Causes

Biologic Causes. Carpal tunnel syndrome (CTS) is considered an inflammatory disorder caused by repetitive stress, physical injury, or medical conditions. 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 biologic mechanisms leading to carpal tunnel syndrome are unknown. Although an overactive immune response that causes inflammation and damage in the joints or muscles is responsible for a number of arthritic conditions, similar problems are not likely to play an important role in CTS. More likely, reduced blood flow and lack of oxygen are important in the process leading to progressive swelling and scarring.

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 such as obesity, smoking, alcohol abuse, or significant mental stress. When such susceptible people are subjected to repetitive hand or wrist work, the risk for CTS can become significant. CTS, then, is very likely to be due to convergences of factors that lead to nerve damage in the hand.

Work-Related Issues and Carpal Tunnel Syndrome

Worker's Compensation and CTS. The issues surrounding workers' compensation are particularly troubling in determining accurately whether labor conditions cause carpal tunnel pain. CTS is a major contributor to workers' compensation cases.

High Force and Vibration. Even though medical and physical conditions may be the initial culprits leading to CTS, certain working conditions are especially related to nerve damage -- if not to pure cases of CTS. 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 hand and wrist result from these work-related movements. They include the following:

• Hand-arm vibration syndrome -- tingling and numbing 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.

Psychosocial Factors in the Workplace. Studies indicate that psychosocial factors in the workplace, such as intense deadlines, a poor social work environment, and low levels of job satisfaction, 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.

Medical Conditions

A number of medical conditions increase the risk for or even cause CTS. The main conditions associated with CTS are diabetes, hypothyroidism, rheumatoid arthritis, osteoarthritis, obesity, and pregnancy. Many of the underlying diseases that contribute to the development of CTS are also associated with more severe forms of CTS.

Diabetes. CTS is a very common feature of diabetic neuropathy, one of the major complications of diabetes. Neuropathy is decreased or distorted nerve function; it particularly affects sensation. Symptoms include numbness, tingling, weakness, and burning sensations, usually starting in the fingers and toes and moving up to the arms and legs. About 6% of patients with CTS have diabetes. A 2005 study reported that an estimated 85% of patients with type 1 diabetes develop CTS. Development of CTS was related to the patient's age and the length of time they had diabetes. The development of diabetes-related complications, such as kidney disease, is not related to the development of CTS in people with diabetes.

Autoimmune Diseases. In autoimmune diseases, the body's immune system abnormally attacks its own tissue, causing widespread inflammation, which, in many cases, affects the carpal tunnel of the hand. Such autoimmune diseases include rheumatoid arthritis, systemic lupus erythematosus, and hypothyroidism. Some experts believe that CTS may actually be one of the first symptoms in a number of these diseases. Studies also suggest that CTS patients with these disorders are more likely to have severe CTS that requires surgery.

Diseases that Affect Muscle and Bones. Arthritis, gout, and other medical conditions that damage the muscles, joints, or bones in the hand may cause changes that lead to CTS. In fact, in one 2000 study, susceptibility to muscle and bone diseases was the major risk factor for CTS in British women. Osteoporosis (loss of bone density), although not a direct cause of CTS, increases the risk for wrist fractures that can lead to CTS.

Injuries and Previous Surgeries. Injuries, fractures, and operations that affect the forearm, wrist, or hand may lead to CTS, sometimes many years after the event.

Structural Abnormalities. Inborn abnormalities in the bones of the hand, wrist, or forearm may contribute to CTS.

Chronic Kidney Insufficiency. People who undergo hemodialysis for chronic kidney damage often experience a build-up in the hand of a certain type of protein called beta 2-microglobulin. This build-up can result in CTS. The longer the person has been receiving hemodialysis, the greater the risk of CTS. Certain drugs and procedures (particularly one procedure called hemodiafiltration) are being investigated as having the potential to reduce microglobulin build-up. It is hoped such new methods will delay the need for carpal tunnel surgery in patients undergoing long-term hemodialysis.

Other Diseases. A number of other medical conditions may cause or increase susceptibility to CTS:

• Down syndrome
• Amyloidosis (a progressive disorder of the connective tissues)
• Acromegaly (a disease that leads to abnormally large hands and feet due to excessive growth hormone)
• Tumor on the median nerve (removal of the tumor often resolves the CTS in such cases)

Medications. According to case reports, a number of medications may increase the risk for temporary CTS. They include certain medications that affect the immune system (such as interleukin-2), and anticlotting drugs (such as warfarin). There has been conflicting evidence as to whether corticosteroids and hormone replacement therapy may increase risk. More research is warranted before a causal association can be established.

Injuries

Bone dislocations and fractures can narrow the carpal tunnel, thereby exerting pressure on the median nerve.

Obesity and Lack of Fitness

Being overweight consistently turns up as a risk factor for CTS and may play a direct causal role on CTS. Greater body mass appears to reduce nerve flow speed into the hand. Obesity is also related to poor physical fitness, which may also increase risk. A 2005 analysis indicated that weight is strongly linked to the onset of CTS in patients under the age of 63 years, but may be a less important factor as they get older.

Hormonal Changes

Hormonal fluctuations in women play a role in CTS. Such fluctuations may cause fluid retention and other changes in the body that cause swelling. Fluid retention is one reason that CTS may develop during pregnancy.

Genetic Factors

CTS is strongly associated with a family history of the disorder. Many of these cases can be attributed to physical characteristics or medical conditions associated with CTS, which also run in families. However, in one study, 17% of family clusters of CTS were not associated with any such medical conditions, suggesting the genetic factors may be important in some people. Carpal tunnel syndrome that develops in young people is most likely to have a genetic component.

A 2000 study suggested that some patients with CTS may have a genetic defect that produces higher levels of a certain collagen subtype. Collagen is the protein used to build all connective tissue, muscle, bones, and ligament. The collagen found in CTS patients tends to produce stiffness.

Other genetic factors that may contribute to this disorder include abnormalities in certain genes that regulate myelin, a fatty substance that serves as insulation for nerve fibers.

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Review Date: 4/1/2006
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital