What Is the Evidence for a Cause-and-Effect Linkage Between Occupational Hand Use and Symptoms of Carpal Tunnel Syndrome?

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Chapter 16 What Is the Evidence for a Cause-and-Effect Linkage Between Occupational Hand Use and Symptoms of Carpal Tunnel Syndrome?

Because the cause of carpal tunnel syndrome (CTS) is in most cases still unknown, a number of explanatory theories have been proposed, one of which has been occupational hand use. The possible association between occupational hand use and CTS has been mainly based on the common belief that the pathophysiologic mechanism in CTS is increased pressure in the carpal tunnel. It has been suggested that certain hand or wrist activities or postures needed to perform occupational tasks may cause increased carpal tunnel pressure, which may lead to CTS. The hand and wrist activities and postures often linked to CTS have been mainly repetitive movements, force (or combinations of these two factors), and excessive flexion and extension.1 The relationship between CTS and work has been argued for a long time2; an early review concludes that “exposure to physical work load factors, such as repetitive and forceful gripping, was probably a major risk factor for CTS in several types of worker populations.”3 The issue, however, has been debated intensely.4,5

Determining the possible association between certain occupations or occupational activities and CTS involves two essential aspects: measuring exposure and establishing the diagnosis (often called CTS case definition). These two aspects have not been managed in a standardized fashion in the literature. With regard to exposure, the methods of measuring occupational activities have mainly been self-report or expert judgment, with few studies using objective measures of hand posture or force (such as measuring number of cycles in a repetition, force exerted, or wrist angles). Other studies have used job titles to compare occupations, implying that they differ in their impact on the upper extremity. However, various work activities may differ even within the same categories, such as the amount of force or repetitiveness used in different activities classified as “low force” or “high repetitive,” respectively. Besides, different types of work usually described as repetitive (e.g., computer work and packaging or assembling) are not similar. Hence, definition of high force versus low force and high repetition versus low repetition may not be consistent. In addition, no attempts have been made to determine the length of exposure time for a given occupational stressor needed to cause disease. Regarding case definition, studies of CTS and occupational exposure have used different case definitions ranging from self- reported symptoms only to combination of symptoms, physical examination, and nerve conduction measurements. The use of different diagnostic criteria inevitably leads to different conclusions, and such studies cannot be considered to involve the same disease (i.e., numbness and tingling in the hand reported through a questionnaire is not the same entity as CTS diagnosed by a physician based on history and abnormal nerve conduction measurements). Demonstrating an association between occupation and symptoms cannot be interpreted as an association with CTS.

It is helpful to review the reported prevalence and incidence rates of CTS in the general population to interpret the results reported in the literature regarding the relationship between work and CTS. Using diagnostic criteria including nerve conduction measurements, researchers have reported the prevalence rates of CTS among women in general population studies to range from 3% to 6% and among men from 0.6% to 2.1%.68 Based on symptoms and nerve conduction measurements, annual incidence rates of up to 0.5% among women and 0.1% among men have been reported.9 If it is assumed that a certain occupation does not increase the risk for CTS, the prevalence of CTS in a random sample of workers from that occupation would likely be lower than that in an age- and sex-matched random general population sample because the latter would include former workers who are sick, disabled, or retired.

STUDY DESIGNS AND BIASES

To evaluate the strength of the evidence derived from the published studies that investigated the association between CTS and occupation, you must recognize the different types of study design and biases related to each study design.

Confounders

It is well established that a number of factors such as age, sex, and body mass index are related to CTS.6,8, 10, 11 If the exposed workers are older or have a greater proportion of overweight persons than the nonexposed workers, finding a greater prevalence of CTS may be related to these factors rather than to occupation. These factors need to be accounted for when comparing the prevalence of CTS in different occupational groups. Few studies have addressed nonoccupational hand use, but the importance of this factor is uncertain. Other medical conditions, such as inflammatory joint disease and diabetes, may also confound the association between CTS and occupation, but because of the relatively low prevalence of these conditions in occupational groups, their impact may not be large.

LITERATURE REVIEW

The evidence regarding the association between CTS and occupational hand use should be based on appropriate epidemiologic studies in which participants were selected from populations of exposed and nonexposed persons (different occupations or occupational activities), and the presence of CTS determined with reasonable accuracy. A review of the studies generated by the literature search showed several common problems that may affect the conclusions and subsequently the evidence derived from them. Inclusion and exclusion criteria were used to select appropriate studies for evaluation of the evidence.