Stone disease may affect the kidney, ureters (ducts carrying urine from the kidneys to the bladder) and the bladder. Stones form in the kidney when urine contains too much of certain substances (e.g. calcium), which create small crystals that become stones. Kidney stones may not produce symptoms until they begin to move down the ureters, when they can block the flow of urine out of the kidneys. This causes swelling of the kidneys and usually severe pain. Kidney stones are common and can occur in premature infants. There are different types of stones, with varying causes, including hereditary. There are several tests for kidney stones and the affected areas may be examined with CT, MRI, ultrasound and other viewing technologies. Treatment varies depending on the type of stone and how severe the symptoms are. Small stones pass out in urine by themselves. Medication can help control the pain of passing the stones. Medication can also decrease stone formation or help break down and remove the material that is causing the stone. Surgery is usually needed if: the stone is too large to pass on its own; it is growing in size; or it is blocking urine flow and causing an infection or kidney damage. Minimally invasive treatments include extracorporeal shock-wave lithotripsy to remove smaller stones located near the kidney. This method uses ultrasonic waves or shock waves to break up stones. The stones then leave the body in the urine. Percutaneous nephrolithotomy is used for large stones in or near the kidney, or when the kidneys or surrounding areas are incorrectly formed. The stone is removed with an endoscope (a flexible lighted tube with instruments attached) inserted into the kidney. Ureteroscopy (inserting an endoscope into the ureter) may be used for stones in the lower urinary tract. Standard open surgery (nephrolithotomy) may be needed if other methods do not work or are not possible.