The carpal tunnel is a passageway that runs from the forearm through the wrist. Bones form three walls of the tunnel and a strong, broad ligament bridges over them. The median nerve, which supplies feeling to the thumb, index, and ring fingers, and the nine tendons that flex the fingers, passes through this tunnel. This nerve also provides function for the muscles at the base of the thumb (the thenar muscles). Usually, carpal tunnel syndrome (CTS) is considered an inflammatory disorder caused by repetitive stress, physical injury, or other conditions that cause the tissues around the median nerve to become swollen. It occurs either when the protective lining of the tendons within the carpal tunnel become inflamed and swell or when the ligament that forms the roof becomes thicker and broader.
Just as stepping on a hose slows the flow of water through a garden hose, so compression on the median nerve fibers by the swollen tendons and thickened ligament slows down the transmission of nerve signals through the carpal tunnel. The result is pain, numbness, and tingling in the wrist, hand, and fingers (except the little finger, which is not affected by the median nerve).
Carpal tunnel syndrome is one of a group of disorders categorized by several different terms: repetitive stress injuries, cumulative trauma disorder, overuse syndromes, chronic upper limb pain syndrome, or repetitive motion disorders. All of these problems are generally associated with repetitive and forceful use of the hands that damage muscles and bones of the upper extremities.
What Causes Carpal Tunnel Syndrome?
It is often very difficult to determine whether the primary cause of CTS is primarily due to work conditions or an underlying medical problem. Carpal tunnel syndrome almost always occurs in adults and most adults work; CTS, then, is very likely to be associated with the work place whether or not it is actually caused by the work itself. Indeed, estimates of work-related CTS vary wildly. Some studies suggest that more than half are due to workplace factors and others that few cases of CTS are actually caused by conditions on the job. In one study, for example, obesity or diseases such as diabetes, hypothyroidism, and arthritis occurred in the majority of people with work-associated CTS. Such conditions are known contributors to carpal tunnel disorder. It is likely that many cases of CTS occur from a combination of factors, including a predisposing medical condition exacerbated by work stress and psychologic and social stressors. In many patients with CTS, an underlying cause for the disorder cannot be discovered.
Work-Related Causes
Disorders related to work that requires repetitive motion are increasing. They account for nearly half of all reported work-related illness, and carpal tunnel syndrome is estimated to account for over 41% of these repetitive motion disorders. Researchers have defined six key risk factors in the workplace for the development of these disorders, including carpal tunnel syndrome:
1. repetition
2. high force
3. awkward joint posture
4. direct pressure
5. vibration
6. prolonged constrained posture
Some experts believe that incorrect posture may play a large role in the development of CTS, particularly in people who work at computer and other types of keyboards. The tendency to roll the shoulders forward, round the lower back, and thrust the chin forward can shorten the neck and shoulder muscles, compressing nerves in the neck. This, in turn, can affect the wrist, fingers, and hand. It has been difficult, however, to obtain reliable data on the direct link between repetitive hand and wrist tasks and carpal tunnel syndrome.
Studies indicate that psychosocial factors in the workplace, such as intense deadlines, interpersonal relationships, and job design, are major contributors to carpal tunnel pain. Such psychosocial conditions are more likely to be important factors in contributing to CTS in office workers, although they also complicate the condition in workers whose work is primarily physical.
Injuries and Medical Conditions
Bone dislocations and fractures can narrow the carpal tunnel, thereby exerting pressure on the median nerve. Certain other medical conditions, such as rheumatoid arthritis, diabetes, and hypothyroidism, can also cause the inflammation in the carpal tunnel that results in median nerve entrapment. These are all autoimmune diseases, meaning that the body's immune system is abnormally attacking its own tissue. Some experts believe that carpal tunnel syndrome may actually be one of the first symptoms in a number of these diseases.
Carpal tunnel syndrome can also be caused by long-term hemodialysis and diseases such as multiple myeloma, Waldenstrom's macroglobulinemia, and non-Hodgkin's lymphoma, which cause a build-up of a waxy starch-like protein called amyloid in the bone and joint tissues. Acromegaly, a disease that causes abnormally long bones, is a cause of CTS. A virus known as human parvovirus was suspected in a few cases of carpal tunnel syndrome that developed in both hands. Certain medications that effect the immune system, such as interleukin-2, which is administered to some cancer patients, may cause temporary CTS. There have been some reports of carpal tunnel syndrome caused by anticlotting drugs, such as warfarin.
Hormonal Changes
Fluid retention during pregnancy or hormonal changes associated with menopause can cause swelling and symptoms of carpal tunnel syndrome.
