A kidney stone is a hard, crystalline mineral material formed within the kidney or urinary tract. Kidney stones are a common cause of blood in the urine (hematuria) and often severe pain in the abdomen, flank, or groin. Kidney stones are sometimes called renal calculi.
The condition of having kidney stones is termed nephrolithiasis. Having stones at any location in the urinary tract is referred to asurolithiasis and the term ureterolithiasis is used to refer to stones located in the ureters.
Who is at risk for kidney stones?
Anyone may develop a kidney stone, but people with certain diseases and conditions (see below) or those who are taking certain medications are more susceptible to their development. It is estimated that one out of every 10 people in the U.S. will develop stones in the urinary tract at some point in their lives. Most urinary stones develop in people 20-49 years of age, and those who are prone to multiple attacks of kidney stones usually develop their first stones during the second or third decade of life.
In residents of industrialized countries, kidney stones are more common than stones in the bladder. The opposite is true for residents of developing areas of the world, where bladder stones are the most common. This difference is believed to be related to dietary factors. Urinary tract stones are about three times more common in males than in females.
The prevalence of kidney stones begins to rise when men reach their 40s, and it continues to climb into their 70s. A Caucasian male has a one in eight chance of developing a kidney stone by age 70. People who have already had more than one kidney stone are prone to developing further stones.
A family history of kidney stones is also a risk factor for developing kidney stones. Kidney stones are more common in Asians and Caucasians than in Native Americans, Africans, or African Americans.
Uric acid kidney stones are more common in people with chronically elevated uric acid levels in their blood.
A small number of pregnant women (about one out of every 1,500-3,000 pregnancies) develop kidney stones, and there is some evidence that pregnancy-related changes may increase the risk of stone formation.
Factors that may contribute to stone formation during pregnancy include a slowing of the passage of urine due to increased progesterone levels and diminished fluid intake due to a decreasing bladder capacity from the enlarging uterus.
Healthy pregnant women also have a mild increase in their urinary calcium excretion. However, it remains unclear whether the changes of pregnancy are directly responsible for kidney stone formation or if these women have another underlying factor that predisposes them to kidney stone formation.
When a tubular structure is blocked in the body, waves of pain occur as the body tries to unblock the obstruction. These waves of pain are called colic. This is opposed to non-colicky type pain, like that associated with appendicitisor pancreatitis, in which movement causes increased pain and the patient tries to hold very still.
- Renal colic (renal is the medical term for things related to the kidney) has a classic presentation when a kidney stone is being passed.
- The pain is intense and comes on suddenly. It may wax and wane, but there is usually a significant underlying ache between the acute spasms of pain.
- It is usually located in the flank or the side of the mid back and may radiate to the groin. Males may complain of pain in the testicle or scrotum.
- The patient cannot find a comfortable position and often writhes or paces with pain.
- Sweating, nausea and vomiting are common.
- Blood may or may not be visible in the urine because the stone has irritated the kidney or ureter. Blood in the urine (hematuria), however, does not always mean a person has a kidney stone. There may be other reasons for the blood, including kidney and bladder infections, trauma, or tumors.
Urinalysis with amicroscope may detect blood even if it is not appreciated by the naked eye. Sometimes, if the stone causes complete obstruction, no blood may be found in the urine because it cannot get past the stone.
There is no consensus as to why kidney stones form.
- Heredity: Some people are more susceptible to forming kidney stones, and heredity may play a role. The majority of kidney stones are made of calcium, and hypercalciuria (high levels of calcium in the urine) is a risk factor. The predisposition to high levels of calcium in the urine may be passed on from generation to generation. Some rare hereditary diseases also predispose some people to form kidney stones. Examples include people with renal tubular acidosis and people with problems metabolizing a variety of chemicals including cystine (an amino acid), oxalate, (a type of salt), and uric acid (as in gout).
- Geographical location: There may be a geographic predisposition, and where a person lives may predispose them to form kidney stones. There are regional "stone belts", with people living in the southern United States having an increased risk of stone formation. The hot climate in this region combined with poor fluid intake may cause people to be relatively dehydrated, with their urine becoming more concentrated and allowing chemicals to come in closer contact to form the nidus, or beginning, of a stone.
- Diet: Diet may or may not be an issue. If a person is susceptible to forming stones, then foods high in calcium may increase the risk; however, if a person isn't susceptible to forming stones, diet probably will not change that risk.
- Medications: People taking diuretics (or "water pills") and those who consume excess calcium-containing antacids can increase the amount of calcium in their urine and potentially increase their risk of forming stones. Taking excess amounts of vitamins A and D are also associated with higher levels of calcium in the urine. Patients with HIV who take the medication indinavir (Crixivan) may form indinavir stones. Other commonly prescribed medications associated with stone formation include dilantin and antibiotics like ceftriaxone (Rocephin) and ciprofloxacin (Cipro).
- Underlying illnesses: Some chronic illnesses are associated with kidney stone formation, including cystic fibrosis, renal tubular acidosis, and inflammatory bowel disease.