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
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
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.
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.
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