Crystal deposits of varying sizes that form in the kidney
- Most common between the ages of 30 and 50
- More common in males
- In some cases, the cause is inherited
- Certain diets and living in a hot climate are risk factors
Normally, the waste products of the body’s chemical processes pass out of the kidneys in the urine. Kidney stones occur when the urine is saturated with waste products that crystallize into stone-like structures, or when the chemicals that normally inhibit this crystallization process are not present. Kidney stones can take years to form.
If the stones are small, they may become dislodged from the kidney and move through the urinary tract, eventually passing out of the body in the urine. Larger stones stay in the kidney but may occasionally move into the ureter (the tube that takes urine from the kidney to the bladder). If a stone becomes lodged in the ureter, it can cause severe pain. A large stone in the kidney is not usually painful, but it increases the risk of urinary infection.
What are the causes?
The risk of stones forming in the kidneys is greatest when there is a high concentration of dissolved substances in the urine. Inadequate intake of fluid increases the risk of kidney stones. When there is too little water in the body, the kidneys conserve water by forming less urine, and as a result the urine they produce is highly concentrated. People who live in hot climates may be susceptible to kidney stones if they do not drink enough to replace the fluid lost through perspiration.
Different types of kidney stone can form, depending on the waste products that crystallize out of the urine. Most are made of calcium salts. These stones may be associated with a diet containing foods rich in calcium or a substance called oxalic acid. They may also develop if your body produces too much parathyroid hormone (see Hyperparathyroidism), a process causing high levels of calcium to build up in the bloodstream. A small percentage of stones contain uric acid and may occur in people who have gout.
Kidney stones may also result from a long-standing urinary tract infection. In such cases, the stones can grow into a staghorn shape and fill the central cavity of the kidney. Rarely, they are formed from cystine, a substance present in abnormally high levels in people who have the inherited disorder cystinuria. Kidney stones are also associated with some drugs, such as indinavir, which is used to treat HIV infection (see Drugs for HIV infection and AIDS).
What are the symptoms?
Very small kidney stones may pass unnoticed in the urine. Larger stones or small fragments of stones that pass into the ureter may cause painful spasms of the ureter wall. The symptoms usually appear suddenly and may include:
Excruciating pain that starts in the back, spreads to the abdomen and groin, and may be felt in the genitals.
Frequent, painful passing of urine.
Nausea and vomiting.
Blood in the urine.
If a kidney stone is passed in the urine, the pain will subside rapidly. However, if a stone lodges in a ureter, it may cause a build-up of urine, which will then result in swelling of the kidney (see Hydronephrosis).
How is it diagnosed?
If your doctor suspects that you have kidney stones, he or she will take a specimen of urine to look for blood, crystals, and evidence of an underlying infection. Stones that have been passed in the urine may be collected and analysed to determine their composition. An ordinary X-ray may be used to look for calcium stones; other types of kidney stone can be detected by taking specialized X-rays of the urinary tract (see Intravenous urography), by ultrasound scanning, or by CT scanning. You may also have a blood test to assess kidney function, and to measure the levels of calcium, uric acid, and other substances in your blood. Collectively, these tests help the doctor to determine the presence and composition of kidney stones.
What is the treatment?
If the stones are small and remain in the kidney, you may simply be advised to take painkillers, and drink plenty of fluids to help flush the stones into the urine. For stones that need treatment, one common method is lithotripsy, in which shock waves are used to break them into small fragments that can be passed in the urine. Another is percutaneous lithotomy, in which an endoscope (viewing tube) is inserted into the kidney through a small incision in the skin and instruments passed through the tube to remove a stone. Surgery is performed only if a stone is very large. Rarely, a staghorn stone is so large that the entire kidney must be surgically removed.
If a stone moves into the ureter, it usually causes severe pain that can last for several hours. You may need to go to hospital for strong painkillers. You may also be given intravenous fluids to increase the volume of urine and help to flush out the stone. If a stone is lodged in the ureter, it may be pulverized or removed by special instruments during cystoscopy, in which a viewing tube is passed into the ureter through the urethra and bladder.
The underlying cause needs to be treated to prevent the kidney stones from recurring. Your doctor will probably advise you to drink at least 2–3 litres (4–6 pints) of fluids every day and to avoid foods that may encourage the formation of stones (see Preventing kidney stones).
What is the prognosis?
More than half the people treated for a kidney stone develop another within about 7 years. However, the self-help measures above may reduce the risk of recurrence. Kidney stones rarely cause permanent damage to the kidneys.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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