium oxalate stones." Oxalate (oxalic acid) is an organic compound found in many plant foods (via Healthline). Oxalate is usually broken down by our friendly gut bacteria, but undigested oxalate can bind to minerals — usually calcium — to create a salt. Normally these salts dissolve when they mix with water in the kidneys, but if there's not enough liquid or too much calcium oxalate, the salts can clump together and become stones.

The other 20% of calcium-based stones are made up of calcium phosphate. It's much less common for calcium phosphate salts to crystalize into stones, and there are often other serious medical issues driving the formation of these stones (via Harvard Medical School). These include hyperparathyroidism (a condition in which the body produces too much parathyroid hormone, which regulates circulating levels of calcium and phosphorous), renal tubular acidosis (a kidney condition that causes a buildup of acid in the body), and urinary tract infections (UTIs).

Uric acid stones

Uric acid is a byproduct of your digestive system breaking down substances called purines (via the National Kidney Foundation). Purines are found in varying amounts within a number of foods, but are most prevalent in meat (particularly organ meats). Uric acid stones can develop if there's either too much uric acid in the urine, or the urine has too low of a pH. Uric acid levels can be too high if an individual's diet is too high in purines, or if they have an inherited condition that affects the way their body processes purines. Alternatively, if a person's urine is too acidic, it can cause even normal levels of uric acid to form crystals because a low urine pH prevents the uric acid from dissolving normally. About 5–10% of kidney stones in the United States are uric acid stones (via UpToDate). In areas with a hot, arid climate, however, they can account for as much as 40% of kidney stones because individuals living in these areas are more likely to have low urine volume and more acidic urine.

Uric acid stones are often a complication of a condition called gout. According to the Mayo Clinic, gout is a type of arthritis marked by sudden and severe pain in a particular joint, often the big toe. Gout is caused by needlelike urate crystals that form in joints or the surrounding tissue. When these urate crystals accumulate in the kidneys, it leads to uric acid stones.

Struvite stones

About 10–15% of kidney stones are struvite stones, composed of magnesium ammonium phosphate (via WebMD). These stones are also known as "infection stones" because they occur when bacteria such as Proteus or Klebsiella invade the upper urinary system. The bacteria feed off of the urea in urine and convert it to ammonia. The ammonia alkalizes (raises the pH) of the urine, which in turn causes struvite stones to form. The ammonia produced by the bacteria can also mix with calcium stones to create "mixed" stones. Because women have shorter urethras and are more prone to UTIs, they're also more likely than men to get struvite stones.

Because the bacteria that cause upper UTIs can multiply rapidly, struvite stones can also grow very quickly and become quite large and painful. They may even cause potentially fatal kidney damage because of their size and number. In addition to the usual symptoms associated with kidney stones discussed below, those with struvite stones will usually also experience symptoms of a UTI, such as frequent, burning urination and fever. If you have an upper UTI, in addition to antibiotics, your doctor may also prescribe medications to keep the urine acidic and prevent struvite stones from forming.

Cystine stones

The fourth and final type of kidney stone is known as a cystine stone. Unfortunately, because of their underlying cause, cystine stones are usually a recurrent, lifelong problem (via the National Kidney Foundation). They can also grow extremely large and thus are usually unable to pass on their own. In fact, some cystine stones may grow as large as a golf ball.

Cystine stones are caused by an inherited genetic condition called cystinuria. In cystinuria, gene mutations affect how the body metabolizes certain amino acids (via the National Organization for Rare Disorders). The disorder causes excessively large amounts of the amino acids cystine, lysine, arginine, and ornithine to leak into the urine. While the latter three dissolve easily in urine and don't cause any issues, cystine doesn't dissolve as readily and may form stones. Individuals must have a copy of the mutated genes from each parent in order to have the condition; those with only one copy are carriers but usually don't have any symptoms (via MedlinePlus). Cystine stones are the rarest type of kidney stone. They account for approximately 3% of kidney stones, and cystinuria affects only about 1 in 7,000 people.

Who's at risk for kidney stones?

Kidney stones can happen to anyone, but some people are more likely than others to get them. In general, men are more likely than women to get stones, and they're more common in non-Hispanic Caucasians than other ethnicities (via the American Kidney Fund). Certain genetic conditions, such as cystinuria or disorders affecting how uric acid is processed, can also increase a person's risk for certain types of stones. Obesity, gastric bypass surgery, and certain medications (including diuretics and calcium-based antacids) also up your risk. Some medical conditions, including polycystic kidney disease and conditions such as Crohn's that cause chronic inflammation in the bowel, may also predispose you to stones. Lifestyle factors that can increase risk include not drinking enough fluids and eating a diet that's high in protein, sodium, or sugar.

When it comes to calcium-based stones, having too much calcium in your blood (hypercalcemia) is a major risk factor. Normally, the body keeps very tight control over the amount of calcium circulating in the blood, but hypercalcemia can occur for a number of reasons (via the Mayo Clinic). These include an overactive parathyroid gland, certain types of cancer, tuberculosis, use of lithium for bipolar disorder, and a rare genetic condition known as familial hypocalciuric hypercalcemia.

