Carpal Tunnel Syndrome

CONDITIONS 

Carpal tunnel syndrome (CTS) is a disorder that causes pain and weakness in the hand and wrist. CTS develops from problems in a nerve in the wrist — not the muscles, as some people believe. The symptoms of CTS can range from mild to 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 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 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).

Wrist anatomy

The median nerve travels through a compartment in the wrist called the carpal tunnel. The ligaments that traverse 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 transmission 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 a buildup of fluid (called synovial fluid) under the lining. Synovial fluid normally 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 press on 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, while 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 of these processes is pain, numbness, and tingling in the wrist, hand, and fingers. Only the little finger is unaffected by the median nerve.

Symptoms

Symptoms of carpal tunnel syndrome usually progress gradually over weeks and months, or sometimes years. Anyone with recurrent or persistent pain, numbness and tingling, or weakness of the hand should consult a doctor for a diagnosis. Symptoms often develop as follows:

  • Initial symptoms include pain in the wrist and palm side of the hand. Problems commonly occur in both hands. (Even when only one hand is painful, the other hand often shows signs of nerve conduction abnormalities when tested.)
  • Early on, the patient also usually reports numbness, tingling, burning, or some combination of symptoms on the palm side of the index, middle, and ring fingers. (Typically the fifth finger has no symptoms.) Such sensations may radiate to the forearm or shoulder.
  • Over time, the hand may become numb, and patients may lose the ability to feel heat and cold. Patients may experience a sense of weakness and a tendency to drop things.
  • Patients may feel that their hands are swollen even though there is no visible swelling. This symptom may actually prove to be an important indicator of greater CTS severity.

Symptoms may occur not only when the hand is being used but also at night when the patient is at rest. Even in cases where work is the suspected cause, symptoms typically first occur outside of work. In fact, the disorder may be distinguished from similar conditions by pain that occurs at night after going to bed.

Causes

Biological Causes. Carpal tunnel syndrome is considered an inflammatory disorder caused by repetitive stress, physical injury, or a medical condition. It is often very difficult, however, to determine the precise cause of carpal tunnel syndrome. No tests are available to identify a specific cause. Except in patients with certain underlying diseases, the biological mechanisms leading to carpal tunnel syndrome are unknown.

Working Conditions versus Medical Problems. Although some studies suggest that more than half of CTS cases are associated with workplace factors, there is no strong evidence of a cause-and-effect relationship. In fact, most studies now strongly suggest that carpal tunnel syndrome is primarily associated with medical or physical conditions such as diabetes, osteoarthritis, hypothyroidism, and rheumatoid arthritis.

CTS also tends to occur in people with certain genetic or environmental risk factors. These risk factors include obesity, smoking, alcohol abuse, or significant mental stress. CTS sometimes runs in families, which suggests that it has some type of genetic origin. When people who are susceptible to CTS are subjected to repetitive hand or wrist work, the risk for CTS can become significant. CTS, then, is very likely to be due to a combination of factors that lead to nerve damage in the hand.

Work-Related Issues and Carpal Tunnel Syndrome

High Force and Vibration. Even though medical and physical conditions may be the initial culprits leading to CTS, certain working conditions may be linked to nerve damage. Work that involves high force or vibration is particularly hazardous, as is repetitive hand and wrist work in cold temperatures.

In addition to CTS, other disorders of the hand and wrist result from these work-related movements. They include the following:

  • Hand-arm vibration syndrome — tingling and numbness that persist even after the vibration stops
  • Cumulative trauma (repetitive stress) disorder
  • Overuse syndromes
  • Chronic upper limb pain syndrome

All of these problems are generally associated with repetitive and forceful use of the hands, resulting in damaged muscles and bones of the upper arms.

 

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