Inherited and Inborn Factors
One study has reported a higher risk for carpal tunnel in people with a family history of the problem, indicating that a genetic susceptibility exists in some people. Some researchers are particularly interested in abnormalities in certain genes that regulate myelin, a fatty substance that serves as insulation for nerve fibers. Some people may be born with abnormalities of the carpal bones.
What Are the Symptoms of Carpal Tunnel Syndrome?
Symptoms of carpal tunnel syndrome usually progress gradually over weeks and months and, in some cases, years. The first symptoms may be pain in the wrist and hand or numbness and tingling of the fingers (except the little finger). Patients may also experience a sense of weakness and a tendency to drop things. They may lose the sense of heat and cold or feel that their hands are swollen even though there is no visible swelling. Symptoms may occur not only when the hand is being used but also when it is at rest. In fact, the disorder may be distinguished from similar conditions by pain occurring at night after going to bed. In some cases, labor-related CTS symptoms first occur outside of work, so patients may fail to associate the symptoms with work-related activity. Anyone with recurrent or persistent pain, numbness and tingling, or weakness of the hand should consult a physician for a diagnosis.
What Other Diseases Show the Same Symptoms as Carpal Tunnel Syndrome?
Accompanying Disorders
About 25% of patients with work-related repetitive stress disorders also have evidence of other similar conditions that resemble -- but are not -- carpal tunnel syndrome. A definitive diagnosis is often difficult. Most require treatments similar to those used for CTS: rest, immobilization, steroid injections, and even surgery if conservative management is unsuccessful.
Nerve Entrapment Disorders
Repetitive work can cause pressure on the median nerve in locations other than the wrist and can also affect other nerves in the arm and hand. The branch of the median nerve that runs through the palm of the hand can be damaged directly by repeated pounding or by the use of certain tools requiring a strong grip using the palm, such as needle-nosed pliers. The median nerve can also be pinched up in the forearm.
The ulnar nerve supplies sensation to the ring and little fingers. Like the median nerve, it too can become trapped as a result of repetitive stress, with subsequent loss of sensation in these fingers and the outer half of the palm. This condition, known as ulnar tunnel syndrome, can be a separate disorder or appear with carpal tunnel syndrome. In the latter case, release surgery for CTS usually also relieves the ulnar nerve entrapment. The ulnar nerve can also be affected at the elbow.
Tendon-Related Disorders
Tenosynovitis (swelling of the tendon sheath) in the hands and fingers is also a repetitive stress injury and can effect various parts of the hand and fingers. One or more fingers may feel painful and stiff, especially in the morning; the wrist may be swollen. Trigger finger (also called snapping finger) is a condition brought on when a tendon thickens, leaving the finger in a bent position. It is a common complication of rheumatoid arthritis; it also may occur in diabetes or for unknown causes. De Quervain's disease involves tenosynovitis at the base of the thumb. These disorders are often present with carpal tunnel syndrome.
Patients who have tendinitis (swelling of the tendon) feel pain in the involved area, which is intensified when they contract the muscles adjoining the tendon or when the physician stretches the affected finger or part of the hand.
Thoracic Outlet Syndrome
Pinched nerves in the neck may also cause weakness in the hands. A disorder known as thoracic outlet syndrome caused by compression of nerves and blood vessels running down the neck into the arm can cause symptoms very similar to CTS. The compression occurs at the first rib in the front of the shoulder. A physician may be able to diagnose the condition by detecting diminished blood flow in the arm as the patient raises the affected hand and turns his or her head toward the opposite side. Although the condition is uncommon, a correct diagnosis is important to differentiate it from CTS. Treatments for thoracic outlet syndrome are useless for carpal tunnel syndrome.
Arthritic Conditions
Arthritic conditions, including rheumatoid arthritis, gout, and osteoarthritis, can all cause pain in the hands and fingers.
Raynaud's Phenomenon
Raynaud's phenomenon produces symptoms of numbness and tingling or pain in the fingers of one or both hands. It is usually brought on by cold or stress and is treated with warmth or, in severe cases, medications that may open blood vessels. People with this disorder, in fact, appear to be at higher risk for carpal tunnel syndrome and there may be some associations between the two conditions.
Who Gets Carpal Tunnel Syndrome?
In 1988, 2.8 million people in the U.K. reported symptoms to their doctors that they believed were those of carpal tunnel syndrome. In a 1998 British study, experts estimated that between 7% and 16% of the population experience CTS. The incidence appears to be increasing. People over age 54 were at higher risk than younger adults. The wide variation in severity and the difficulty in diagnosis make it hard to pinpoint specific figures.
