The cause of carpal tunnel syndrome is pressure put on the median nerve. The median nerve is a mixed nerve, meaning it has a sensory function and also provides nerve signals to move your muscles (motor function). The median nerve provides sensation to your thumb, index finger, middle finger and the middle finger side of the ring finger. Pressure on the nerve can stem from anything that reduces the space for it in the carpal tunnel. Causes might include anything from bone spurs to the most common cause, which is swelling or thickening of the lining and lubricating layer (synovium) of the tendons in your carpal tunnel.

The exact cause of the swelling usually isn't known, but a variety of conditions and factors can play a role. Some examples include rheumatoid arthritis, certain hormonal disorders — such as diabetes, thyroid disorders and menopause — fluid retention due to pregnancy, or deposits of amyloid, an abnormal protein produced by cells in your bone marrow.

• menopause
• Rheumatoid arthritis

Repetitive flexing and extending of the tendons in the hands and wrists, particularly when done forcefully and for prolonged periods without rest, also can increase pressure within the carpal tunnel. Injury to your wrist can cause swelling that exerts pressure on the median nerve.

Physical characteristics also may play a role. It may be that your carpal tunnel is more narrow than average. Carpal tunnel syndrome may be associated with people who have square wrists, where the width and thickness are about the same, versus the more common rectangular wrist. Other less common causes include a generalized nerve problem or pressure on the median nerve at more than one location.

Some studies suggest that carpal tunnel syndrome can result from overuse or strain in certain job tasks that require a combination of repetitive, forceful and awkward or stressed motions of your hands and wrists. Examples of these include using power tools — such as chippers, grinders, chain saws or jackhammers — and heavy assembly line work, such as occurs in a meatpacking plant. Some studies have also indicated that typing or computer use may cause carpal tunnel syndrome, but the scientific evidence for this is weak.

Although it's not clear which activities can cause carpal tunnel syndrome, if your work or hobbies are hand-intensive — involving a combination of awkward, repetitive wrist or finger motion, forceful pinching or gripping and working with vibrating tools — you may be at higher risk of developing the condition.

Women are three times as likely as men are to develop carpal tunnel syndrome. The incidence in women peaks between the ages of 45 and 54, and the risk of carpal tunnel syndrome also increases with age in men.

Certain health conditions can increase your risk, including some thyroid problems, diabetes, obesity and rheumatoid arthritis. Women who are pregnant, taking oral contraceptives or going through menopause also are at increased risk, most likely due to hormonal changes. Fluid retention may be a cause of carpal tunnel syndrome during pregnancy. Fortunately, carpal tunnel syndrome related to pregnancy almost always improves after childbirth. People who smoke cigarettes may experience worse symptoms and slower recovery from carpal tunnel syndrome than nonsmokers do.

• menopause
• Obesity
• Rheumatoid arthritis