What is a kidney stone?
A kidney stone is a small stone, usually made up of calcium crystals, that forms inside the part of the kidney where urine collects. The stone usually causes little problem until it falls into the ureter, the tube that drains the kidney into the bladder, and causes an obstruction, preventing urine from draining out of the kidney and often causing severe pain.
One of the roles of the kidney is to remove waste from the body by filtering blood and making urine. That urine flows from the kidney into the bladder through the ureter, a thin tube that connects the two. The bladder empties through the urethra, a tube much wider than the ureter.
A variety of minerals and chemicals are excreted in the urine and sometimes these combine to form the beginning of a stone. Over time, this can grow from an invisible speck of sand into a stone that can be an inch in diameter or larger.
There are different terms for kidney stones depending upon where they are located within the urinary tract:
- Urolith: A stone anywhere within the urinary tract
- Nephrolith: A stone within the kidney
- Ureterolith: A stone within the ureter
- Cystolith: A stone within the bladder
What are the different types of kidney stones?
Calcium oxalate is by far the most common type of kidney stone. Less frequently, uric acid can cause stones. (This is the same chemical compound that is the cause of gout.) Struvite, xanthine, and other types of stones are rare. Some stones are combinations of these minerals and chemicals.
What are the signs and symptoms of kidney stones?
A kidney stone does not usually cause symptoms when it remains in the kidney. There, they can sometimes become infected leading to serious kidney infection called pyelonephritis. When the kidney stone passes from the urine collecting system within the kidney into the ureter, it can act like a dam, preventing easy flow of urine from the kidney into the bladder. This causes urine to back up, increasing pressure and swelling within the kidney.
Pain from a kidney stone can be excruciating, particularly as the stone is passing through the ureter. Kidney stone pain of this type is referred to as renal colic and its intensity is often described as akin to the pain of childbirth. The pain often begins in the back or flank of the side of the low back. It may radiate to the front of the abdomen and, in males, may cause testicular or scrotal pain. The pain is often intermittent in waves causing the affected individual to writhe or move constantly to find a comfortable position. There can be associated nausea, vomiting, and sweating.
The intense pain can be continuous or it can wax and wane as the stone passes toward the bladder. Often, in between the intense pain phase, there remains a dull ache in the back or flank. Once the stone passes into the bladder, the obstruction is relieved, urine can flow freely and the pain resolves. The dull flank ache can remain for a few hours or days after the stone has passed.
Since the urethra is much wider than the ureter, passing the stone while urinating is usually not an issue and most patients cannot tell when they have eliminated the stone from their bladder.
The quantity and severity of pain is not related to the size of the stone but rather the amount of obstruction and kidney swelling present.
Sometimes, there can be blood visible in the urine as the kidney stone passes and irritates the lining of the urinary tract. Most often, the urine is clear to the naked eye and red blood cells are only visible in the urine when it is analyzed under the microscope.
Medically reviewed by Michael Wolff, MD; American Board of Urology
Tintinalli, J. E., et al. Tintinalli's Emergency Medicine: A Comprehensive Study Guide. 7th ed. McGraw-Hill Professional, 2010.