Carpal tunnel syndrome and computer exposure at work

By Professor A. Descatha

wrist hurting carpal tunnel work workplace injury

The boom in computer use and concurrent high rates in musculoskeletal complaints and carpal tunnel syndrome (CTS) among users have led to a controversy about a possible link. Most studies have used cross-sectional designs and shown no association. The present study used longitudinal data from two large complementary cohorts to evaluate a possible relationship between CTS and the performance of computer work.

Carpal tunnel syndrome (CTS) is a common, painful and costly disease among working adults. It is also the most common entrapment neuropathy of the upper limb.1 ,2 Studies on CTS have reviewed the potential risk factors and confirmed its relationship with biomechanical exposure at work;3–6 CTS occurs more frequently among workers in occupations with high physical exposures, that is, forceful and repetitive hand movements combined with awkward posture.5 ,6

However, the growing use of computers and the concurrent increase in musculoskeletal complaints among users have led to a long-running controversy over the role that exposure to computer use may play in the development of CTS. Some past studies have shown a possible link between computer exposure at work and musculoskeletal disorders including CTS.7 ,8 Recently, published studies have shown opposite results,5 ,9–11 and several reviews concluded that there was no evidence for such an association.5 ,12–15 Only one study has used a longitudinal design, and found a positive association between right-handed mouse use (over 20 h/week) and symptoms of CTS.16 Some countries, including France, consider CTS to be potentially caused by computer use at work, so computer workers with CTS are eligible for compensation by Workers’ Compensation Insurance.8 ,17

We set out to explore the possible association between computer use and CTS using longitudinal data from two large cohorts, one from Europe and one from the USA. Each study used a specific case definition of CTS and followed participants for several years.

The Cosali study (Cohorte des Salariés Ligériens, Principal Investigator YR) gathered data prospectively on a large sample of workers from the Loire Valley area of West Central France. This area represents 5.6% of the French workforce, whose demographic characteristics and distribution of employment are similar to that of the national workforce.19 ,20 At the time of the first survey, all salaried employees in France, including temporary and part-time workers, were required to undergo a mandatory annual health examination by a qualified occupational physician. Each physician was in charge of the medical surveillance of a group of companies. Participants were randomly selected from workers aged between 20 and 59 years who underwent an annual health examination between April 2002 and April 2005 by 1 of the 83 regional occupational physicians who volunteered to recruit workers to the study. The distributions of jobs and industries among the participants were similar to those of the region. Participants filled out self-administered questionnaires before each health examination. They were invited to receive a repeat examination between 2007 and 2010.

The Cosali cohort recruited 3710 participants and completed follow-up on 41.8% of the participants (figure 1). The PrediCTS cohort enrolled 1107 participants at baseline; 711 (64.2%) had complete exposure and outcome data at follow-up, and did not have prevalent CTS at baseline (figure 2). Comparing participants who completed the follow-up visit with participants lost to follow-up in the Cosali cohort, a prior publication showed that those followed were older and employed for a longer period in the initial job, with fewer temporary workers and workers employed in the agriculture sector.20 ,31 For the PrediCTS cohort, there were no statistically significant differences at baseline for age, gender, body mass index, medical history or baseline job category between those with follow-up data and those lost to follow-up.32 Similar group comparisons showed no difference in prevalent CTS between those followed or lost to follow-up in either cohort. Both cohorts were predominantly male. Participants were primarily employed in service (56.9%) and clerical (37.3%) jobs for the Cosali cohort, while they were more often employed in construction (40.8%) and clerical jobs (36.9%) at baseline for the PrediCTS study. As summarised in table 2, the subject’s age ranged between 20 and 59 years in the Cosali cohort, while more than 50% of the PrediCTS participants were aged less than 30 years.

This study evaluated the potential relationship between computer use and CTS using simple measures of computer use (time spent at the computer and the importance of computer use in the job) and found no association in two separate prospective cohort studies. Few of the workers employed in jobs dominated by computer use developed CTS. Both cohorts included workers from many different industries whose tasks involved various other biomechanical exposures (ie, hand force, repetition, vibration) that have previously been associated with CTS. Within these diverse working populations, any association between using a computer and CTS was too small to be detected.

There were some limitations to this study. In the Cosali cohort, only 41.8% of the baseline cohort participants remained in the follow-up assessment. This attrition parallels the decline in employment observed in the region during the economic crisis of this period, with a reduction in employment by 3.4% of regular workers and 33.7% in temporary employment agencies.31 The lowest participation rate was among young workers, workers in temporary employment and those with a short length of service. Even though this situation might affect the incidence of musculoskeletal disorders,33 its impact was unlikely to have a major effect on the observed exposure associations.34 Furthermore, the simulations to compute hypothetical estimates of associations between CTS and computer work among participants lost to follow-up indicated that unrealistically high risks of CTS associated with computer work would be needed in these participants to affect the results. In addition, a prior study of workers with CTS showed that few patients leave their jobs following a diagnosis of CTS.35

The case definition in the Cosali cohort did not include NCS, which are generally recommended for improving diagnostic accuracy, and could potentially have led to some disease misclassification. Previous comparisons between different case definitions of CTS showed relatively good agreement, including agreement between the case definition used in the Cosali cohort and a definition including NCS.36 ,37 Furthermore, the two independent studies showed similar associations despite using two different case definitions of CTS.