Workers at High Risk
At high risk are those whose occupations combine force and repetition of the same motion in the fingers and hand for long periods. Such workers include those in the meat and fish packing industries and workers using vibrating tools, like jackhammers or chain saws. Meat packers complained of pain and loss of hand function as long ago as the 1860's.
Even today, the incidence of carpal tunnel syndrome in meat, poultry, and fish packing industries may be as high as 15%. Workers in these industries and those who assemble airplanes have the highest risk of CTS, according to one study. In addition, high risk for CTS has been reported in other assembly line workers (such as food and beverage processing), cake decorators, postal workers, dentists, and dental technicians -- virtually any workers who use their hands and wrists repetitively.
Even though the increased number of people using computer keyboards has provoked much publicity about their risk for carpal tunnel syndrome, it is actually lower than those in occupations involving heavy labor. The force of the movement may, however, be a particular factor for CTS in typists. One study observed that typists with carpal tunnel syndrome struck the keys with greater force than those without the disorder. Some workers may not even be aware of the amount of force they exert while performing their jobs. For example, the fingers of typists whose speed is 60 words per minute exert up to 25 tons of pressure each day.
People who engage intensively in certain domestic occupations, including knitting, sewing and needlepoint, cooking, housework, carpentry, and extensive use of power tools, are also at risk. Many leisure activities in the home can contribute to the development of CTS, including computer games, sports, and card playing.
Gender
Many -- but not all -- studies indicate that women have a significantly higher risk for carpal tunnel syndrome than men do. The explanation for this greater risk is unknown. The hand-intensive nature of housework and typing may contribute to a higher incidence in women. Hormonal changes, however, appear to play a major role as evidenced by an increased incidence in CTS symptoms while taking oral contraceptives and during pregnancy, the postpartum period, and menopause. In one study of pregnant women with CTS, the condition developed at any point during pregnancy; no single trimester posed a higher risk than others
Physical Characteristics
Studies indicate that obesity is highly linked with carpal tunnel syndrome. CTS is also more common in those with square wrists (the thickness and width are about the same) than in those with the more common rectangular wrists, although other studies have failed to confirm this.
There has been some suggestion that the size of the carpal tunnel may play a role in the disorder, but a recent study found no difference in size between women who had CTS and those who did not have it. Some researchers claim that poor upper back strength makes people more susceptible to injuries in the upper extremities, including carpal tunnel syndrome.
Medical Conditions and Treatments
People with certain autoimmune medical conditions are at risk for CTS, including hypothyroidism, rheumatoid arthritis, and systemic lupus erythematosus. In one study, 15% to 25% of people with diabetes had carpal tunnel syndrome. People who undergo hemodialysis are at risk for carpal tunnel syndrome caused by build-up in the hand of certain proteins called beta 2-microglobulin. One study found that people with hepatitis C sometimes have symptoms of carpal tunnel syndrome. People with Down's syndrome are also at high risk for CTS.
Other Factors
Cigarette smoking, poor nutrition, previous injuries, and stress can increase one's risk for carpal tunnel syndrome.
How Serious Is Carpal Tunnel Syndrome?
Carpal tunnel syndrome can range from a minor inconvenience to a disabling condition, depending on its cause and persistence and the individual characteristics of the patient. Many cases of CTS are mild, and some resolve on their own. Once a pregnant women gives birth, for instance, the swelling in her wrists subsides and so do the CTS symptoms. Proper treatment of other medical conditions that cause carpal tunnel syndrome can often help reduce wrist swelling.
If severe cases are left untreated, however, muscles at the base of the thumb may atrophy and sensation may be permanently lost. CTS can become so crippling that people can no longer do their job or even perform simple tasks at home. The syndrome may force people to undergo surgery and miss many days of work and can even prevent them from working at all if their hand functions are permanently impaired.
Occupation-related causes of CTS have had a severe impact on American businesses. Workers with CTS become easily fatigued, experience pain and discomfort, and may not perform up to par. In one study, nearly half of all employees diagnosed with CTS had changed jobs or were absent 30 months after the diagnosis. Because of the difficulty in determining the exact cause of CTS, employers are concerned about high worker's compensation costs due to CTS, which may or may not be due to working conditions.
Those receiving compensation are also more likely to be absent from work longer than those not being paid, particularly if the employer is contesting the case. The medical costs and loss of productivity because of carpal tunnel syndrome has been estimated
to average $29,000 per injured worker. Work-related injuries, including carpal
tunnel syndrome, that involve joints and muscles cost the country about
$20 billion every year.