Unfortunately, the biggest risk factor for getting a kidney stone is having had one in the past. According to Intermountain Healthcare, individuals have a 10–15% chance of getting a second kidney stone within one year of their first, a 35–40% chance by the five-year mark, and a 50% chance within a decade of their first stone.

Symptoms of a kidney stone

The main symptom of a kidney stone is pain (via Intermountain Healthcare). The good news is that small stones may not be painful at all, or may cause only mild discomfort. Even large stones are usually painless for much of their existence. Stones don't typically cause pain as they form in the kidney unless they're extremely large. It's when the stone must pass through the very narrow ureter that the pain level spikes. Most stones don't cause any pain once they reach the bladder and, since the urethra is at least double the diameter of the ureter, they usually exit the body without any trouble. Where the stone is on its journey through your urinary system will affect the type of pain you feel (via American Family Physician). If you have any pain at all while it's in your kidney, you're most likely to feel a vague pain in your side. When it enters the ureter, the pain heightens and is usually felt in the side and upper abdomen. Once it reaches the middle and lower portions of the ureter, the pain is most noticeable in the side and the front of the abdomen. Pain can also radiate to the back or groin.

Because they interfere with the normal flow of urine, kidney stones can also cause symptoms when you go to the bathroom (via the Mayo Clinic). Urine may be tinged pink, red, or brown, or may be cloudy or foul-smelling. A feeling of constantly having to go, painful urination, urinating more often than usual, and urinating in small amounts are also symptoms. The pain the stones cause can lead to nausea and vomiting. In the case of struvite stones, the underlying UTI may cause a fever and chills.

Complications of kidney stones

In addition to the severe pain they cause, untreated kidney stones may lead to complications such as kidney infections and loss of kidney function (via the National Institute of Diabetes and Digestive and Kidney Diseases). Complications are rare, though, if a person seeks help from a healthcare professional. Having multiple kidney stones also increases your risk for chronic kidney disease (CKD). In fact, a 2020 study published in BMC Nephrology found that those who'd had kidney stones were 1.82 times more likely to get CKD than those who'd never had a stone.

CKD is a progressive condition in which the kidneys gradually lose their ability to adequately filter waste from the blood (via Nephcure). Many people don't realize they have kidney issues until the damage is moderate or severe. Once a person's kidneys are severely compromised, they require dialysis (a procedure in which blood is passed through a machine that removes waste products) or a kidney transplant. It's important to note that even though kidney stones increase your risk for CKD, that doesn't mean they're the most common cause of CKD. Diabetes (both type 1 and type 2), high blood pressure, polycystic kidney disease, and conditions that produce chronic inflammation in the kidneys are more frequently the root of CKD (via the Mayo Clinic).

Diagnosing kidney stones

If you go to the doctor because you suspect you have a kidney stone, the diagnosis process usually begins with a discussion of your personal or family history with kidney stones (via American Family Physician). The doctor will also conduct a physical exam and ask for a urine sample. The pH of the urine, the levels of various minerals, and the presence of blood or crystals are all important clues to diagnosing a kidney stone. There are many other conditions that can cause abdominal pain and changes in urine, so it's important that the doctor rules out other potential issues. Ordering imaging to get a better look at what's going on in your urinary system is the best way to determine if stones are present or if there are other factors at play.

But not all imaging options are created equal. Ultrasound and x-rays are generally inexpensive and readily available, but they don't provide good images of certain types of stones in some parts of the urinary system, particularly the ureters (where stones are most likely to cause pain that motivates someone to go to the doctor in the first place). An x-ray with contrast dye is much more useful for imaging kidney stones. Although much more expensive, CT scans offer the most detailed images to help doctors visualize the number, size, and location of kidney stones.

Passing a kidney stone

Even though it can be painful, passing a stone naturally is usually preferred if it's feasible, since surgeries and other interventions to remove stones carry their own risks (via Intermountain Healthcare). But whether or not a stone can pass, and how long that process takes, is highly dependent on its size. The average ureter is 3-4 mm wide, so stones that are less than 4 mm in diameter have about an 80% chance of passing naturally. This takes an average of 31 days. Stones between 4 and 6 mm have roughly a 60% chance of passing, since the ureter can stretch slightly to accommodate them. Stones in this size range take an average of 45 days to pass. Stones that are larger than 6 mm have only a 20% chance of passing, and the journey takes an average of 12 months — not exactly the most enjoyable way to spend a year.

Luckily, there are things you can do to make passing a kidney stone less painful. Drinking lots of water (as much as three liters a day) will help move the stone along and prevent the stone from getting larger, as well as prevent additional stones from forming (via the Mayo Clinic). Over-the-counter medications such as ibuprofen or naproxen can ease pain. You may also be prescribed an alpha blocker, a type of medication that relaxes muscles, including those in the ureters. When the muscles are relaxed, the stone can pass through faster.

