Gout.

Figure 1 - Joint with gout versus a normal joint

Causes and effects

Causes

The pain and swelling of gout are caused by uric acid crystals that deposit in the joint. Uric acid is a substance that normally forms when the body breaks down waste products called purines. Uric acid usually is dissolved in the blood and passes through the kidneys into the urine. In people with gout, the uric acid level in the blood is so high that uric acid crystals are deposited in joints and other tissues. This causes the joint lining (the synovium) to become inflamed.

After several years, the uric acid crystals can build up in the joint(s) and surrounding tissues. They form large deposits called tophi that look like lumps just under the skin. Tophi often are found in or near severely affected joints, on or near the elbow, over the fingers and toes, and in the outer edge of the ear. If the tophi are not prevented or treated, they can damage joints (see figure 1).

Uric acid crystals can form stones in the kidneys, in the ureters (tubes connecting the kidneys and bladder), or in the bladder itself. Several factors may cause the formation of these deposits. For example, the deposits may be caused by not drinking enough liquids. Because of this lack of fluid, the urine is unable to dissolve all the uric acid. Deposits also may form as a result of metabolic abnormalities, such as the body's inability to make urine-less acid. In other instances, diet may be the culprit. With some people, a diet rich in foods that further increases the production of uric acid may increase their chances of developing kidney stones. If your doctor suspects that diet is a contributing factor, you may be asked to collect several urine samples. Tests then will be conducted to determine the amount of uric acid your body produces. These tests are particularly helpful because some people with gout produce and eliminate a large amount of uric acid. These people may be more likely to develop kidney stones. People with gout also may have high blood pressure or kidney infections. Since these problems can cause kidney damage, your doctor will check for signs of these problems and treat them if they occur.

Almost all people with gout have too much uric acid in their blood, a condition called hyperuricemia. However, there are many people who have hyperuricemia but not gout. Hyperuricemia is caused by one or both of the following:

• The kidneys can't get rid of uric acid fast enough.
• The body makes too much uric acid.

Hyperuricemia often is caused by using diuretic medications ("water pills"). Diuretics are used to get rid of excess body fluid and to lower high blood pressure. However, diuretics can hamper the kidneys' ability to remove uric acid, thus raising uric acid levels in the blood.

Other factors, such as inherited traits and environmental factors (such as weight, alcohol use, and diet), also can play an important role in causing gout.

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