It is unclear how much misclassification may have been introduced by the measures used to assess the exposure to computer use.38 ,39 In the Cosali cohort, a self-administered questionnaire was used to assess computer use during a typical workday in the preceding 12-month period, and awkward postures were presented to the subjects in a diagram or picture to facilitate the participants’ understanding, and increase the reliability of self-assessment of posture. There was very high concordance (over 0.9 κ value) between exposures defined as viewing a computer screen and as use of data entry devices. Furthermore, Ijmker et al40 compared self-reported assessment of computer use to measured use. In spite of the low correlation between the two assessment methods (r 0.2), neither method showed an association between computer use and musculoskeletal symptoms.41 ,42 The PrediCTS cohort used a job exposure matrix based on exposure values from the Occupational Network (O*NET) database. O*NET is a publicly available online database that describes occupational demands across US job titles; it has recently been used successfully to estimate workplace physical and psychosocial exposures and organisational characteristics in epidemiology studies.17 ,43 However, the computer-related item from O*NET, “How important is working with computers to the performance of your current job?”, does not specifically address the biomechanical factors for the intensity, duration or work positions of computer work tasks. In addition, the question in the Cosali study did not differentiate exposures from various input devices (keyboard, mouse, stylus). Although exposure misclassification, confounding or other interactions may have modified the observed associations in both cohorts, very large effects would be required for computer use to constitute a risk factor for CTS among our study populations.

Preliminary analyses found that forceful work exposures and computer work exposures were strongly negatively associated, so jobs were dominated by one exposure more than the other. In the Cosali study, hard exertion to exhaustion decreased with the increased daily duration of computer use, and office workers had a relatively lower proportion of new CTS than other jobs in the PrediCTS cohort. Since the cohorts contained workers from a wide range of jobs but only the exposure for computer use was added to the models, the OR below 1.0 suggests that workers in jobs requiring computer use have a lower incidence of CTS than workers with other occupational risk factors that were not included in the model. Computer work involves various tasks and durations of exposure, including keyboarding/typing and use of a computer mouse in different work situations. In some companies, computer work may involve awkward sustained postures or other conditions which may be associated with an increased CTS risk. The low associations seen in our two cohorts suggest that the risk found for computer work in general is less important compared to the risk factors for CTS found in other job sectors.

The strengths of both cohorts were their prospective, longitudinal study design and the rigorous case definitions of incident CTS cases. Each cohort had specific strengths: the Cosali cohort was representative of a French region’s working population and used standardised questions specific to computer duration exposure, whereas the PrediCTS study followed a large group of workers over a long period of time with low attrition, and used a robust case definition that included NCS. These studies have different methodologies and populations that made it undesirable to pool the data from the two samples; however, the similar association values found in the two studies is noteworthy.

Other studies have found similar results. Atroshi et al44 conducted a cross-sectional study on 2465 participants aged between 25 and 65 years randomly selected from the population register of south Sweden. The results showed a similar pattern with no significant associations between CTS and the duration of daily computer use (OR<1 h/day 0.9 (0.5;1.7), OR1–4 h/day 0.6 (0.2–1.2) OR>4 h 0.5 (0.2–1.2)). Thomsen et al45 found a positive association between computer work and CTS in a cross-sectional analysis, and they had only two incident cases and were thus unable to conduct the longitudinal study. Andersen et al16 conducted a longitudinal study with a 1-year follow-up in a cohort of 9480 participants recruited from a Danish trade union of technicians. At the 1-year follow-up, case definitions based on the presence of new or worsened symptoms showed that the incidence of possible right-hand CTS was 5.5% (n=198) and the onset of possible CTS was associated with mouse use. However, the use of case definition based solely on symptoms may have created a misclassification of cases, 46 and the authors concluded that “computer use did not pose a severe occupational hazard for developing symptoms of CTS.”

In conclusion, two large prospective cohort studies from different countries found that computer work was associated with lowered risks of CTS when compared to workers across multiple industries. This study does not rule out the possibility that specific biomechanical exposures in some types of computer work may increase the risk for CTS, especially within worker groups without exposure to any other hand-intensive work. It is important to improve any work conditions that place computer workers in sustained awkward postures or lead to other symptoms; surveillance of workers using new technologies is also necessary. However, compensation programmes for work-related disorders should recognise that computer users are at lower risk for CTS compared to workers in food processing, manufacturing, service work, construction and other occupations where repeated or sustained exposures to forceful hand exertions pose a strong increased risk for CTS.47