Carpal Tunnel Syndrome - Introduction

Introduction

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

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 an anatomical 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 called carpals, which 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 to muscles of 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 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 called the carpal tunnel in the wrist. The ligaments that transverse the nerve are not very flexible. If there is any swelling within the wrist compartment excessive pressure can be put on structures such as the blood vessels and the median nerve. Excessive pressure can constrict bloodflow and cause nerve damage. The symptoms from the compression causes 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 through the wrist becomes altered. In general, carpal tunnel develops when the tissues around the median nerve become swollen and press on the median nerve. Early in the onset of 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 compresses the median nerve fibers, just as stepping on a hose slows the flow of water through it. The effect is to reduce blood flow and oxygen supply to the nerve, slowing 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.)

Click the icon to see a depiction of carpal tunnel syndrome.