Stones (or calculi) can form in any part of the urinary tract, from the kidneys to the ureters (the tubes leading out of the kidneys), to the bladder. They're quite common (there are about 40,000 cases each year in the UK) and are three times more likely in men than women. The problem often recurs - about 60 per cent of people will develop another stone within seven years. In most cases, there's no obvious cause, although dehydration, prolonged bed rest and urinary tract infection may all play a part.

Different types of kidney stone

In about 70 per cent of cases, the stone is made of calcium oxalate and/or calcium phosphate. Oxalate is a product of metabolism naturally found in urine. A diet rich in oxalic acid (found in rhubarb, coffee, spinach and leafy vegetables, for example) can increase the risk.

Occasionally, calcium stones occur due to problems with the parathyroid glands, which control calcium metabolism in the body.

In about one in five cases, stones are caused by chronic urinary tract infections. The bacteria break down urea in the urine to form ammonia and make the urine alkali. This leads to the formation of stones containing calcium, magnesium and ammonium phosphate. In severe cases, the stones fill the middle of the kidney, causing a large stone known as a staghorn calculus because of its shape.

About one in 20 stones are uric acid stones, often linked to gout, but also to some cancers and chronic dehydration. Very rarely, stones are the result of metabolic disorders such as cystinuria.

Symptoms

These depend largely on where the stone is.

• Small stones in the kidney itself may cause no symptoms, and go unnoticed. But once stones enter the ureter, they can cause extreme and sudden pain in the loin (in the lower back), which moves down towards the groin. This pain, known as renal colic, is very sharp, may come in waves, and can cause nausea and even vomiting.
• You may notice blood in your urine.
• Once the stone reaches the bladder, the pain may disappear, but you may now find you have problems passing urine, with a slow stream and dribbling, as the stone can block the exit to the bladder. There's also an increased risk of urinary tract infection developing.

Prevention

Prevention depends on the cause, but includes:

• Avoiding getting dehydrated (high fluid intake of at least two to three litres a day can help to keep the urinary tract flushed out)
• Taking care with your diet to avoid an excess of oxalic acid
• Rapid treatment of urinary tract infections
• Specific treatments for metabolic disorders and gout

This article was last medically reviewed by Dr Trisha Macnair in April 2008