Using shock waves to break up a kidney stone

For stones that are too large to pass on their own, extracorporeal shock wave lithotripsy (ESWL) is the most commonly used treatment (via the National Kidney Fund). In ESWL, the stone is bombarded with 1,000–2,000 shock waves, which break up the stone into very small fragments that can then be easily passed. The procedure takes 45–60 minutes and patients go home the same day and can resume normal activities within a day or two.

There are advantages and disadvantages to ESWL. On the plus side, no incision is needed and the procedure is relatively quick. On the other hand, only about 50% of those who get ESWL are stone-free a month after treatment, so oftentimes a more invasive procedure is required. The shock waves needed to break up the stone are quite painful, and either sedation or local or general anesthesia is still needed, which carries some risks. Common side effects after ESWL include blood in your urine and abdominal pain or cramping. In rare cases, ESWL may cause severe bleeding or kidney damage.

ESWL isn't a good fit for everyone. Stones that are larger than 20 mm are usually too big to be broken up with shock waves. Those who are pregnant, morbidly obese, or have a bleeding disorder, infection, or other kidney abnormalities usually will require a different treatment.

Surgical removal of kidney stones

If a stone can't pass on its own and ESWL either isn't a viable option or wasn't effective, the stone will need to be surgically removed through one of several options (via WebMD). All require general anesthesia. Often, a temporary stent (tube) will be placed in the ureter after the procedure to ensure urine can drain from the kidney into the bladder. Because these procedures are more invasive, they carry a greater chance of risk. Infections, excessive bleeding, narrowing of the ureter, and damage to the kidney, ureter, bladder, bowel, or liver are all possible (though rare) complications of kidney stone surgery.

In ureteroscopy, the doctor passes a thin, flexible scope up through the urethra, bladder, and into the ureter. Small stones can be gathered in a basket attachment at the end of the scope. For larger stones, a laser is passed through the tube and used to break up the stones so that they can be collected. Most people can go home the same day as their ureteroscopy. Percutaneous nephrolithotomy also uses a small tube-like scope to reach the stone, but it's moved into place through a small incision in your back or abdomen. Shock waves can also be delivered through the scope to break up large stones in a procedure called percutaneous nephrolithotripsy. The stone or stone fragments are suctioned out through the tube. With or without the additional use of shockwaves, the procedure lasts only about 20–45 minutes, but patients must stay in the hospital for a day or two afterward. Today, open surgery, which entails a much larger cut into the abdomen, is rarely used to remove kidney stones. It carries the greatest risks and the longest recovery time.

Changing your diet to prevent kidney stones

The good news is that there are a number of things you can do to prevent kidney stones from forming, even if you've had them in the past (via the National Kidney Foundation). Many of these strategies center around dietary changes. Regardless of stone type, the most important thing you can do to prevent kidney stones is drink plenty of water. Aim for 2–3 quarts daily. Eating a diet that contains moderate amounts of protein is best, since high levels of protein increase the likelihood of calcium stones as well as uric acid and cystine stones in individuals with certain inherited conditions. A low-sodium diet reduces the amount of calcium in the urine and promotes proper fluid balance. While you might expect that individuals who get calcium stones would be advised to steer clear of calcium-rich foods, that's not the case. It's important to get plenty of dietary calcium, although calcium supplements and high-dose vitamin C supplements can increase the risk of calcium stones.

People who get calcium oxalate stones (the predominant type of stone) may need to follow a low-oxalate diet, although some experts now question whether this is actually necessary to prevent kidney stones (via Healthline). Those on a low-oxalate diet should aim to keep their oxalate intake to no more than 40 or 50 mg daily. Foods extremely high in oxalates that must be avoided on a low-oxalate diet include kiwis, oranges, spinach, potatoes, kidney beans, almonds, walnuts, brown rice, and soy products.

Medications to prevent kidney stones

In some cases, over-the-counter or prescription medications may help prevent kidney stones. As guidelines published by Informed Health in 2016 explained, medications are usually used for those at greatest risk for recurrent kidney stones. This includes people who got their first kidney stones in childhood or adolescence, those who have a strong family history of stones, those with certain underlying kidney conditions, and individuals with an overactive parathyroid gland. Since uric acid, struvite, and cystine stones are more likely than calcium stones to reoccur, people who've had one of these types of stones may be prescribed medications to prevent future attacks.

There are a few possible medications used to stop kidney stones from forming. Citrate salts bind to calcium in urine and prevent it from forming crystals or stones. Citrate salts can also prevent uric acid and cystine stones. They're available without a prescription and come in many forms, including capsules, powders, and dissolvable tablets. Thiazide diuretics reduce how much calcium enters the urine from the bloodstream and promote the production of large amounts of urine that can dissolve substances before they form stones. Allopurinol inhibits the breakdown of purines into uric acid, minimizing the chances of uric acid stones.