The cause of carpal tunnel syndrome is pressure 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.
The carpal tunnel is about as big around as your thumb and is a narrow passageway bound by bones and ligaments located on the palm side of your wrist. This tunnel protects a main nerve to your hand and nine tendons that bend your fingers. Pressure placed on the nerve produces the numbness, pain and, eventually, hand weakness that characterize carpal tunnel syndrome.
Carpal tunnel syndrome typically starts gradually with a vague aching in your wrist that can extend to your hand or forearm. Other common carpal tunnel syndrome symptoms include tingling or numbness in your fingers or hand, especially your thumb and index, middle or ring fingers, but not your little finger. This sensation often occurs while holding a steering wheel, phone or newspaper or upon awakening. Many people "shake out" their hands to try to relieve their symptoms. As the disorder progresses, the numb feeling may become constant. The symptoms can also include pain radiating or extending from your wrist up your arm to your shoulder or down into your palm or fingers, especially after forceful or repetitive use.
The following tests may be ordered to diagnose carpal tunnel syndrome:
An electromyogram, which measures the tiny electrical discharges produced in muscles. A thin-needle electrode is inserted into the muscles your doctor wants to study. An instrument records the electrical activity in your muscle at rest and as you contract the muscle. This test can help determine if muscle damage has occurred.
Nerve conduction study. In a variation of electromyography, two electrodes are taped to your skin. A small shock is passed through the median nerve to see if electrical impulses are slowed in the carpal tunnel.
These tests are also useful in checking for other conditions that might mimic carpal tunnel syndrome, such as a pinched nerve in your neck. Imaging tests, such as MRI or X-ray, generally aren't used to diagnose carpal tunnel syndrome.
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