Kidney stones are abnormal, hard, chemical deposits that form inside the kidneys. This condition also is called nephrolithiasis or urolithiasis.
Kidney stones are often as small as grains of sand. They pass out of the body in urine without causing discomfort.
However, the deposits can be much larger—the size of a pea, a marble or even larger. Some of these larger stones are too big to be flushed from the kidney.
Some kidney stones manage to travel into the ureter. This is the narrow tube between the kidney and bladder. The stones may become trapped in the ureter. Trapped kidney stones can cause many different symptoms. These include:
Blocked urine flow
Bleeding from the walls of the urinary tract
There are several different types of stones. They form for a variety of reasons. Kidney stones are grouped into four different families, based on their chemical composition:
Calcium oxalate stones — These stones account for most kidney stones. Several factors increase the risk of calcium oxalate stone formation in the kidney:
Low urinary volume
High concentrations of calcium in the urine
High concentrations of oxalate in the urine
Low amounts of citrate in the urine (citrate acts to inhibit stone formation)
Medical conditions that increase the risk of calcium oxalate kidney stones include:
Extra parathyroid hormone (hyperparathyroidism)
High uric acid levels in the blood (as in people who have gout)
Surgery for obesity
Struvite stones — These stones are made of magnesium and ammonia (a waste product). They are related to urinary tract infections caused by certain bacteria. Struvite stones are less common now that urinary tract infections are better recognized and treated. Struvite stones are more common in women than in men. They develop frequently in people who have long-term bladder catheters.
Uric acid stones — Uric acid stones form because of an abnormally high concentration of uric acid in the urine. They are more likely to occur in people who have gout because of an over production of uric acid. Gout is a disorder in which uric acid builds up in the blood and gets deposited in joints.
Cystine stones — These rare stones are the least common type of kidney stones. They are composed of the amino acid cystine. Cystine is a building block of proteins. Cystine stones are caused by an inherited defect.