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?

ISAM. ATROSHI, MD, PHD

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%.⁶^–⁸ 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.

Randomized Studies

Randomized studies investigating whether occupational hand use is associated with clinical CTS would probably be difficult to perform given the nature of the research question and the large samples needed; to my knowledge, no such studies have been performed.

Prospective Cohort Studies

Workers without CTS are enrolled and followed over a period during which occupational exposure is measured, and then the incidence of CTS is determined and compared between the exposed and the nonexposed persons (results usually shown as relative risks and their confidence intervals). The advantage of the prospective cohort study is that exposure is measured before the onset of disease, allowing determination of a causal relationship. A problem may be attrition of the sample during follow-up and the need for large samples that can generate an adequate number of CTS cases to allow firm conclusions.

Case–Control Studies

Workers who are CTS cases are identified through various methods and compared with control subjects (persons without CTS), ideally from the same population, regarding previous exposure (results usually shown as odds ratios and their confidence intervals). Problems associated with these studies include difficulties in obtaining representative samples of cases and controls, and accurate measurement of past occupational exposure.

Cross-sectional Studies

A sample from a general or occupational population is examined to identify CTS cases and measure occupational exposure at the same time, and the prevalence of CTS is compared between different occupations or between groups performing different occupational hand activities (results usually shown as odds or prevalence ratios and their confidence intervals). The problems with these studies include difficulties in obtaining a representative sample and in accurately measuring exposure. Moreover, cross-sectional studies cannot provide direct evidence for a causal relationship.

Reporting Bias

When exposure measurement and search for CTS cases are done at the same time, persons with hand symptoms, including those with CTS, may report exposure differently than persons without hand symptoms. The risk for reporting bias would be greater if the persons are asked specifically about hand symptoms and about activities that involve the hands at the same time. If workers with CTS report more exposure than workers without CTS because their reporting is influenced by the presence of disease and their belief that their symptoms are related to their occupation, this may result in a false-positive association. This type of bias should be considered in studies that show a positive association, especially when exposure is based on self-report. Case–control studies that rely on subjects’ recollection of occupational exposure also are subject to reporting bias.

Misclassification

The magnitude of misclassification can be minimized with a more specific case definition that includes neurophysiologic examination and with objective measures of exposure.

Selection Bias

If there is a true causal relationship between a certain occupation and CTS that leads affected workers to leave that occupation, then a cross-sectional sample would mainly include persons without CTS (healthy worker effect). This bias may underestimate a true association between a certain occupation and CTS. Studies that use only healthy asymptomatic persons as control subjects carry the risk for overestimation of the association because control subjects should be as similar to the cases as possible except for the occupational exposure.

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.

Inclusion Criteria

Study Design.

Although it is recognized that, in general, strong evidence would be based on Level I and II studies, the majority of the studies concerning the association between CTS and occupational hand use were Level III case–control or cross-sectional studies with few Level II prospective cohort studies and no Level I randomized studies. It was therefore believed appropriate to include Level III studies, but to take this fact into consideration when judging the strength of the overall evidence.

Selection of Participants

All studies that sampled participants from worker populations or general populations were considered. However, a number of studies selected participants from workers compensation or similar databases, a method subject to selection bias.

Carpal Tunnel Syndrome Case Definition

Many studies used self-reported symptoms, physical findings (usually Tinel’s sign and Phalen’s test), or both without nerve conduction measurement. Symptoms were usually self-reported either through questionnaire or interview often done by research assistants rather than by physicians experienced in the diagnosis of CTS. In addition, studies differed in their definition of symptoms required for the diagnosis with regard to location, severity, and frequency. The issue of CTS case definition was considered to be essential for the purpose of this report. A case definition based entirely on symptoms would probably lead to substantial misclassification. The prevalence of symptoms (numbness or tingling) in the hands is relatively high, and only a proportion of these symptomatic persons actually have CTS.^8,¹²

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