Introduction:

Carpal tunnel syndrome (CTS) is a disorder marked by weakness and pain in the hand and wrist. CTS occurs in the nerves of the hands -- not the muscles, as some people believe. The symptoms of CTS can be incapacitating.

The Carpal Tunnel and Median Nerve

To understand how carpal tunnel syndrome arises, it is important to know the parts of the hand and wrist that are involved.

The Carpal Tunnel. The carpal tunnel is a passageway that forms beneath the strong, broad transverse ligament. This ligament is a bridge that extends across the lower palm and connects the bones of the wrist (carpals) that form an arch below the tunnel.

The Median Nerve and Flexor Tendons. The median nerve and nine flexor tendons pass under the ligament bridge and through the carpal tunnel (similar to a river). They extend from the forearm and up into the hand:

• The flexor tendons are fibrous cords that connect the muscles in the forearm to the fingers (two to each finger) and one to the thumb. They allow flexing of the fingers and clenching of the fist.
• The median nerve plays two important roles. It supplies sensation to the palm to side of the thumb, index, middle, and ring fingers, and to the flexor tendons. It provides function for the muscles at the base of the thumb (the thenar muscle).

The median nerve travels through a compartment in the wrist called the carpal tunnel. The ligaments that transverse the nerve are not very flexible. Any swelling within the wrist compartment can put excessive pressure on structures such as the blood vessels and the median nerve. Excessive pressure can constrict blood flow and cause nerve damage. The symptoms from the compression cause pain, loss of sensation, and decreased function in the hand.

The Carpal Tunnel Syndrome Process

It is not completely known how the process leading to carpal tunnel syndrome actually evolves, and how nerve conduction (the passing of the nerve signal) through the wrist changes. In general, carpal tunnel syndrome develops when the tissues around the median nerve swell and press on the nerve. Early in the disorder, the process is reversible. Over time, however, the insulation on the nerves may wear away, and permanent nerve damage may develop.

The following events have been observed in the hands of people with carpal tunnel syndrome:

• The protective lining of tendons (called the tenosynovium) swells within the carpal tunnel. Some research suggests that this swelling is caused by build-up of fluid (called synovial fluid) under the lining. Synovial fluid lubricates and protects the tendons.
• The transverse ligament, the band of fibrous tissue that forms the roof over the median nerve, becomes thicker and broader.
• The swollen tendons and thickened ligament compress the median nerve fibers, just as stepping on a hose slows the flow of water through it. This compression reduces blood flow and oxygen supply to the nerve, and slows the transmission of nerve signals through the carpal tunnel. Some cases of carpal tunnel syndrome may be due to enlargement of the median nerve rather than compression by surrounding tissues.

The result is pain, numbness, and tingling in the wrist, hand, and fingers. Only the little finger is unaffected by the median nerve.