Extracorporeal shock wave lithotripsy (ESWL) uses high-energy shock waves to break up kidney stones so they can be passed more easily through urine. Kidney stones are solid masses of crystals that form when urine becomes too concentrated. ESWL is the most frequently used procedure for treating kidney stones, according to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).

This technique is also sometimes used to break up stones in the gallbladder and pancreas. ESWL has also been used as an experimental therapy in treating gallstones, pieces of hard matter that form when components of bile (cholesterol and bilirubin) crystalize in the gallbladder.

Also known as lithotripsy, ESWL may be performed on either an inpatient or outpatient basis in a hospital, outpatient clinic or mobile treatment unit. Patients are typically instructed not to eat or drink anything for at least six hours prior to the procedure. They may also be asked to alter their medication regimen for several days before undergoing ESWL.

The procedure itself is usually performed in one of two ways. In the more common method, patients lie on a soft cushion as the waves pass through them. In other cases, patients sit in a tub of lukewarm water as the shock waves are sent into their bodies.

Between 1,000 and 2,000 shock waves are usually required to crush the stones, which are pulverized into pieces as small as grains of sand. The process itself is painless, although patients commonly experience temporary side effects such as minor bruising or blood in the urine.  ESWL treatment usually takes between 45 minutes and one hour.

Between 70 and 90 percent of patients who undergo ESWL are free of stones within three months of treatment, according to the National Kidney Foundation. Treatment is most effective for stones located in the upper portions of the urinary tract, such as the kidney or upper part of the ureter (narrow duct that moves urine from the kidney to the bladder).